Dilshan.Jayakody

Hello, I am Dilshan R Jayakody, 4S6DRJ, an embedded systems engineer from Sri Lanka. My interests range from Electronics, astronomy, amateur radio, Meccano modeling, and woodworking.

[jayakody2000lk]

Building the TD4 4-Bit CPU

The TD4 is a famous 4-bit CPU featured in the book How to Build a CPU by Kaoru Tonami. The book focuses on constructing a functional processor entirely from basic 74-series TTL logic ICs. While the book is unfortunately only available in Japanese, a friend from Japan sent me a copy along with a TD4 PCB. I believe the PCB is based on the open-source design files available on BG5DIW's GitHub repository.

Recently, I finally found the time to build and experiment with it. The project took a few months, as I had to translate the book myself to grasp the core concepts. The overall design is simple and elegant, offering a set of 12 instructions and a 16-byte ROM (implemented via DIP switches) for programming. The board operates on 5V and can be powered via USB. Most components were sourced from local shops, though I had to order a few 74HC-series ICs online. Later, I tested the circuit by replacing some 74HC components with 74LS series chips, and it worked without any issues.

The PCB is straightforward to solder. The only SMD components are four LEDs and a Micro-USB socket. If you haven't done SMD soldering before, the USB socket might look intimidating, but you only need to solder pins 1 and 5 (the remaining pins are not used in this design).

The most time-consuming part of the assembly was soldering the 1N4148 diodes. The book suggests using a 1S1588 or a D8-1A diode network, but these are difficult to source locally or online. Therefore, I opted for individual 1N4148 diodes. In the TD4 architecture, these diodes function as a Diode Matrix ROM. Since the "program" is stored via physical DIP switches, the diodes act as one-way gates. They prevent current from back-feeding into other parts of the circuit when multiple switches are flipped, effectively creating the logic gates necessary to decode the 16-byte memory addresses into specific 4-bit instructions for the CPU to execute.

Once assembly is complete, the TD4 is ready for immediate use; it requires no firmware or calibration. The PCB offers two power options: the easiest is via the Micro-USB port, while the second is a 2.54mm pin header. I used the pin header to attach the power leads for my logic probe, which was essential while I studied the circuit's operation alongside the book.

The TD4 is a hard-wired logic CPU, meaning it doesn't use a microprogram; every instruction is executed by direct electrical paths through the logic gates. The chipset typically consists of:

74HC161: Used for the A and B registers and the Output port.

74HC161: Another 4-bit counter used for the Program Counter (PC).

74HC153: Multiplexers that act as the selector for the ALU inputs.

74HC283: Configured as 4-bit ALU.

74HC10 and 74HC32: NAND/AND gates used for the instruction decoder.

The clock is generated by a simple 74HC14 oscillator circuit, allowing you to step through instructions manually or run them at a very low frequency to watch the LEDs toggle as data moves between registers.

The most interesting part of this project is programming the system. Due to the limited address space, the tasks you can perform are modest. I experimented with various counter systems, and all worked as expected. I initially wrote the code on paper and calculated the DIP switch positions manually based on the book's instruction set.

To simplify this process, I developed a small web-based utility using JavaScript that converts assembler source code directly into DIP switch positions. This allowed me to iterate quickly and try out more complex logic. You can find this TD4 Assembler utility here: github.com/dilshan/td4-assembler.

This is one of the counter programs I tested with the CPU. This specific routine reads the input from the DIP switches and increments the value with every clock cycle. Once the counter reaches 0xF, it resets back to the user-specified starting value and resumes counting.:

in b out b add b, 1 jnc 1 jmp 0

The TD4 is the perfect "Hello World" for computer architecture because it strips away the complexity of modern processors. There are no hidden layers-no microcode, no complex cache, and no operating system. By building it, you see exactly how a bit moves from a switch, through a multiplexer, and into a register. It demystifies the relationship between physical hardware and software logic, making it an invaluable educational tool for anyone interested in embedded systems or electronic engineering.

Details for How to Build a CPU can be found at the publisher's support site. A TD4 emulator (Windows only) is also available for download at the same link.

[jayakody2000lk]

TDA2005 as a DC-to-DC Converter

The TDA2005 is a very popular audio amplifier chip that many of us have used for years in car stereos and home DIY amplifiers. While it is mainly built to amplify AF signals, its internal design is actually strong enough to be used for other interesting projects like this DC-to-DC converter. By using the two amplifier channels inside the chip to create a high-frequency signal, we can turn the IC into a power driver that works with a rectifier circuit.

The way this circuit works is quite simple. The TDA2005 stages (U1A and U1B) are set up to produce a high-frequency AC signal. This signal goes through 2200µF coupling capacitors and then into a full-bridge rectifier made of 1N5822 Schottky diodes. I chose these specific diodes because they have a very low voltage drop, which helps keep the efficiency high. To keep the TDA2005 stable while it drives this kind of load, I included 1Ω resistors and 0.1µF capacitors as stability networks. This stops the chip from oscillating at very high frequencies, which could otherwise cause it to overheat or fail. The final output is cleaned up by a 4700µF filter capacitor and a small RC snubber to give a steady DC voltage.

Even with this design, there are a few things to keep in mind. Since the circuit does not have a feedback loop, the output voltage will change a bit depending on how much load you connect or how the input voltage varies. Also, heat is a major factor. In my testing, I found the overall efficiency to be around 80%, but the TDA2005 will still get hot during long use. Because of this, use a proper heatsink and make sure the 1Ω stability resistors are 1W rated so they don't burn out under stress.

When assembling the PCB, start by soldering the smaller parts like the 1W carbon film resistors and ceramic capacitors first. Then, you can add the larger electrolytic capacitors. Please make sure all capacitors are rated for 35V or higher just to be safe. Finally, always bolt the TDA2005 to a good heatsink with some thermal paste before you turn it on.

Schematic and PCB files of this project are available in this Google drive location.

[jayakody2000lk]

ZZRX-40 Receiver build

I found the ZZRX-40 receiver while going through some old issues of QEX magazine. Designed by Craig Johnson (AA0ZZ), this is a very simple and easy-to-build 40m direct conversion receiver. Even though it is a simple circuit, it performs quite well for its size.

For this project, I decided to make my own PCB design because I wanted to use components that I could easily find in our local shops. Since I couldn't find any PCB files online, I drew the schematic in KiCad and designed a custom board. If you are interested in building this, I have shared the Gerber files and KiCad project files here: https://github.com/dilshan/zzrx40-clone.

The heart of this receiver is the popular NE602 (or SA602) mixer chip. This single chip handles both the local oscillator and the product detector, which makes the whole circuit very compact. Although it is designed for the 40-meter (7 MHz) ham band, my prototype was also able to pick up 41-meter band shortwave stations, especially strong signals from China Radio International (CRI).

