Zero cross using optocoupler 220vac

[Vindhyachal.takniki]

1. I have to detect zero cross using optocoupler of 185Vac-265Vac & 50Hz. Attached is the circuit. Have to feed the signal to MCU pin.
Will make MCU pin to give interrupt on both rising & falling edge.

2. Is circuit ok. What should be best value for R1 & R3.
I have put 4.7K each.
But consider max voltage = 265vac,
then Vp=  374.76Vac
current flow = around 39ma. (via R1, led, & R3 consider led drop zero)
Each resistor watt = 7W


Value comes out to be very high?
If I increase the value of resistor, zero cross distorts.

[Gyro]

The circuit, as drawn will expose the Optocoupler LED to full mains voltage in the reverse direction on alternate half cycles, this will immediately kill it. You at least need to put a diode across the LED with reverse polarity to prevent this.

[Andy Watson]

You at least need to put a diode across the LED with reverse polarity to prevent this.

Yes. I would also put a diode in series - one that can withstand the peak reverse voltage - this would half the (not inconsiderable) power dissipation in the resistors.

[bktemp]

This circuit needs more parts, but is more accurate and consumes much less power:
http://www.dextrel.net/diyzerocrosser.htm