How to detect 24vac from my AIRCON ?



I'm wanting to detect 24vac (without using a relay) from my AIRCON unit so I can tell if the fan/compressor etc is actually running.

My thinking was to use an opto.. so 24vac full wave bridge rectified via the Opto to turn it on. However this is going to be pulsed as there is no smoothing and hence I suspect the output will also be pulsed, not what I want...

Does anyone have any experience in this, can I simply add a small smoothing cap and if so, what value ? I'm trying to dig out my old theory books to see if there is something I can find.

Is there a better way to do this ?

I'm, "all ears" :)

I love Relay-In-A-Box from Functional Devices. I use a bunch of them for HVAC and pool system interface since both use 24VAC. 24VAC relays are not a common hobbyist item.

Enclosed Relay 10 Amp SPDT with 10-30 Vac/dc/120 Vac Coil
check ebay ($13 for buy it now) or a distributor.