The light activated switch circuit works on a very basic principle, that of amplification of the current produced by a photo-transistor when light impinges upon it. The current is amplified by op-amp 741 that drives T2, energising a relay. The circuit shown can be used to activate a relay with a conventional torch up to reasonable distances for use at home. The controlled apparatus will remain switched on (or off) as long as the torch’s light falls on T1
It will be noted that the base of photo-transistor Tl is not connected., The photo-transistor must be protected from ambient light by some sort of protective tube, or it might switch on the relay when sufficient IR light is available. The output of the op-amp is sufficient to energise a sensitive relay. Transistor T2 may be eliminated, if the relay is capable of working on currents less than 8mA. However, if T2 has to be used, almost any type of silicon switching transistor may be used for amplification; the particular make depends on the power consumed by the relay. The LED power-on indicator requires about 10mA, and may be eliminated if so desired.