In building alarm systems, two wires are used to communicate to, and power all the sensors on the line. A large amount of capacitance is used on each sensor to hold power during the communication period. I suggest you use a capacitor that will hold power while transmitting is active. Best Regards, Fred -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu