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

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.
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.
222fbj (author) 3 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:
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 !


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