The receiver has two oscillator modes. The Crystal-Controlled (VXO) mode is definitely the most stable way to use it. By using the tuning potentiometer, I could shift the frequency a few kHz up or down from the crystal's main frequency. I tested this with many different crystals from 2E0HQU Lester Russell's collection, including 7.000, 7.020, 7.030, 7.040, 7.050, 7.1424, 7.159, and 7.200 MHz, and they all worked perfectly.

There is also a VFO mode, which uses a 50pF trimmer capacitor for calibration. In my experience, this mode is a bit "drifty" compared to the crystals. You have to do a lot of fine-tuning to keep the station locked in. After the mixer, the signal goes through a low-pass filter and then to an LM386 audio amplifier. I tested both LM386N-1 and LM386N-4 versions, and both worked fine. You can use a small 8Ω speaker, but using low-impedance headphones gives much better audio when listening for weak stations.

What I really like about the ZZRX-40 is that it's not just a receiver, but a great tool for learning radio design. I added on-board jumpers to my PCB so I can bypass the internal oscillator. This is very useful because you can plug in an external oscillator, like a Si5351 VFO or an Arduino signal generator, to see how it performs.

The PCB for this project was fabricated by PCBWay, the project's generous sponsor, which provides comprehensive manufacturing, assembly, CNC, and 3D printing services. The PCB for this project is also available on PCBWay for direct ordering. Check out the PCBWay website for its manufacturing capabilities and pricing.

For my testing, I used a 9V DC power supply and my 50Ω outdoor wire antenna. Since I used the 1A version of the 7806 regulator in this design, it is safe to feed a 12V or 15V DC input to the circuit. For voltages under 12V, no heatsink is required for the 7806. Like the original ZZRX-40, this design also supports battery power, but it switches to battery only if the DC jack is removed (the manual power source selection switch was removed in my design).

The receiver is excellent for listening to CW and shortwave broadcasts. However, like many simple direct-conversion designs, SSB signals proved a bit more difficult to tune, and the audio was less intelligible in the 7.040 - 7.060 MHz range compared to CW.

Building the ZZRX-40 was a great experience. It shows that you don't need a very complicated circuit to start listening to the shortwave radio bands. Even with a few limitations in SSB mode and VFO drift, the ability to experiment with different crystals and external oscillators makes this a perfect project for any electronics hobbyist.

[jayakody2000lk]

A DIY build of a two-tube FM receiver kit

I recently received a DIY two-tube FM receiver kit from one of my Chinese component suppliers for review. The circuit is elegantly simple, designed around a 1A2 heptode and a 2P2 pentode. The kit includes all the necessary components for the build, featuring a single-sided FR4 PCB with dimensions of 100mm × 100mm.

The primary hurdle when starting this project was the complete lack of proper assembly instructions. To assist others, this article documents the exact steps I took to assemble and test the receiver. I have also reverse-engineered the schematic based on the PCB layout; a high-resolution PDF version is available for download here.

The radio is designed around two main stages to handle frequency conversion and audio amplification: The 1A2 heptode - functions as a super-regenerative detector. The 2P2 pentode acts as the audio power amplifier, which receives the demodulated signal and drives the headphones via an output transformer.

I began by soldering the resistors, capacitors, and 33µH inductors. During assembly, I noticed that several component footprints did not perfectly match the provided parts, specifically regarding lead spacing and component boundaries.

The most significant mechanical issue was mounting the tuning capacitor. The capacitor pins did not align with the PCB holes, requiring me to slightly trim the sides of the pins to fit. To secure the capacitor firmly to the board, I used two M2.5 × 5mm screws, which were not included in the kit.

For the inductors L1 and L2, I used 22 SWG enameled copper wire. Both are wound on a 14mm former: L1 - 2 turns and L2 - 4 turns.

Interestingly, the kit utilizes a standard 230V to 6V step-down transformer as the AF output transformer. The primary (230V) winding connects to the 2P2 pentode, while the secondary (6V) winding connects to the audio output jack.

The circuit operates on two distinct power supplies: a 1.35V DC source for the filaments and a 67V DC source for the anode/plate. Power consumption is notably low, with the filaments drawing about 80mA - 85mA and the 67V line using under 10mA.

For the audio output, I used standard low-impedance (32Ω) stereo earphones.

Tuning this receiver is not a straightforward task. I used an RF signal generator to calibrate it to known frequencies. Overall, the performance is modest; it struggles to clearly receive standard FM broadcast stations. Achieving the best possible results requires a "balancing act" between the C2 variable capacitor and the R2 potentiometer. The potentiometer adjusts the regeneration level, effectively acting as a sensitivity control.

During my tests, I could only pick up a few stations, even when coupled with a 50Ω outdoor FM antenna. However, the receiver performed well with FM microphones and low-power FM transmitters over short distances. A notable limitation is the frequency coverage; sensitivity deteriorates significantly at the higher end of the band. While it performs adequately between 75MHz and 99MHz, it fails to cover the full 88MHz - 108MHz broadcast range effectively.

I would not recommend this kit for those looking for a reliable way to listen to FM broadcasts. However, it is an excellent choice for educational or experimental purposes. It successfully demonstrates the principles of FM reception with a minimal component count, though it understandably trades sensitivity and stability for simplicity.

[jayakody2000lk]

OpenHamClock Kiosk

For an amateur radio operator, having a real-time dashboard is essential for monitoring band conditions and identifying DX opportunities. While several solutions exist, I recently completed a project centered on OpenHamClock, an open-source, web-based successor to Elwood Downey’s (WB0OEW) original HamClock. My goal was to move beyond simply opening a browser tab and instead create a dedicated, always-on kiosk for my workshop/shack. This setup allows me to monitor live DX cluster spots, PSKReporter reception, and satellite X-ray flux in real-time, helping me decide exactly when to jump into a radio session.

The heart of this project is a two-component system: a centralized server and a remote kiosk unit. For the server, I utilized a PC equipped with an AMD Ryzen 7 series CPU and 32GB of RAM. While this machine hosts multiple other services, it serves as the perfect, reliable host for the OpenHamClock backend.

The installation process is straightforward, following the official repository guidelines to get the software running on Node.js. To tailor the dashboard to my specific needs, I configured a .env file in the root directory with my preferred settings. Once I verified the deployment was accessible via the default port 3000, I transitioned the application into a formal system service to ensure it runs continuously and resumes automatically after system restarts.

When creating the custom openhamclock.service script, it is vital to update the Environment, WorkingDirectory, and ExecStart paths to match your specific installation. If you use a Node.js version manager other than NVM (such as fnm or brew), ensure you modify the service file accordingly. Additionally, update the User and Group variables to reflect your Linux credentials. After a quick daemon-reload and enabling the service via systemctl, the backend was up and running without any issues.

With the server established, I turned my attention to the kiosk hardware. I chose the Banana Pi M4 Zero, based on the Allwinner H618 with 2GB of DDR4 RAM - paired with a 10.1-inch display. This SBC provides ample power for a dedicated web interface while maintaining a tiny footprint. Because the kiosk is intended for long-term operation, thermal management and power stability were my primary hardware priorities.

I am running this unit in a workshop environment where ambient temperatures typically stay between 29°C and 31°C. To prevent overheating during extended uptime, I installed a commonly available Banana Pi M4 Zero heatsink kit. After 24 hours of continuous operation, the board temperature stabilized at approximately 38°C, which is a very safe range for this chipset. Powering the unit requires a reliable source to avoid mid-session crashes; I used a 5V 3A power supply unit to drive the board.

The software side of the kiosk required a "set it and forget it" approach. I used Armbian to craft a minimal OS image built on the X Window System using the Openbox window manager and Chromium. This image requires no keyboard or mouse and boots directly into the dashboard. To protect against SD card wear and file system corruption, the OS is designed to launch with a read-only rootfs. While the initial launch takes a moment, subsequent boots are impressively fast, typically reaching the dashboard within 30 to 35 seconds.

Integrating the Banana Pi into the display unit involved some custom fabrication. I designed a mounting unit using a 2mm acrylic sheet to securely attach the SBC to the back of the 10.1-inch display. Using a laser cutter, I precision-cut the sheet and all necessary mounting holes. This mounting panel drawing available in the download pack.

The final touch was optimizing the UI for the hardware. Given the display’s 1024×600 resolution, I configured OpenHamClock to run in "tablet mode", which scales the information perfectly for this screen size. The unit has been running continuously in my workshop for a few weeks now, allowing me to monitor the ionosphere, HF band traffic, and track SOTA/POTA activators in real-time.

All mentioned service scripts, environment files, and mounting drawings are available in my Google drive, and the OS build system can be found on https://github.com/dilshan/openhamclock-kiosk.

[jayakody2000lk]

Discrete CMOS digital desk clock

This project began with a simple idea: to build a fully functional digital desk clock using only classic CMOS logic ICs that are still readily available in local electronics markets. No microcontrollers, no firmware, no programmable logic - just counters, flip-flops, logic gates, and seven-segment decoders doing exactly what they were designed to do. The result is a 24-hour digital clock powered from a 5V, 1A supply, drawing roughly 200mA, and assembled entirely by hand on a perfboard using point-to-point wiring.

At the heart of the clock is a traditional timebase derived from a 32.768kHz crystal oscillator. This frequency is widely used in timekeeping because it can be divided cleanly to exactly 1Hz. A CMOS frequency divider, such as the CD4060 and CD4013, performs this task, producing a precise one-second pulse that serves as the master clock for the entire system. From this point onward, time is counted purely in hardware, one pulse at a time, just as early digital clocks did.

The one-second pulses are fed into a cascade of CMOS BCD counters. The first pair of counters handles the seconds, counting from 00 to 59. Once the count reaches 60, additional logic resets the seconds counters back to zero and generates a carry pulse for the minutes section. This same structure repeats for minutes, which again count from 00 to 59 before rolling over and advancing the hours counters. For the hours, the logic is slightly different, as the clock operates in a 24-hour format. Dedicated reset logic ensures that the count wraps correctly from 23 back to 00.

Each digit of the clock is displayed using a seven-segment LED module driven by a BCD-to-seven-segment decoder. These decoder ICs take the four-bit BCD output from the counters and translate it into the appropriate segment pattern. This greatly simplifies the design, as there is no need to generate segment patterns manually using discrete logic. Current-limiting resistors are placed on each segment line to protect the LEDs and keep the brightness consistent across all digits.

User interaction is also handled entirely in hardware. Push buttons allow manual setting of minutes and hours. When pressed, these buttons temporarily override the normal 1Hz clock signal and inject faster pulses into the relevant counters, allowing the time to be adjusted quickly. A stop/run control lets the clock be paused during setting, preventing accidental increments while adjustments are being made. All switch inputs are conditioned with simple RC networks and Schmitt-trigger inverters to ensure clean, bounce-free transitions. The clock is designed to allow time adjustment while running, and when setting minutes, the seconds are automatically reset to zero.

Powering the clock is straightforward. The entire circuit runs from a regulated 5V supply, with local decoupling capacitors placed near every IC to maintain signal integrity and suppress noise. Despite the number of chips involved, the total current consumption stays around 200mA, well within the capability of a 1A power adapter. This makes the clock reliable for continuous operation without excessive heat or stress on components.

One of the most enjoyable aspects of this project is how it was physically constructed. Instead of using a printed circuit board, the entire design was built on a perfboard using point-to-point wiring. While this approach requires patience and careful planning, it offers tremendous flexibility and a very satisfying hands-on experience. Once completed, the finished circuit was mounted inside a homemade picture frame built from molded PVC picture frame bars, giving the clock a unique, personal enclosure that blends electronics with DIY fabrication.

Using discrete CMOS logic for a clock like this comes with clear advantages. The design is fully transparent - every function can be understood by following the signals from one IC to the next. There is no firmware, no debugging tools required, and no dependency on proprietary development environments. The components are inexpensive, widely available, and electrically robust. From an educational perspective, this approach provides deep insight into digital design fundamentals such as counting, decoding, gating, and timing.

At the same time, this style of design has its limitations. Compared to a microcontroller-based clock, the circuit is physically large and component-heavy. Adding features like alarms, date display, or power-saving modes would significantly increase complexity. Power consumption is also higher than that of a modern low-power MCU solution, especially due to the continuously driven LED displays. Any functional change requires hardware modification rather than a simple firmware update.

Despite these trade-offs, this CMOS digital desk clock succeeds exactly at what it set out to do. It demonstrates that reliable, accurate timekeeping is possible using nothing more than classic logic ICs and careful design. Beyond the technical aspects, the main motivation behind this project is its retro-style design and construction technique. I primarily designed it as a fully handmade piece to decorate my home workshop and to celebrate traditional, hands-on electronics.

Schematic of this project is available in https://github.com/dilshan/cmos-clock. This project is released as an open hardware design under the Creative Commons Attribution 4.0 International (CC BY 4.0) license. You are free to use, modify, and share the design with proper attribution.

[jayakody2000lk]

FA-5 VizControl

The BG7TBL FA-5 frequency counter has become recognized as an excellent choice for experimenters and radio enthusiasts, offering high-spec performance comparable to branded units. But it remains remarkably accessible due to its low cost. 

The FA-5 has powerful hardware. However, most existing control applications lack a good user experience. Many of them rely on scripting languages, such as Python, while others utilise simple command-line interfaces. These limitations inspired us to create FA-5 VizControl. This native application provides a comprehensive, intuitive, and portable desktop experience for managing the FA-5.

