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Read 24 Vac signal with a Raspberry Pi

Electrical Engineering Asked by Riccardo Minati on July 25, 2020

I want to read a 24 Vac signal coming from my old automatic gate with my Raspberry Pi. The signal tells me if the gate is OPEN->24 Vac or CLOSED->0 Vac. Raspberry Pi input are 3.3 Vdc tollerant. My first idea was to use a rectifier to get a DC signal and then use a voltage dividers to get the 3.3 Vdc. The second one was to use a octocoupler to achieve the same results. Has anyone any suggestion on what type of such devices should I use to achieve my goal?

Best regards

Ric

One Answer

Instead of using a slightly larger number of inexpensive multiple-sourced parts, you could consider using a part designed for this kind of application, the TI ISO1211.

See Figure 3. I think values of Rsense = 560 ohms, Rthr = Rshunt = 1K and C = 100uF/50V would work, but check that yourself. It will run off the 3.3V Pi power supply.

enter image description here

You might want to use a bridge rated at a bit higher voltage.

Answered by Spehro Pefhany on July 25, 2020

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