I'm looking for a simple, cheap device to detect low ac current. It will be used to detect irrigation valves. These valves operate @ 24 vac (60 hz) typically 100 - 200ma. I just need a go-no go signal, I'm not interested in the actual value. The device needs to be a "clamp on" type with no physical connections. I'm open to rolling my own, this is a one-off project. Perhaps something with a hall sensor, clothes pin type of thing. Any ideas out there?
I have done this in the past using an old toroidal mains transformer. Just loop one of your valve leads once or twice round the transformer (not strictly "clamp-on" I know) and use the voltage induced in the primary. You should get sufficient voltage to rectify with a diode bridge and a capacitor/comparator will give you your output.
I'd use a snap-on ferrite core that can be clamped on the original wire (in picture brown) without interrupting or electrically connecting to it.
Add some additional windings (in picture red) to pick up a voltage signal that will be present when alternating current goes though the primary wire. If neccesarry amplify the signal, rectify, low-pass-filter and feed it into a Schmitt-trigger to get an on/off signal.
I've tried the assembly and found it works fine. I got about 10mVpp signal per 40 windings on the secondary side and 100mA primary current (at 50Hz). So amplifying the signal by factor 100 gives about 1Vpp which can easily be turned into a on/off signal.