Has electronically modulated infrared light source and synchronous phase sensitive demodulator pick up unit. Pulsed-light technique overcomes adverse effects of continuous or varying ambient light.Alarm goes off if power supply or interconnecting wires are tampered with.Floating 12-v battery takes over load only if power supply fails.C1 tunes T1 to 55-cps oscillator frepuency.-S.Bagno and J.Fasal,Intruder Alarm Uses Phase-Sensitive Detector,Electronics,31:7,p 102-105.