We developed FA-5 VizControl using Lazarus and the FPC (Free Pascal Compiler). This choice ensures maximum compatibility and portability. The application functions as a standalone, portable executable on both Linux and Windows operating systems. There is no need for complex installation routines. Users simply download and launch it. Connectivity with the FA-5 uses virtual serial port emulation over USB. Usually, no specialized device drivers are needed.

The GUI provides clear access to all FA-5 control options. Users can select the input source (including the internal 10MHz reference), configure input impedance, and set gate time. Additional toggles are available for essential device parameters, such as input impedance and LPF settings. The GUI also allows users to directly program the default configuration to the FA-5's internal EEPROM via a dedicated menu option.

The lower portion of the FA-5 UI features a real-time data plotting graph, which maps the frequency over sampled points. This immediate graphical feedback is useful for analyzing signal stability or drifts. The user can also view measurements in different scales, ranging from Hz to GHz.

FA-5 VizControl is an open source application. The source code and compiled binaries are available at https://github.com/dilshan/fa5-vizcontrol.

[jayakody2000lk]

Surge arrester array

This is a simple surge arrester array that I made in my free time. Built using readily available components, the array provides effective shielding against lightning-induced surges and static discharges that can damage sensitive transmitting and receiving systems.

The primary goal of this project was to create a robust, low-loss surge protection unit that can handle multiple RF lines simultaneously. By integrating gas-discharge based lightning arresters with a solid grounding infrastructure, the array ensures that transient over-voltages are safely diverted to ground without affecting normal signal transmission.

The core of this project relies on a few key components:

At the heart of the design are eight BL-2000 Lightning Arresters, each incorporating a GDT (Gas Discharge Tube). These units are designed to conduct high-voltage transients from the coaxial line directly to ground within microseconds of a surge event. The BL-2000 series is available in several barrel configurations; for this build, PL259 female-to-female models were selected for compatibility with standard coaxial systems.

Each arrester is rated for 50Ω impedance and 400 W power handling, supporting a frequency range from DC to 1000 MHz, making it suitable for most HF to UHF amateur radio applications.

Technical specification of the BL-2000 is listed below (copied from supplied datasheet):

Frequency range: DC ~ 1000MHz

VSWR: 1.1 or less

Loss: Below 0.2dB 

Rated power: 400W PEP

Minimum discharge voltage: DC230V ±15%

Discharge voltage: 1000V

Maximum discharge current: 6000A

Repeated discharge capability: DC230V ±20%

Insulation resistance: 10000MΩ and above

External dimensions: 77×41×21 (W×H×D) mm

Weight: 114g

Two 2.5 mm-thick copper bus bars are used as the main conductive path of the array. These copper strips - commonly used in electrical distribution and lightning protection systems—provide a low-impedance route for surge currents. The arresters are mounted such that their metal bodies are both mechanically and electrically bonded to the bus bars using 12mm stainless steel M6 screws. To minimize corrosion and ensure long-term conductivity, a battery terminal protection compound is applied to all connection points.

The entire assembly is mounted on a galvanized metal or aluminum plate, which serves both as a mechanical support and as an additional conductive surface. In this setup, the mounting plate from a panel board enclosure was repurposed to provide structural stability and grounding continuity.

A 7/1.70 gauge grounding conductor connects the copper bus assembly directly to a dedicated earth ground. This ensures that surge energy is rapidly and safely dissipated into the earth during a lightning or static discharge event.

During normal operation, the GDTs inside the BL-2000 arresters remain in a non-conductive state, maintaining the integrity of the RF signal path. However, when a surge or high-voltage transient occurs, the GDTs ionize instantly, creating a conductive path that shunts the voltage spike from the coaxial center conductor to the arrester body. The surge current is then conducted through the copper bus and grounding wire into the earth, effectively bypassing and protecting the connected transceivers and receivers.

The array was tested with several HF transceivers across multiple frequency bands. No measurable change in S.W.R. was observed, indicating minimal signal loss and excellent impedance matching.

[jayakody2000lk]

Arduino Mini Amplifier

The Arduino Mini Amplifier is a compact audio system that merges vintage analog and modern digital components. It combines a Philips TDA8425 audio processor and an Arduino Nano, supporting Bluetooth and line-in audio, spectrum analysis, stereo modes, and software-controlled tone adjustment.

The Arduino Mini Amplifier is organized into six major subsystems: the audio processor, spectrum analyzer, Bluetooth source, power amplifier, control interface, and power management unit. This modular setup allows each part to operate with minimal interference from the others, making designing and PCB routing easier. The system uses separate paths for analog and digital ground signals (these are called GNDA for analog ground and GNDD for digital ground). Their connections are limited to specific points, which helps prevent unwanted electrical noise from the MCU from reaching the analog audio circuits.

The audio section is centered on the TDA8425, a dual-channel audio processor developed by Philips in the late 1980s. Originally used in televisions and high-end audio receivers, this device offers precise analog tone control and volume control with excellent linearity and low total harmonic distortion. Despite its age, the TDA8425 remains highly regarded for its analog fidelity and simplicity. Although this chip was manufactured in the 1980s, it can still be purchased at local and online electronics stores.

In this project, it performs input source selection, tone adjustment, and stereo effect management, while maintaining a fully analog signal path. The Arduino Nano communicates with the TDA8425 via an I²C interface, issuing control commands to configure volume, bass, treble, and stereo modes in real-time.

A distinctive feature of the amplifier is its 16-channel real-time spectrum analyzer. The spectrum analyzer in this amplifier is a pure software implementation. Rather than relying on analog filter banks or external ICs, the amplifier’s Arduino Nano samples the audio waveform directly through its ADC, processes the data using an FFT algorithm, and visualizes the results on the LCD as a bar-graph style spectrum.

To prevent weak signals from being invisible and strong signals from saturating the display, the firmware applies a dynamic scaling process, and it adjusts the gain level of the spectrum data based on the overall amplitude of the audio input. As a result, the spectrum display maintains consistent visibility regardless of the input volume level.

Finally, the analyzer data is rendered on a 16×2 LCD as a continuously updating frequency display.

Because the NE5532 buffers offer excellent noise performance, high slew rate, and flat frequency response, the analyzer introduces negligible distortion, ensuring the display remains accurate and responsive without affecting the main audio path.

To accommodate modern audio sources, the amplifier integrates an AC6939-based Bluetooth module capable of receiving stereo A2DP audio streams. The module’s outputs are capacitively coupled and routed into the TDA8425 input selector, where they are treated as one of the primary sources alongside the wired line input.

The Yamaha YDA138, a compact Class-D audio power amplifier IC, serves as the final stage of the system. It delivers up to 2×10 W RMS into 8 Ω loads, providing sufficient power for desktop or bookshelf speakers while maintaining excellent efficiency and low thermal output.

