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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?

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    \$\begingroup\$ Will you be using this to check different places or leave it connected once it is installed? \$\endgroup\$ – Joe May 26 '11 at 13:20
  • \$\begingroup\$ It will be left connected at one location. \$\endgroup\$ – SteveR May 26 '11 at 13:51
  • \$\begingroup\$ Damn, I was thinking you were trying to make a devices that could help someone find where the valve was instead of having to dig up a yard (as I have once done before) \$\endgroup\$ – Kellenjb May 26 '11 at 13:53
  • \$\begingroup\$ As an alternate I would consider a contact-less voltage detection circuit, like those pens used by electricians. Problem is I haven't seen any that work as low as 24 vac. If I could find a circuit, I might be able to redesign for my needs. I assume they detect RF! The AC interference gives and output? In any case it would need to be reliable with no false triggers. \$\endgroup\$ – SteveR May 26 '11 at 16:58
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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.

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  • \$\begingroup\$ Yeah, I have ct's that I could use, but the problem is it must be a clamp on. I can not physically make or break the line. Thanks. \$\endgroup\$ – SteveR May 26 '11 at 13:49
  • \$\begingroup\$ @Steve R - I was afraid you would say that! Looking around, I see the Magnelab SCT-0750 (for around £28 UK). Very convenient, but the highest sensitivity (1A) model only gives 33mV/100mA. You might be able to loop a few turns if you have enough slack. \$\endgroup\$ – MikeJ-UK May 26 '11 at 14:18
  • \$\begingroup\$ Yeah that one is a little pricey @ 45 usd each.The thing is that the project needs to be disposable, therefore cheap. I face the real possibility of it not being returned. \$\endgroup\$ – SteveR May 26 '11 at 15:37
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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.

enter image description here

EDIT:
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.

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  • \$\begingroup\$ Your approach will only work bat high frequencies. \$\endgroup\$ – Autistic Feb 19 '16 at 20:25
  • \$\begingroup\$ @Autistic: What makes you think it would work only at high frequencies? \$\endgroup\$ – Curd Feb 20 '16 at 9:54

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