Photo Detector Circuit

This post contains the design and circuit of a simple photo detector or a simple circuit to detect light energy .This circuit can be used for various purposes like controlling street lights ,  detecting humans or opaque objects and can also be used to count numbers of items with some extra aided digital circuits.

Photo Detector Circuit:

The circuit diagram for the Photo Detector is given below: ( Click on the image to enlarge it )

photo detector circuit

photo detector circuit


The components used in the circuit are:

1> LDR.

2> Potentiometer, Value: 50 Kilo ohms

3> Resistor, value: 1 Kilo ohms

4> Relay, Value: 6 Volts

5> Transistor, value: BC 548 see below for the orientation of emitter , base and collector.


You can choose the sensitivity level of the circuit by rotation the potentiometer. And you can use the switch of the relay to switch on and off any any device according to the intensity of the light falling on the LDR.



A simple LDR or Light Dependent Resistor is used in the circuit as the light sensor.



LDR: It is a special type of resistor whose resistance decreases with increasing intensity of light falling on it. Or the more light falls on the LDR the more electrical energy it conducts through it.  It is made up of a photo sensitive material “cadmium sulfide” thus it exhibits such electrical properties.


Potentiometer or variable resistance:



Potentiometer or a variable resistance is a three terminal resistance , where the resistance of the middle terminal with the remaining terminal varies when the slider or the rotor is rotated. Here in the circuit we are using a Potentiometer in series with the LDR to make a voltage divider from which the voltage is provided to the transistor for switching purpose.





Transistor is a semiconductor device used to amplify or switch electrical signals.

In our case we are using the NPN transistor numbered or labeled “BC548″ for amplifying purpose which amplifies the signal in it’s base and provides the signal to relay for switching on any device.


A transistor are of Type NPN and PNP , here we are using NPN transistor “BC 548″ .

A transistor have three terminals which are named “Emitter” , “Base” and “collector” the orientation of Emitter , base and collector of transistor bc 548 is given below:

bc548 transistor

bc548 transistor

So , place the BC5448 transistor facing the flat face toward you , then number the terminal from left to right 1 , 2 and 3.
The the first or “1″ terminal is Collector , the middle or “2″ terminal is Base and the last or terminal “3″ is Emitter.


A relay is simply a switch which is electrically operated. In our case we are using a 6 V relay.


After the circuit is ready , you can use it for any purpose. This Photo Detector Circuit can be used for simply switching on the lights when the surrounding is dark and switch it off when the surrounding is bright.  You can also output the amplified signal from the collector terminal of the transistor to any other device instead of relay and use this circuit as photo detector circuit aid on to any other device or circuit.

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