The amplifier output is routed through an LC low-pass filter to remove PWM switching components before reaching the output connectors. A relay-controlled switching system, driven by the Arduino, toggles between speaker and headphone outputs. This ensures safe operation and prevents simultaneous loading of the Class-D output stage. Additionally, a digital mute control line is managed by firmware, enabling silent mode transitions and power-on muting to eliminate switching pops.

The core component of the system, Arduino Nano, is responsible for orchestrating all signal routing, mode selection, and user interface tasks. The Arduino Nano interacts with the TDA8425 and other subsystems through I²C, manages ADC sampling for the spectrum analyzer, and updates the 16×2 LCD display. Four front-panel tactile buttons (UP, DOWN, OPTION, and MUTE) provide intuitive navigation through system parameters and tone adjustments.

The firmware, developed using PlatformIO, is based on standard Arduino libraries. The codebase utilizes non-blocking state machines and timer-driven routines, ensuring smooth LCD refresh, responsive button handling, and real-time spectrum updates without introducing latency or jitter.

Through this user interface, the user can control virtually every function, from tone adjustment to input source selection.

The high-quality PCB for this project was fabricated by PCBWay, the project's generous sponsor, which provides comprehensive manufacturing, assembly, CNC, and 3D printing services. The complete PCB design files are available for direct order through the PCBWay project page. Check out the PCBWay website for its manufacturing capabilities and pricing.

The Arduino Mini Amplifier PCB is designed using KiCad and optimized for manual assembly without requiring specialized equipment. Although the board primarily uses SMD components to achieve a compact layout and short signal paths, all component footprints are carefully selected to be hand-solderable using a fine-tipped soldering iron and standard tools. (A hand-soldering guide is included in the project documentation, outlining the recommended soldering order.)

The Arduino Mini Amplifier project is an open hardware project. All its design files, firmware source code, documentation, and other resource files are available at the git repository: https://github.com/dilshan/arduino-mini-amp.

[jayakody2000lk]

A simple transistor based electronic siren

For some time, hobbyists have relied on dedicated sound generator ICs, such as the UM3561, UM3562 to build compact alarm and siren circuits. Unfortunately, this device is becoming increasingly difficult to find in the market. The manufacturer of the chip, United Microelectronics Corporation (UMC), is still around. However, they are no longer producing the UM3561 or UM3562. As part of a fun experiment, I decided to design a replacement circuit using only discrete transistors and passive components. This avoids the need for specialized ICs altogether. The result is a simple yet effective transistor-based siren. It can reproduce the familiar rising and falling alarm effects typically associated with police cars, fire trucks, or ambulances.

The design philosophy behind this project was straightforward: recreate the functionality of the classic two-timer based siren circuit (NE556/NE555 pair), but instead of timers or sound generator ICs, use only transistors in a multivibrator and modulation arrangement. By carefully selecting resistor and capacitor values, we were able to replicate the frequency modulation effect that gives sirens their distinctive sweeping tone.

The circuit of this siren can be divided into three functional sections: the LF oscillator section, built around Q1 (BC548) and Q4 (BC558), which generates a slow modulation waveform. The rate of this oscillator, which controls how fast the siren "sweeps" up and down, can be adjusted via the 500k trim-pot (RV1). Capacitors C2 and C6, along with resistors R7 and R8, shape this low-frequency oscillation.

The next most crucial section is the audio frequency oscillator. Built around Q3 (BC558), this section is responsible for generating the actual audio tone. The output frequency is determined by the capacitor selected through jumper J1. By switching between C1, C3, and C5, different frequency ranges can be chosen, producing distinct siren effects. This is conceptually similar to selecting different alarm sound profiles on the UM3561.

The final section amplifies the modulated audio signal and drives a small speaker (16Ω - 32Ω). Diodes D1 and D2 provide bias stabilization for the push-pull transistor stage. This improves linearity and reduces distortion. The coupling capacitor C4 ensures that DC is blocked while allowing the audio signal to pass into the speaker.

This circuit is designed to operate from a 6V to 12V DC power source. The recommended supply voltage is 9V. The output frequency of this siren is heavily dependent on the supply voltage. Therefore, to produce stable output, this circuit needs a regulated "clean" DC power source.

To make the project more accessible, we designed a dedicated PCB for this circuit. The board is compact and uses commonly available through-hole type components. For convenience, standard 2.54mm pin headers are provided for the speaker, power input, and jumper settings.

We gratefully acknowledge PCBWay’s sponsorship of this project. Their reliable PCB manufacturing, assembly, and 3D printing services make it easy to produce both prototypes and final products, ensuring consistent quality and reducing the risk of errors or durability issues. The PCB of this siren is available to order at the PCBWay website.

To complement this project, we also created a short video showing the assembly process and testing of the siren. The demonstration highlights how the different jumper settings produce distinct alarm effects, and how adjusting the rate control potentiometer changes the siren’s sweep speed.

All the resources of this project, including KiCAD design files and schematics, are available to download at https://github.com/dilshan/simple-electronic-siren.

[jayakody2000lk]

Electronic fluid level controller

Managing liquid levels in tanks is a common challenge across home, agricultural, and industrial automation systems. Whether dealing with a rooftop water tank or a chemical reservoir in a processing plant, maintaining the correct fluid level is crucial to prevent overflow, avoid dry-run damage to pumps, and reduce unnecessary power consumption.

To address this challenge, we have designed a compact and reliable electronic limit switch module. This microcontroller based controller automates fluid level regulation with precision and ease.

Traditional fluid level control methods often rely on mechanical float switches and simple relay logic. While effective, these methods can suffer from issues such as bounce noise, relay chattering, and wear over time. This module modernizes this process by integrating microcontroller based debounce filtering, hysteresis, and control on/off delays.

To perform its operation, the module utilizes two float-type limit switches: a lower limit switch and an upper limit switch. Both are normally closed (NC) switches, meaning they open when floating in liquid and close when the liquid falls below their designated levels. This module supports not only mechanical float switches but also electronic sensors (such as IR or ultrasonic).

The integrated microcontroller continuously monitors the state of these switches, processes the information using hysteresis logic, and controls the onboard relay accordingly. For instance, if the tank level drops below the lower limit, the relay activates the pump. Once the upper limit is reached, the relay turns off the pump. This hysteresis mechanism prevents rapid oscillation of the relay if the fluid level hovers near a switch threshold, ensuring a smooth on/off cycle without causing wear-inducing chatter.

