Stupidly simple go/no-go DC voltage tester

Thread Starter


Joined Jan 27, 2006
Hey guys,
I'm trying to put together a brainlessly simple test circuit that simply checks a PSU's output for DC from 24-30 V and lights up an LED to tell you if you're good, and another to tell you if you're over-voltage. Sort of a stupidly oversimplified bargraph, if you will. My initial thought was just to use an LED with an appropriate resistor in front of it so that it'll take 24 V to hit the turn-on voltage for the first LED, and 30 for the second, but my worry there is what damage might be caused if the voltage hits significantly higher than that range. Can I just do something as simple as this and then add a diode that's above the turn-on voltage but below the "whoah, you don't really want to do that to the LED, buddy!" voltage to protect it?

Again, I'm not going for precision, just need to be able to tell the end user if the voltage is too high or low. In fact, it really doesn't even need to be able to differentiate between high vs. low, it can truly be a "light on if it's 24-30 V, light off if it's outside that range" thing.

The other thought I had was two relays in series, the first being NO and taking 24 V to open, and then the second NC and opening at 30 V, but I feel like there might be a better way?

Thanks in advance,


Joined Jun 30, 2006
Put a 30 volt zener in the circuit in series with a transistor base so that when enough voltage trips the zener it will turn on the transistor and light a red LED.
You can put another zener on the base to protect the transistor from over voltage.

A duplicate circuit with a 24volt zener can be used for the green LED, it will still be on when the red lights.


Joined Apr 20, 2004

Two zeners and a red plus a green LED are the simplest way to go.

Mouser is a good source -

Red indicator LED - 645-558-0101-007F @ $1.74
Green indicator - 645-558-0201-007F @ $1.74
24 volt zener - 512-1N5252B @ $.50
30 volt zener - 512-1N5256B @ $.50
3K resistor (get 2)- 291-3K-RC @ $.08

Thread Starter


Joined Jan 27, 2006
Indeed, I suspect that'll do the trick nicely, and I'm kicking myself for not figuring it out myself (it's that whole knowing enough to get myself into trouble, but not using the knowlege often enough to actively remember it thing, LOL).

Actually, I could even go with a dual-color LED, the 24V feeding the green side, and the 30V feeding the red, so that green=good and orange=bad. That will certainly do the trick for my purposes...much obliged!