Light sensitive switch - a de-light-full circuit!

7-14 - elektor july/august 1982 There is a wide range of applications for light sensitive switches: staircase light timers, outdoor illumination, automatic door openers by means of a light beam, alarm systems and so on. Many of our readers will be familiar with the single transistor opto-switch where a LDR is placed between the base and either ground or supply depending whether a "normally on" or "normally off" function is required. This simple circuit gave way to more complex arrangements involving the use of opamps with the advent of the supercheap 741! Another, not so well- known, method of opto-detection uses a bridge circuit operating on the principle that current flow across the bridge will be zero when the four impedances have been calculated correctly. The "bridge is in balance" when this occurs. The latter principle is used in the circuit here. The opto-detector is situated in a bridge circuit and a comparator is used as a "bridge is in balance" indicator. The comparato...