To implement the control logic, we use the PIC10F200 microcontroller. This ultra-low-cost, 8-bit microcontroller from Microchip is ideal for small, single-purpose control applications such as this module. The PIC10F200 features a simple architecture with 256 words of flash memory, 16 bytes of RAM, and a compact instruction set. The firmware for this project is written in C and can be compiled using Microchip’s MPLAB X IDE along with the XC8 compiler. To flash the MCU, use a compatible programmer like PICkit 3, PICkit 4, or MPLAB SNAP via the ICSP interface located on the PCB.

This project is released as open hardware. All content, including schematics, PCB design files, microcontroller firmware, and compiled binaries, is available in the project repository on GitHub. Complete project documentation is available at the wiki section of the project repository.

[jayakody2000lk]

Ecowitt weather station to APRS bridge

Ecowitt2APRS is a new open-source Linux service that I have been working on for the past few weeks. This project is designed to seamlessly integrate weather data from Ecowitt compatible professional weather stations into the global Automatic Packet Reporting System Internet Service (APRS-IS) network, transmitting this information in the standardized format of APRS weather packets.

APRS, an amateur radio-based system, facilitates real-time tactical digital communications, covering applications such as GPS position reporting and the dissemination of meteorological data. Ecowitt2APRS addresses a specific need within this ecosystem: providing a lightweight and efficient method for weather stations to contribute to this data stream.

The Ecowitt2APRS service is developed as a minimalist daemon for Linux environments. It ingests live weather data from Ecowitt compatible stations, converts it into the required APRS weather packet format, and then relays these packets to the APRS-IS network. One of my primary objectives was to create a service that operates effectively on minimal Linux systems, such as low-power SBCs like the Raspberry Pi Zero. This enables dedicated, continuous weather reporting with minimal operational overhead and power consumption.

I'm testing this system with my Ecowitt WiFi weather hub - GW2000C. The live feed from my Ecowitt2APRS instance is available as 4S6DRJ-13, and I am hosting the Ecowitt2APRS daemon on the Orange Pi Zero LTS board.

As an open-source initiative, all source code, documentation, and installation instructions are publicly available in the project repository on GitHub. This page also provides pre-built binaries of Ecowitt2APRS for ARM64, ARMv7, and AMD64 architectures. Anyone interested in deploying this can download the binaries or compile the source code themselves. The source code has limited external dependencies and can be easily built using the GCC toolchain. The compilation steps are outlined in the project's readme file.

To manually deploy the downloaded package, follow these steps:

1. Extract the files:

tar -xvf ./ec-aprs-wx-linux-<PLATFORM>.tar.gz

2. Copy the necessary files:

sudo cp ./ec-aprs-wx /usr/local/bin/ec-aprs-wx  sudo cp ./config.cfg /etc/ec-aprs-wx.cfg  sudo cp ./ec-aprs-wx.service /etc/systemd/system/ec-aprs-wx.service

Before starting the service, make sure to open the /etc/ec-aprs-wx.cfg file and edit the fields: APRS_CALLSIGN_SSID, APRS_PASSCODE, APRS_LATITUDE, and APRS_LONGITUDE. The details for these fields are covered in the readme file of the project.

To start the service, issue the following command:

sudo systemctl daemon-reload

To enable the service to start automatically on system reboot, issue the following command:

sudo systemctl enable ec-aprs-wx.service

To trigger this service, the Ecowitt device needs to be configured, with the configuration steps detailed in the project's readme file.

If all configuration steps are correct, you should be able to observe the weather data feed from APRS monitoring sites like https://aprs.fi or https://aprs.to.

[jayakody2000lk]

Open source 24-channel USB high-voltage driver

When it comes to automation and control systems, there's often a need for multiple digitally controlled output terminals with high-voltage handling capabilities. Many existing modules are bulky, expensive, or require numerous additional components to function. To address this gap, I've developed a fully open-source, USB-controlled 24-channel high-voltage driver. This device provides precise, flexible control in a compact and user-friendly package. The project is open hardware, released under the CERN-OHL-W license, ensuring transparency from hardware schematics to firmware code. The driver module communicates via USB using a simple virtual COM port, eliminating the need for special drivers and complex setups.

At the core of the system are three TPIC6B595 shift registers, each supplying eight open-drain outputs that can handle up to 50V and sink currents of up to 150mA per channel. These registers are daisy-chained to achieve a total of 24 outputs. The outputs are designed for low-side switching and include integrated clamping diodes, making them suitable for driving inductive loads such as relays and solenoids. Data is clocked into the registers through serial input from a microcontroller, allowing for fast and reliable state updates across all channels with just a few lines of code.

The logic and communication for this module are managed by the STC15W204S microcontroller, a cost-effective yet powerful 8051-based MCU with enhanced UART performance and an integrated oscillator. This chip is paired with a CH340N USB-to-UART bridge, which presents the device as a standard virtual COM port to the host PC. Upon connection, the microcontroller listens for a set of AT-style commands sent over the serial connection. These commands are straightforward and user-friendly, for example, "ON=65280" activates the middle 8 outputs, "CLR" turns off all channels, and "VER" retrieves the firmware version. Additionally, there is a command to save the current output state to the built-in EEPROM, enabling the system to restore its output to a known state after power cycles. This interface design is perfect for scripting, automation, or integration with software tools such as Python, LabVIEW, or custom control GUIs.

The PCB is designed using KiCad and features a 2-layer layout measuring 75.25mm × 33.75mm. It includes 2.54mm pitch headers for output connections and is equipped with a USB Type-C connector. Power can be supplied through either USB or an external regulated 5V source, which can be selected via onboard jumper settings. The layout ensures clean signal routing and minimizes crosstalk or interference, even when switching high-voltage loads. Careful decoupling and protection components provide robustness for real-world applications.

The PCB for this module was fabricated by PCBWay, who generously sponsored this project. PCBWay offers high-quality PCB manufacturing and assembling services. Also, they offer CNC and 3D printing services. The PCB of this module is available to order from PCBWay. Check out the PCBWay website for its manufacturing capabilities and pricing.

The firmware for the STC15W204S is written in C using SDCC. It is easy to expand the command set, introduce new communication modes, or add timed control logic as needed. The current implementation allows full 24-bit output control using a base 10 numerical mask, making it both scriptable and human-readable. Thanks to the preloaded bootloader of the STC15W204S, firmware updates can be performed through the same serial interface. Details about this process are covered in the project documentation. Like the hardware, the firmware is released under the MIT License and is available in the project repository.

The system has been tested with a variety of 12V and 24V inductive and resistive loads, including relay banks, solenoids, and LED arrays. Since the outputs are open-drain, external voltages up to 50V can be safely switched on each channel making it ideal for a range of industrial, laboratory, or artistic applications. Output timing is reliable, with clean edge transitions observed during scope testing, and no signal integrity issues even during full 24-channel toggling. It is recommended to use individual heatsinks for the driver ICs when driving high-current inductive loads with this module. While the printed circuit board has heat transfer traces, the addition of individual heatsinks can increase the durability of the module.

