Detecting an energized AC solenoid coil

Developing a test tool that will need to be able to detect a 10 to 15 watt solenoid coil, it may either be 24vac or 115vac. I attempted to use a split coil current sensor, but the current is too low to provide reliable results. I've thought about either a magnetic reed switch or hall effect sensor placed near the coil, but wondering if a non-contact voltage detector maybe better. I only need a digital feedback signal to an arduino pro-mini 3.3v, for my needs.

Has anyone had experience with similar & can offer a suggestion?

You could wrap some turns (start with 20 or 30) of fine magnet wire around the coil, and use a diode/RC filter to create a DC signal as shown below.

The more turns, the higher the voltage. You will need at least 3.4V to trigger a 5V digital input, or much less to get a usable signal on an analog input. Also, you might add a 10K resistor between the output of the detector and the input, to protect from overvoltage.


Thank you very much, I like the suggestion!

Note that when powering at 115V you need almost 5 times as many windings as when powering at 24V. This is a current sensor, the output voltage varies with the current in the wire, and at 24V the current is 4.8 times higher than at 115V.

The sense coil is a "changing magnetic field" sensor, not a current sensor.

Since the number of turns and impedance in a 24VAC coil will be much less than in the 115VAC coil, but the input power is about the same, I suspect that the same sense coil will perform similarly on either solenoid.

Alternatively, the solenoid and sense coil make up a transformer, and if the core is not saturating, it should be roughly true that Nin/Nout = Vin/Vout, where N and V are the number of turns and rms voltage across each coil. That leads to the same conclusion.