All About Circuits Forum  

Go Back   All About Circuits Forum > Electronics Forums > The Projects Forum


The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

Reply   Post New Thread
Thread Tools Display Modes
Old 12-04-2006, 10:40 PM
SoundGuyAndy SoundGuyAndy is offline
New Member
Join Date: Jan 2006
Posts: 6
Default Stupidly simple go/no-go DC voltage tester

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,
Reply With Quote
Old 12-05-2006, 02:53 AM
thingmaker3's Avatar
thingmaker3 thingmaker3 is offline
Super Moderator
Join Date: May 2005
Location: Rural, Oregon GMT -8
Posts: 5,072
Blog Entries: 6

Would it work to put appropriately rated zener diodes in parallel with the indicator circuits?
"I want to establish in your mind very clearly that you must not think I deny all that I do not admit. On the contrary, I think there are many things which may be true, and which I shall receive as such hereafter, though I do not as yet receive them; but that is not because there is any proof to the contrary, but that the proof in the affirmative is not yet sufficient for me"
- Michael Faraday
Reply With Quote
Old 12-05-2006, 06:43 AM
mrmeval mrmeval is offline
Senior Member
Join Date: Jun 2006
Posts: 833

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.
Reply With Quote
Old 12-05-2006, 06:33 PM
beenthere's Avatar
beenthere beenthere is offline
Senior Member
Join Date: Apr 2004
Location: Missouri, USA (GMT -6)
Posts: 15,815
Blog Entries: 10
Default Parts for indicator


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
Reply With Quote
Old 12-05-2006, 11:41 PM
SoundGuyAndy SoundGuyAndy is offline
New Member
Join Date: Jan 2006
Posts: 6

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!
Reply With Quote
Reply   Post New Thread

, , , ,

Related Site Pages
Section Title
Worksheet CMOS logic gates
Worksheet TTL logic gates
Worksheet Summer and subtractor opamp circuits
Worksheet Negative feedback opamp circuits
Worksheet Open-loop opamp circuits
Worksheet Junction field-effect transistors
Worksheet Class A BJT amplifiers
Worksheet Bipolar junction transistors as switches
Worksheet Series DC circuits
Textbook Special-purpose diodes : Diodes And Rectifiers

Similar Threads
Thread Thread Starter Forum Replies Last Post
Testing an Industrial AC and DC Variable Speed Drives NiCeBoY General Electronics Chat 13 12-08-2013 02:11 PM
Op amp mad_mat222 General Electronics Chat 27 08-18-2013 12:05 PM
Simple DC DC converter for electrical machine mmehdi98 General Electronics Chat 1 08-08-2013 11:46 AM
Dc to Dc voltage booster Ascendant The Projects Forum 1 03-28-2011 07:50 PM
LED lamps getting hot MingJae Homework Help 20 02-25-2010 01:54 PM

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

All times are GMT. The time now is 06:15 AM.

User-posted content, unless source quoted, is licensed under a Creative Commons Public Domain License.
Powered by vBulletin
Copyright ©2000 - 2014, vBulletin Solutions, Inc.