Potential use cases for this module include automated test benches, home automation systems, signal routing for instrumentation, nixie tube multiplexing, and other high-voltage control tasks. The command-based protocol makes it easy to script operations or integrate this module into a larger system.

For those who wish to explore the schematics, command protocol, design rationale, and usage examples in greater depth, I have published comprehensive documentation and resources in the project wiki. This includes detailed assembly instructions, firmware flashing guidance, and tips on customizing the firmware for enhanced functionality.

All source files - including schematics, PCB layout, firmware code, and the bill of materials - are freely available at https://github.com/dilshan/24ch-usb-high-voltage-driver.

[jayakody2000lk]

4-Element Yagi antenna for the 2m band

This blog post discusses the engineering design, construction, and performance evaluation of a 4-element Yagi-Uda antenna specifically optimized for the 2-meter amateur radio band, focusing on the frequency range of 144MHz to 145.5MHz. The project was initiated to establish strong and reliable communication with distant VHF repeaters of the RSSL (Radio Society of Sri Lanka), which are located approximately 58km and 96km from my location.

The primary objective was to develop a high-gain, directional antenna with a superior front-to-back ratio. This directionality is essential for maximizing signal capture from the desired repeater while minimizing interference and noise from unwanted directions, ultimately improving the SNR and the quality of the communication link.

VHF communication at these distances typically relies on line-of-sight propagation, making antenna gain a critical factor in overcoming path loss and achieving reliable signal levels.

The antenna design was carried out using MMANA-GAL, a well-known and validated software suite based on the Method of Moments (MoM) for antenna analysis. While MoM simulations require significant computational resources, they deliver accurate predictions of antenna performance. This is achieved by breaking down the antenna structure into small segments and solving Maxwell's equations to determine the current distribution. This approach enables precise modeling of antenna impedance, radiation patterns, gain, and front-to-back ratio.

Key design parameters were iteratively optimized within MMANA-GAL to achieve the desired performance:

Target Frequency Band: 144MHz - 145.5MHz, encompassing the primary 2m amateur band frequencies.

Characteristic Impedance: 50Ω, to ensure impedance matching with standard RG-58 coaxial transmission line and transceiver equipment, minimizing reflected power and maximizing power transfer to the antenna.

Front-to-Back Ratio Optimization: Aiming for a high front-to-back ratio to minimize reception from the rear hemisphere, reducing interference and improving signal clarity, especially in noisy RF environments.

SWR Minimization: Achieving a VSWR as close to 1:1 as possible across the target frequency band. Low VSWR indicates efficient impedance matching and minimal power reflection back to the transmitter.

The simulation process involved adjusting the lengths of the elements and the spacing between them for the reflector, driven element, and directors of the Yagi antenna. This antenna operates on the principles of constructive and destructive interference of electromagnetic waves. The reflector, positioned behind the driven element, reflects waves forward, while the directors, located in front of the driven element, help to focus the radiated energy toward the main lobe. This arrangement increases both the forward gain and directivity of the antenna.

The optimized design, after multiple simulation iterations in MMANA-GAL, predicted the following performance metrics:

Simulated Gain: 13.1dBi. This represents a significant gain increase over a dipole antenna (approximately 2.15dBi) and translates to a substantial improvement in signal strength.

Simulated Front-to-Back Ratio: 15.35dB. This indicates that the power radiated in the forward direction is over 15dB stronger than in the backward direction, providing good directivity and rejection of rearward signals.

Simulated Input Impedance: Close to 50Ω across the 144MHz - 145.5 MHz band, ensuring a good match to standard 50Ω transmission lines.

The construction of the antenna focused on durability, lightweight design, weather resistance, and good electrical conductivity, all while keeping costs reasonable. We used 10mm diameter aluminum alloy tubes sourced from Lanka Aluminum. This diameter was selected because it is commonly available at many aluminum stores.

For the boom, we use a rigid 19.05mm (3/4 inch) square aluminum box bar, also sourced from Lanka Aluminum. This material is made from a similar aluminum alloy to ensure structural integrity and to serve as a common ground plane for the parasitic elements. The square profile enhances torsional stiffness compared to a round boom.

To effectively secure the components, we use 10mm ABS element holders. These holders are commonly utilized here to construct VHF/UHF TV antennas. Both the plastic holders and 10mm end caps are sourced from Kumarasinghe Radio.

The Polychrome junction box, measuring 85×85×50 mm and designed for outdoor use, is waterproof with an IP55 rating. This rating indicates that the box provides protection against dust ingress (although some dust may enter, it won’t cause harm) and shields against water jets from any direction. This makes it an essential choice for long-term outdoor deployment, ensuring the critical dipole feedpoint remains protected.

We used corrosion-resistant M3 stainless steel nuts, bolts, and washers to secure all elements and the IP box.

Precise assembly is crucial for achieving the intended performance of a Yagi antenna. All elements, except for the driven dipole, were directly and electrically bonded to the aluminum boom. This is a standard practice for Yagi antennas, as it utilizes the boom as a common ground, simplifying construction. Electrical connections were established using M3 stainless steel bolts that passed through pre-drilled holes in the aluminum tubes and boom, ensuring reliable electrical contact. The element holders provided mechanical support and maintained precise spacing between the elements, as well as a 90° angle with the boom, in accordance with the design specifications from MMANA-GAL.

The IP55 box containing the dipole was securely mounted to the underside of the aluminum boom using M3 stainless steel fasteners. To ensure consistent height with other elements, we used a strip of 10mm acrylic sheet.

Antenna VSWR and input impedance were measured across the frequency range of 144MHz - 145.5MHz using a VNA. The measured SWR of 1.1 at 145.025MHz confirms excellent impedance matching at the desired operating frequency. A Smith chart, generated by the VNA, visually represented the antenna's impedance characteristics throughout the band. It demonstrated that the impedance remained close to the target of 50Ω, indicating a broadband match across the intended operating frequencies.

The antenna was deployed at the designated operating location and oriented towards the target RSSL VHF repeaters. A subjective performance evaluation was conducted by attempting communication with the repeaters at distances of 58km (Yatiyanthota) and 96km (Piduruthalagala). 

Reports from repeater users consistently indicated an S9+ signal strength, showing a significant improvement in reception when compared to a baseline mobile whip antenna.

You can find the complete design of this antenna, including all dimensions, in the PDF linked here. The Fusion 360 design file for the antenna is also available here. Additionally, the MMANA-GAL design file (in .mma format) can be downloaded from this link.

[jayakody2000lk]

RDA5807M FM stereo receiver module

This project focuses on a simple yet powerful FM stereo tuner module. The core component of the module is RDA5807M, a single-chip broadcast FM stereo tuner that has gained popularity among designers in recent years due to its high level of integration. This integration significantly reduces the need for external components, simplifying circuit design. The RDA5807M supports a wide FM band, covering frequencies from 50MHz to 115MHz, making it adaptable to global FM standards.

