Hello, I am designing electronic circuit board, and i want to only detect that the utility voltage is on. I don't want to measure the rms value of the voltage, only i wan to make voltage detection. The voltage range should be from 50VAC to 250VAC. I am looking for a cheap, small size, and a way where there could be isolation also. So i thought to use a resistor and optocoupler as in the attached drawing. In order to get the value of the resistor R, i used 1.2 v for the optocoupler, and 20mA will pass on the optocoupler for a minimum voltage of 50VAC. The resistor R value was 3.5K. For this value the current at 250VAC will be 0.1A. -> the resistor power should be 35 watt, which is a very huge resistor. Also bearing in mind that the optocoupler should handle 250√2 V when it is reversed biased. So i don't know if i can find an optocoupler that can hold this value also. If some one has any idea to share about the cheapest way to detect utility voltage.