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Re: how to sense 220v input -solved- I hope

Wed Jul 19, 2017 8:17 am

I'm stupid with electronics too. Generally, if I build anything with mains, I hide behind the chair the first time I switch it on.

The obvious thing to test is the voltage across the LEDs. You'll probably want to attach (and double-check) the probes before turning it on. If the LEDs have failed you may get mains voltage across them, so start out with the meter on a high setting. If everything is working well then you'll get 1-2V AC.
The next worry I would have is the capacitor. Try disconnecting it. The GPIO should then see a 50 or 60Hz square wave when power is on.

Also be aware of the comments further up the page that the 100K resistor looks rather large for the application. If you do have 1-2V across them and no signal with the capacitor out of circuit, I would try reducing it to 2W 50K.

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Re: how to sense 220v input -solved- I hope

Mon Apr 23, 2018 6:04 am

One tip, using a cap on the input to lower the voltage with much less heat is great if you are connected to the grid where you have a nice clean sine wave.

I sometimes need to monitor the output of a 240VAC modified sine wave inverter in an off grid system. I find that using a cap in this situation lets the smoke out of my interface. So I go with a resistor or two at 2 - 3 Watts.

I also wanted an indicator lite and some input protection against spikes.

I did a write up on the design I have been using for a few years if you want to look over what worked for me. ... pberry-pi/

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