Let me introduce an improved version of a blackout alert beeper, which is, in fact, a low-cost mains failure alarm designed to help protect stock and equipment. This device, extremely useful when you are away and need to be alerted, can be used in all types of domestic and commercial applications as it sets off a noticeable
alert when the electricity fails. For example, it can be either wall- or pole-mounted to protect your incubator so that you can avert an accidental loss of fertile eggs and chicks, but it is also useful in keeping an eye on your farm security system like the electric fence!
Circuit Description
The core component of the design given here is one revered 14-stage ripple-carry binary counter/divider and oscillator CMOS IC — CD4060BE (IC1). As shown in the schematic, components R3 and C2 set the clock frequency of IC1 close to 10 Hz, and there’s a blue indicator (LED1) to show the clock activity. The Q4 output (Pin 7) of IC1 drives an active piezo-buzzer (PZB1) and a red indicator (LED2) through a small NPN transistor S8050 (T1). The circuit also includes a standard, on-rechargeable, 6F22 9-V battery (BAT) in power supply and a low-voltage slide switch (S1) as the master power on/off switch.
The front end of the design is a simple AC230V mains presence sensor built around one popular photocoupler PC817B (OC1). The output of OC1 is routed to the “master-reset” terminal (Pin 12) of IC1 to disable its clock operation when the AC mains supply is available (standby mode). There are so many ways to convert an AC voltage into a DC voltage required by a photocoupler, and traditionally, this has been done with a step-down transformer. However, a low-cost alternative — capacitive power supply — is employed here because it’s more cost-effective and significantly smaller. In the AC mains presence sensor, a 100-n/250-V AC capacitor (C5) is the key element, and the 100-Ω resistor (R3) is to limit inrush current. The remaining components include a rectifier (D2), buffer (C4), and a protector (ZD1). The 5.1-V Zener diode (ZD1) is added intentionally to deter the blowup of C4 in case of an open-load condition at the output of the capacitive power supply (all resistors are ¼-W type).
As you might have observed, there’s a two-way jumper (JP1) in the circuit for “normal—latch” mode selection with “normal” as the default. In default mode, LED1 starts winking instantly if there’s a blackout, and PZB1 (LED2 as well) beats rhythmically (~0.6 Hz) for a short time (eight cycles). Thereafter, only LED1 remains lit until the input is reinstated. Nonetheless, LED1 and PZB1(LED2 too) wakes up instantly in “latch” mode while there’s a blackout but remains steady until the input is reinstated (watch my quick test video).
Construction Note
A flake of perforated circuit board is enough for construction of the entire electronics. Remember to not only make it fit perfectly in a prototype enclosure but to isolate the high-voltage and low-voltage areas in the circuit board well with a fairly large headroom. The finished circuit board must be mounted in such a way that it’s not in direct contact with the enclosure even if the enclosure is a hard-plastic (ABS) type (plastic standoffs will be helpful). This is the enclosure model for reference — see below:
It’s also possible to connect a weatherproof external siren to the system by any two-core low-voltage cable. This allows the alert sounder to be installed outside the location being monitored to increase the effectiveness of sound travel. Fortunately, low-voltage/low-current–type electric hooters (~120 dB) are now easily available through many online sellers (the prototype was tested with an old 9-V/88-dB PCB-mount buzzer).
The term decibel (dB) and the dB scale are used worldwide for the measurement of sound levels. It is very important to realize that the term “dB” can have different meanings and the value depends on the context in which it’s used. Here is an example (only for reference) of different sound intensities as expressed in decibels hearing level (dB HL):
0 dB: Softest sound we can hear
60 dB: Ordinary conversation
120 dB: Rock band
Finally, two random shots of the muddy prototype, from my workbench!
Note: This circuit deals with high voltage, AC circuits. If you are not experienced in working with these circuits, please understand that they can cause bodily harm in the case of mishandling or not following proper safety practices.&n-
bsp; Please do not attempt to manufacture this circuit unless you have taken the proper safety precautions.
It's often necessary to power a low voltage circuit such as a microcontroller with HV AC line current. For example, most fire alarms are powered this way. In these situations, a transformer is both bulky and expensive especially if the device is a consumer product. The solution is to use a reactance to limit the current, rectify the voltage with a diode, regulate the voltage with a zener, and use a large electrolytic capacitor to filter out the ripples.
The main disadvantage of transformerless supplies is that they don't offer isolation from the HV line and present more of a safety issue.
We present four circuits which use this principle and a calculator which determines the power capability for the circuit. These circuits are based on the www.microchip.com App Note AN954.See the application note for the equations upon which the calculator is based.
Supply Type Pros and Cons
Capacitive Higher cost than resistive, more efficient than resistive. The zero crossing is delayed.
Resistive Lowest cost, least efficient. The zero crossing is not delayed.
Bridge Highest cost, but highest current capacity and efficiency. The output voltage isn't referenced to line or neutral so TRIAC control isn't possible.
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