Detecting if 120v AC wire is live with a 2 or 3 VDC line

by Timberwolf   Last Updated July 12, 2019 12:25 PM - source

For a background, I have 3 DPST bistable (latching flip-flop) relays that I have 120VAC input and 2 outputs. On one side, I have a 120VAC appliance. On the other side I need to find a way to feed it back into a Raspberry Pi's GPIO pins to detect whether it's live. What is the simplest way to accomplish this? I thought about step-down transformers and induction, but the transformer would get expensive, since I can't seem to find any other way to do it besides chaining 120vac to 5vdc and 5vdc to 3vdc and I have no idea how to use induction.

Answers 1

If you know how to make compute an impedance voltage transformer with caps and resistors like a scope probe, you can divide 100:1 and rectify with some slow rise or fast risetime specs and threshold to logic and shunt Neutral to 0V with another cap.

A suitable detection window for time and frequency must be specified by you for noise immunity, depending on your noise environment.

If you define your input , output and range / tolerances & thresholds, then a solution can easily be defined. Vin min(on), Vin max(off), Logic “0” Vil(max), Logic “1” Vih(min,max), Time detect, min, max) Noise reject at 1kHz to 1MHz= __ dB, ( nearest interference dI/dt, dV/dt and distance to input.)

Sunnyskyguy EE75
Sunnyskyguy EE75
July 12, 2019 12:08 PM

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