Light/dark relay switch

Posted on Mar 13, 2013

This kit is the most basic, practical circuit to build using an LDR to turn on a relay. The two transistors connected as a Darlington pair give the circuit enough sensitivity, while the trimpot give sensitivity adjustment. The switching point of the relay is dependent on the supply voltage and temperature. This circuit is satisfactory if the changes in light level to be detected are large and the transition is quick - for example, a person walking past a doorway. But an inherent problem of the circuit is chattering of the relay for slowly changing light levels just at the transistion point between turning on/odd and vice versa.

Light/dark relay switch
Click here to download the full size of the above Circuit.

Look what happens when the relay turns on: the input voltage drops slightly (say around 20 mV) when the relay turns on and loads the circuit. If the voltage applied to the base-emitter junction of Q1 is only just sufficient to turn it on then this slight drop will immediately start to turn Q1 off. But then with the relay load reducing the supply voltage will start to rise & Q1 will start to turn on etc. This leads to the relay chattering as it rapidly turns on/off. (This problem is overcome in K79B by the built-in hysteresis of the op-amp, and the Schmidt Trigger arrangment of the circuit in K79C.)

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