Additionally, its use of a low-IF digital architecture streamlines the design process by requiring fewer external parts. The inclusion of an AGC (automatic gain control) feature ensures consistent signal reception, even in environments with fluctuating signal strengths.

The chip's digital stereo decoding capabilities deliver high-quality audio output, providing a rich stereo sound experience. Moreover, the energy efficiency of the RDA5807M makes it suitable for portable, battery-powered devices. The I2C control interface allows for easy integration with microcontrollers, enabling straightforward digital control.

This module is designed as a ready-to-use FM receiver solution, emphasizing compactness and ease of integration. It incorporates the RDA5807M IC and the STM8S003F3P MCU, along with the necessary supporting passive components, standard headers for connections, and flexible antenna connectivity options. The module is engineered to connect with commonly available 0.56-inch, 4-digit common cathode LED seven-segment display modules without any soldering.

The compact footprint of the module makes it ideal for projects where space is at a premium. Importantly, its open-source design allows for complete customization and modification.

It provides standard stereo audio output with separate left and right channels. The antenna input options allow for various antenna types, ensuring optimal signal reception. During testing, we utilized a 30cm long wire as the antenna and successfully received all FM stations clearly.

The tuner module features push switches to perform automatic station scans in both directions. The firmware limits the tuning range of the module to between 87MHz and 108MHz. Additionally, it includes digital volume control with 15 levels and a memory manager for storing up to 10 FM radio stations. The current tuner frequency is displayed on the segment display module, and there are two LEDs to indicate stereo reception and memory manager status.

The output level of this receiver module is not sufficient to drive speakers directly. Therefore, to achieve audio output through speakers, this module needs to be paired with a stereo audio amplifier. During testing, we successfully used the commonly available PAM8403 3W class-D audio amplifier with this module.

Power is supplied via a screw terminal, typically requiring a DC power source between 5V and 9V. The dimensions of the module are 72.85mm × 32.0mm.

This project is available on GitHub, providing schematics, PCB files, firmware source code, and comprehensive documentation. The hardware design of this project is licensed under CERN-OHL-W, while the firmware is licensed under the terms and conditions of the MIT license. All other resources, including documentation, are released under the Creative Commons Attribution 4.0 International license.

[jayakody2000lk]

Resolving Keyboard Issues on the Sony ICF-SW7600GR

The Sony ICF-SW7600GR, a well-known portable shortwave receiver, has built a solid reputation for its performance and reliability. I purchased this receiver 20 years ago, and it is likely the last portable shortwave receiver manufactured by Sony. I primarily use it for shortwave listening, and it has functioned continuously for two decades. However, like all electronic devices, it is subject to component degradation over time. This post outlines the diagnostic and restorative procedures I undertook to address a complete failure of the keypad on this radio.

Initially, the radio exhibited a gradual decline in keypad functionality, which eventually led to a complete lack of response. I observed intermittent key input before all keypad operations ceased entirely. This symptom suggested a possible discontinuity or short circuit within the keypad interface circuitry.

To investigate further, I disassembled the unit to inspect the internal components. My primary focus was the flexible flat cable (FFC) connecting the front control panel to the main circuit board. A visual inspection revealed significant damage to the FFC. Specifically, the protective dielectric film had delaminated from the conductive traces, leading to potential short circuits between adjacent conductors. The FFC in question is an 18-pin cable with a forward direction, a 1.25mm pitch, and a length of 37mm.

Finding a replacement FFC with these specifications was challenging, both in local markets and popular online electronic component stores. Eventually, I consulted with a manufacturer called Shenzhen Yiwei Technology and was able to obtain a nearly identical cable. The only difference was that the new cable is 100mm long. Since this cable is located closer to the speaker, I carefully bent it and positioned it underneath the speaker magnet. Now, this cable is also available for purchase directly from the AliExpress store of the above mentioned manufacturer.

After replacing the FFC, I reassembled the unit and conducted tests. I was pleased to find that the keypad had fully restored functionality, with all keys responding as designed.

Compared to modern electronic repairs, this repair was quite straightforward. Thanks to its modular design, I was able to complete the repair using just a few Phillips screwdrivers and tweezers.

[jayakody2000lk]

Analog pink noise generator

Pink noise is an essential tool in audio testing, widely used for analyzing speaker systems, room acoustics, and crossover networks. Unlike white noise, which has equal energy across all frequencies, pink noise features equal energy per octave, making it ideal for audio response testing. This characteristic ensures that pink noise offers a flat frequency response when analyzed on a logarithmic scale, providing a more accurate representation of audio systems.

We developed a this pink noise generator to meet various audio testing requirements. This device combines simplicity and performance, featuring a minimal component count for ease of assembly without compromising accuracy and usability. This pink noise generator design uses a reverse-biased emitter-base junction of a 2SC945 transistor as the noise source. In this configuration, the transistor behaves like a noisy zener diode, producing a broad spectrum of white noise.

In this design the NJM4558 op-amp is used to amplify and buffer the generated noise, ensuring high input impedance, stability, and consistent performance. At this stage above generated white noise is converted into pink noise using a 3dB/octave filter, which ensures equal energy distribution per octave over the audio frequency range.

The unit operates on a single-rail DC power source, compatible with 12V to 18V inputs. At our testing we found that this kit with 2SC945 from Matsushita electric starts to generate output at 8.4V and above. As we noticed this behavior is changing from vendor to vendor. For example some 2SC945's (It's manufacturer is difficult to identify) produces output at 9.1V. By considering most of the datasheets we decided 12V as the safest voltage which guaranteed the output.

To evaluate the performance of the pink noise generator, we paired it with the Simtelic KT0001 (LM386 power amplifier) module. This setup ensured that the output noise was sufficiently amplified for practical testing scenarios. This kind of setup is useful to identifying anomalies in speakers and crossover networks, measuring and optimizing room responses for audio clarity, and for microphone calibrations. When connecting this kit to an amplifier, pre-amplifier, or other analog audio equipment, always use shielded audio cables (UL1185 or equivalent) with the shortest possible length. Long, unshielded cables may introduce hum, oscillations, and distortion in the output.

The PCB for this module was fabricated by PCBWay, who generously sponsored this project. PCBWay offers high-quality PCB manufacturing and assembling services. Also, they offer CNC and 3D printing services. The pink noise generator PCB is available to order from PCBWay. Check out the PCBWay website for its manufacturing capabilities and pricing.

This kit is also available for purchase from Simtelic as a DIY kit. It is designed with through-hole components and can be assembled and tested without the need for specialized electronic assembly tools or instruments. For more details, refer to the kit's user manual provided on the Simtelic website.