4 Isolated AC Power Line Interface

This simple circuit will help you monitor your 220V or 110V AC line if it is available or not. The circuit can be connected to a microcontroller or computer for monitoring purposes. Circuit uses opto-coupler thus the AC line is isolated from your monitoring device.
schematic diagram
Part List:
R1 - 470 kilo ohms 1/4W
R2 - 1.2 Mega ohms 1/4W
R3 - 5.6 kilo ohms 1/4W
C1 - 100nF Mylar capacitor
D1 - 1N4007 or any diode with minimum 400V PIV
Q1 - 9013, 2N222 or similar NPN transistor
Opto - 4N25 or similar opto-coupler

4N25 Opto pinout
9013, 2N2222 pinout
Note: The circuit is connected directly to AC supply. Do not touch any part without double checking the connection.


  1. Nice one, thank you very much!

  2. no resistor serial to C1 might kill opto. No constant output signal due to switching opto with net frequency...

  3. Power rating of R1 is to close without safety margin .. P = (Vpeak)^2 / R = (325V)^2 / 470000 = 0.22W should be 1/2W. Place two 220Kohm in series since the max voltage of R1 typically is 200V.

  4. Can this need to be connected serially with the AC device or need to be connected in parallel?
    What about connecting an isolated 5v power supply in parallel to the AC device with a resistor and fuse? Can that make the circuit a little simpler?