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.
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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