Mains voltage circuit for micro

[1icarus1]

Hi there,

I would like to build a circuit for changing the mains voltage value to a lower 0-5 Vdc or pulse in order to connect it to micro and have info about the mains voltage value. The product will be class 2 which means that it must be isolated with maximum leckage current 0.5mA.
I have buid some circuits using transformer, reset ics, mosfet, optocouplers but I would like to have low cost as lower as possible.

Thanks for your time.

 

[davidgrm]

An optocoupler followed by a schmitt trigger should be fine. You need to choose your resistor on the LED side according to the current rating for the optocoupler. Being an ac source you should also put a diode in reverse across the LED to protect it from reverse voltage. There are other ways to do it but it is not worth compromising safety to save a few cents.
The other way is to take a ac from in front of your circuits rectifier bridge / diodes. Feed this through a schmitt trigger (might need a cap & resistor before schmitt trigger) This way you are using the same transformer that powers your circuit and it is also safe.