Detect AC voltage - 24-240VAC

Discussion in 'The Projects Forum' started by 1999F250, Oct 19, 2010.

  1. 1999F250

    Thread Starter New Member

    May 14, 2010

    I would like to monitor my well pump and heating system with a microcontroller.
    My well pump is 240VAC (I am in the USA so 60 Hz)
    The pumps on my boiler are 120VAC.
    The zone valves are 24VAC.

    I saw a circuit somewhere that used a capacitor to pass a small current that was used to drive a LED (In my case the input of an optocoupler, with the photo transistor in the optocoupler driving a pin on a microcontroller). But I can't find that circuit now.

    Ideally I would like a circuit that would accept 24-240 VAC and when present drive the LED. I don't care about zero crossing.

    I have 6-8 things I would like to monitor so inexpensive and compact would be good.

    I assume this has been discussed before but just can't seem to find the proper search string - either here or Google.

    I plan to tap into the voltage parallel to the load.

    Last edited: Oct 19, 2010
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    Ideally you'd want to build two different circuits, one for 24 and one for 240 however a hall effect device could be used.
  3. DonQ

    Active Member

    May 6, 2009
    There are certainly optocouplers you could use that could be made to cover the range from 24-240VAC, but it might just be easier to put a small signal transformer at each higher voltage location to convert to 24VAC before you do the sensing. You could even use some variety of 'wall-wort' type power-supply, for a few bucks each. Doesn't even have to be exactly 24V or does it need to be AC. Just some reasonable voltage at a tiny available current.

    You will probably be needing to run wires to the remote locations where the pumps are, and it would be better to run a bunch of low voltage wires (some telephone cord) rather than string a bunch of 240VAC wires (risking electrocuting someone, fires, etc).

    Then, everything coming into the microcontroller is 24VAC. Now it's easy.