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How to install an LED on/off Indicator to Monitor 24v AC line? Answered

I want an LED indicator that will lite when an AC circuit is on. The line/output is 24vAC at .5A. This is an irrigation controller, which sends 24vAC to activate a water/sprinkler controller valve.   I need a basic cicuit design and/or instructions.
Googling led me to the 3 pages below, but I am not sure what to do.
Any advice appreciated.  Thank you.



Put two LEDs back to back, anode to cathode, cathode to anode. Add a series resistor, put resistor and LEDs across the output.

Cute! Elegant way to get continuous illumination from AC. Again, the difference between a true working engineer and a physicist.

I think you may be adding some unnecessary components. Im not a very smart individual so i didn't figure this out on my own I just have seen things before and said ill try that myself BUT, Square D used to sell a relay for HVAC that had an LED indicator, the power was 24vac just in parallel with the coil load. all it is, an LED and a 5.5kohm resistor no "additional" diode. Im a little confused as to why an LED would be damaged by the revers polarity as mentioned several times; "Diode" is in the name LED, remember when you were tinkering your first led circuit and you switched anode and cathodes on your led in a dc circuit, no damage just didn't work. Ive ran this successfully on 24vac circuits without issue, honestly unless the lights are off in the room you dont notice a flickering(at american 60hz dont know about 50hz) with an "indicator" LED you know the dull green and red types. Maybe for lighting LED's and something to produce very solid bright lights you would deff need a cap to reduce flicker. Just hoping to add to this old thread because some options are none the less correct but dont miss the forest for the trees. Hope this helps someone and saves them some time and components.

Hmmm, seems that I should be able to throw a capacitor across the LED to reduce the flicker. The bigger the capacitor, the less the flicker, and the less responsive the circuit. Correct? Would a 100uf realistically do it?

I have almost the exact same desire, I want to monitor the zones with my oil fired boiler. However, I also want to attach an arduino and do some realistic tracking. 'Seems to me that I should be able to monitor across the led as well. Correct?

A tiny 10µF will do too for the LED ;)
For the monitoring on the Arduino you might want to consider a transformer.
Similar to those on big amp meters or the "spy ears" for the old analog telephones.
Easy to make from an old core found in many power supplies and filters.

A 1.8K 1/2W resistor in series with the LED is all that you need. That will supply about 10 to 15 mA depending on the LED forward voltage.
For an AC line polarity doesn't matter. The LED will flicker though.

so if I make the circuit as you suggest with just one LED and the resister, the LED won't be damaged on the "reverse" pulse of the AC wave?

Hence Steve's elegant solution of two opposed LEDs in a common housing.


7 years ago

Thank You! Does this image represent the circuit you describe (except you are using 2 LEDs instead of an LED and a diode.)? Image source: http://www.merg.org.uk/led/index.htm


That's it. I have these on my heatpump so I can tell what it's doing. (I don't want the AUX heat on until I'm below 1 deg F).

The reverse diode protects the LED against reverse polarity voltage that would blow it out. Instead of one resister, I used two. I needed to do that because of the wattage with one exceeded the power rating of the resister. I only had 1/8 watt resisters handy.

In my case I mounted them up higher than the thermostat, because the resisters are like little heaters and might otherwise throw things off.

EXACTLY what I mean. Well done for finding that !