Light sensor circuit diagram

In life we may have come across many of the detector circuits.But how it will be when we can make our own detector circuits? Here is a light sensor circuit diagram which can be build so easily to detect the falling of light on it.This circuit can cause less cost.It comprises of a transistor,Potentiometer,LDR(Light Dependent Resistor) and a relay circuit with a protection diode within it.


The working of this circuit is very simple and uses only less number of components in it. This simple circuit using a single transistor turns ON the relay when light falls on the LDR.The Light Detector resistor has the characters of giving low resistance when light falls on it.And high resistance when light is obstructed and this forms the backbone of working of this circuit.

The potentiometer is adjusted for the sensitivity of the circuit.And a relay circuit which is an electrically operated switch which may be used as a indication in this circuit.This is used along with a diode which is called as protection diode which protects the other electronic components from the brief high voltage produced when a relay coil is switched off.

When light falls on the LDR its resistance is low therefore the transistor enters into saturation state and turns  the relay circuit on.When the light is obstructed LDR offers a high resistance to the current flow thus the potentiometer shorts the transistors base to the ground and it is cut off.Hence this makes the relay off.Thus we can detect the falling of light on any object.

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ozzyFrank DonaldDon Warner Recent comment authors
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Does it matter if the 10k pot is logarithmic or linear?

Don Warner
Don Warner

I am building a chicken coop with an automatic door that opens at sun up
and closes at sun down. For power I will have a solar panel charging a 12
volt auto battery. Can your circuit run on a 12 volt battery?