Generalities learned from the LM3914N - Courtesy of @JohnS_AZ
So this IC, as many others, have a whole lot of pins. And I'm glad that @JohnS_AZ knew what they were and how they worked and I realized that it was because of that knowledge, that things "made sense". It wasn't just oh, this works because the schematic says it's supposed to. Thanks to John, I've put a slightly more comprehensive explanation together:
So I wanted to go through the labeled pins and see what new lessons I could learn.
Pin 2 (V-): This, as in most ICs, usually means the negative power rail - and this is usually set to ground.
Pin 3 (V+): This, as in most ICs, usually means the positive power rail. This is usually pretty much just what is required to power the chip and leds.
Pin 4 (RLo): This is essentially the low end of the internal voltage dividing tree. If you're making a voltmeter, and you want to have the meter measure from 12v to 24v, RLo would be 12v. You would adjust this reference level by placing a resistor between Rlo and V-.
Pin 5 (Sig): This is the signal level - this is the signal whose voltage levels you're measuring.
Pin 6 (RHi): This is the Reference High and sets the high end of the internal voltage dividing tree. The voltage on this pin also determines how much it will take for the LED bar to light up all of the way (all LEDs lit up). So if you wanted the meter to measure from 12v to 24v, Rhi would be connected to 24v. Anything above 24v would still make the LED bar light all the way up.
Pin 7 (Ref Out): By default, this is the + lead of an internal 1.25v source. LED brightness is controlled by changing how much current flows through the Vref pin.
Pin 8 (Ref Adj): By default, this is the - lead of an internal 1.25v source. This can be adjusted anywhere up to 12V. But this is done by adding a resistor or pot between RefAdj and ground. I thought it might be possible to put RefOut to V+ and have a pot in between to adjust. This may be important especially if RHi is attached to RefOut.
Pin 9 (Mode): If this is placed to ground, you have DOT mode. And if it is placed to the power rail, you have BAR mode.
A few extra notes:
This chip is one you can daisy chain together - to make a super long and more sensitive voltmeter. So Chip1 Rlo would go to ground, Chip1 Rhi connects to Chip2 Rlo , and Chip2 Rhi connects to some reference voltage for a two-chip 20 LED display.
Audio scale only ever gets to 1 or 2v, so we need the top of the divider tree to be lower, in the neighborhood of what we expect the maximum input voltage to be - hence our use of hooking Pin 6 RHi to Pin 7 RefOut
The LM3914 chip is linear - it turns on in even steps. The 3915 is a log chip, the 10 LEDs light at different voltages representing a log scale. And then 3916 is a special LOG scale set up for use as a true VU meter. If you look at page 7 on each of the data sheets, you will see that the -ONLY- difference between the chips is the value of the resistors in the divider tree. So how that divider network, with or without additional resistors on Rhi and Rlo TOTALLY determine what voltages the individual LEDs represent.
A "basic" lesson. A comparator compares two voltages - for our purposes, one is a reference off of a voltage divider tree and the other is from the signal. If the signal is greater than reference, then a HI/ON output goes forth from the opamp, otherwise the output is LO/OFF.
If the + input of the comparator is higher than the - input, the output is off. If the - input is higher than the + input, the output turns on, and the LED lights do as well.
So all of the - inputs of the 10 comparators of the chip are tied together and that's where the input signal goes.
Each + input is connected to a different point in the divider tree, so each one sees a different voltage. In this chip, each resistor is 1k. So if Rlo is ground and Rhi is 10v, the first comparator will see 1v, the second will see 2v, then 3v, etc.
All of those internal resistors add up to 10k ohms. So if you connect a separate 10k resistor between RHi and 10v, the TOP comparator will now see 5V instead of 10v. So now, instead of the top LED lighting at 10v, it will light at 5v.
If you connect a RHi to 10v, and connect RLo to a separate 10k resistor to ground, now the TOP comparator will see a 10v again, but the bottom one will see 5v. So now the bottom one LED will not light until the input is over 5v.
I still have some questions (though I'm not sure how to word them) on the Ref Adj and Ref Out, BUT, for now I'm satisfied :). And of course, thanks to @johns_az as he was the one who gave me most of this information and answered my questions in an understandable manner.
@atdiy/@tymkrs