pc controlled ac devices

[lythanhthuan]

please help me how to build a circuit that will indicate whether an ac device is on or off????the output of the cicuit is 5v dc so that i can use it as an input for my program...ive got an idea to use a transformer in series with the ac device and use it to activate a circuit that will send a 5v dc signal whenever an ac device is turned on..but it might affect the voltage and current coming into the ac device itself and also its impractical if i have plenty of device to check whether its on or off.........
thanks

 

[IanP]

Use an optocoupler ..
Connect its "primary" in parallel with an AC device and its "secondary", in most cases an open-collector transistor ( additional pullup resistor should be connected to +5V ), to digital input ..

Here are some examples:
http://i2c2p.twibright.com/datasheet/6n139_an.pdf

Regards,
IanP