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.

Could someone please give me some advice with regards to the best way to go about this?

  • \$\begingroup\$ 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? \$\endgroup\$
    – Oldfart
    May 1, 2018 at 17:59
  • \$\begingroup\$ Try searching "window comparator" \$\endgroup\$
    – EE_socal
    May 1, 2018 at 18:12
  • \$\begingroup\$ 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? \$\endgroup\$
    – jonk
    May 1, 2018 at 18:50
  • \$\begingroup\$ Thanks, window comparator seems to come up with the right sort of info, I'm investigating now. \$\endgroup\$
    – Terry
    May 1, 2018 at 19:20
  • \$\begingroup\$ 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. \$\endgroup\$
    – Terry
    May 1, 2018 at 19:31

2 Answers 2


OK, thanks for the tip on using the bargraph meter chip, I've persevered and come up with this:

LM3914 15v Monitor

I found this article on Nuts and Volts which gives details on using the LM3914 and getting the best out of it: http://www.nutsvolts.com/magazine/article/led-graph-circuits.

The circuit works pretty well, the sensitivity can be adjusted by varying the 'window' set with the trimmers R2 & R3, and also by choosing different LED outputs. The green 'normal' LED needs to be on LED_5, pin 14.


I've made something similar for motorcycles using an ATTiny chip. A simple board with a duo LED on it that indicates the board voltage.

I used the ATTiny10 to be precise, which is a fun little 6 pin microcontroller.

The chip is ridiculously limited in capabilities, with on 1024 bytes of ROM and 32 bytes of RAM. You can barely compile C for it. But what it has to do is also very simplistic, so a perfect fit.


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