# What is the best way to design voltage monitor with tri-colour LED?

I've built a power supply with regulated +15v, +12v, +5v, -12v and -15v outputs and I'd like to build a voltage monitor to let me know if any of the rails are above or below their stated voltage.

I have some tri-colour RGB LEDs (common cathode), the plan is that when the rail is at the correct voltage, the LED is green, it goes red when it detects over-voltage and blue if there's under-voltage (say, ~20mV either way).

I've tried building a circuit with the LM3914 bargraph driver chip but the problem with this is it requires a common anode LED if I want to use an RGB. Also the chip is a 10 LED output and it seems wasteful to only use 3 outputs.

I've done some research and it seems like I could get something to work with op-amp comparators and either zener diodes or regulators as a reference but I don't really know enough about this yet.

• When you type 'voltage comparator' in google you get 13 million hits, many with descriptions and schematics. What do you want us to add to that 13Million? May 1 '18 at 17:59
• Try searching "window comparator" May 1 '18 at 18:12
• By "best" do you mean to include "gold-plated" as in traceable to NIST voltage standards? You mention $-20\:\text{mV}$ as being enough to be under-voltage. Even the ISL21009BFB850 considers $\pm\:2\:\text{mV}$ initial accuracy to be "something good," though you can buy even better. Are you serious about the $\pm 20\:\text{mV}$ accuracy window?
– jonk
May 1 '18 at 18:50
• Thanks, window comparator seems to come up with the right sort of info, I'm investigating now. May 1 '18 at 19:20
• I'm not sure I need to go so far as to satisfy NIST voltage standards, but the more accurate the better, I suppose. the PSU is for a modular synth so any voltage drop will affect tuning etc. s Ideally, less than +/-20mV, maybe as little as +/-5mV. The idea is to have a monitor to let me know when something is not right. I don't really want to go as far as having a voltmeter on each rail because I can't really dedicate that much front panel. May 1 '18 at 19:31