The circuit diagram present here is that
of a street light that automatically
switches ON when the night falls and
turns OFF when the sun rises. In fact you
can this circuit for implementing any
type of automatic night light.
The circuit uses a Light Dependent Resistor (LDR) to sense the light .When there is
light the resistance of LDR will be low.
So the voltage drop across POT R2 will
be high.This keeps the transistor Q1 ON.
The collector of Q1(BC107) is coupled to
base of Q2(SL100). So Q2 will be OFF
and so do the relay. The bulb will remain
When night falls the resistance of LDR
increases to make the voltage across the
POT R2 to decrease below 0.6V. This
makes transistor Q1 OFF which in turn
makes Q2 ON. The relay will be
energized and the bulb will glow.
Circuit diagram with Parts list.
1-POT R2 can be used to adjust the
sensitivity of the circuit.
2-You can use bulb of any wattage,
provided that relay should have the
3-The circuit can be powered from a
regulated 9V DC power supply.
4-Click Here to get the power supply
circuit for this project.
5-The relay K1 can be a 9V SPDT relay