Hi Shahzad,

I don't think using a PC power supply is a good idea in this case, because it needs a certain minimum load to make it operate properly. Go with Andy's suggestion and use a 'wall-wart' adaptor.

The alarm output is a Normally Closed relay contact, so yes, you can connect it directly to your 8051. Configure it as a switch to Ground. I assume the detector is connected through some length of wire, so you will need to add a pull-up resistor to the input pin for noise immunity.

Best regards,
Rob.