Smoke detector - the optical method of detection

7-34 — elektor july/august 1982 Smoke detectors are part of any sophisticated alarm system. Most of the professionally made ones use some form of gas-sensor, ionisation- chamber, or radio-active element. The circuit described does not use any of these rather complex components but makes good use of two light detecting resistors ( LDRs), and a LED. A special IC LM1801, allows the cir- cuit to be constructed using the mini- mum of components. It is an IC designed specifically for use in smoke detectors, containing among other things, an internal supply zener, two reference voltage outputs, a voltage comparator, and a 500 mA output transistor with clamp diodes. The complete circuit is connected to the mains supply. Diode D1 rectifies the supply, R7 reducing the voltage to a workable level for the IC. Capacitor C2 smoothes this, and the internal zener of the IC stabilises it. The circuit uses a pair of balanced light detecting resistors (LDRs). By using these in a bridge arrangement...