LDR Circuit Diagram

This LDR circuit diagram shows how you can make a detector of light / dark .

LDR circuit diagram

An LDR or " Light Dependent Resistor " is a resistor where the resistance decreases with the strength of the light.


How The LDR Circuit Diagram Works
The circuit diagram of LDR works:

When there is light , the LDR has low resistance. So the voltage at the base of the transistor is not high enough to turn the transistor on .

Therefore, no current will go from the collector to the emitter of the transistor . All current instead pass through the potentiometer and LDR .



Light dependent resistor

In the darkness, the LDR has high strength . This means that there will be a voltage drop across the LDR . If you look at the circuit you will see that the LDR is connected from the base to the emitter of the transistor.

This means that we will also have a voltage drop from base to emitter. And if you know a little about transistors , you know this is what it takes to turn on the transistor.

So from the transistor turns on, the current will flow through R1, the LED and from the collector to the emitter of the transistor. This will make the LED lights .

Some notes about the components
The resistor R1 controls the amount of current passing through the diode . It is a current limiting resistor .

If you are using a LED voltage drop of 1V ( which is common) , will have a 5V voltage drop in resistance when the transistor is on. Using Ohm's law we have the power to :

I = \ frac { U} { R } = \ frac { 5V } { 330 \ Omega } = 15 mA

And 15 mA is usually a good value for ordinary LED current .

If you want to feed the circuit with a 9V battery for example, then you need to change the value of a different resistance . Use Ohm's law for this.

The variable resistor R3 is used to change the trigger point for the LED . That is, the amount of light needed for the LED is on or off.

Light dependent resistors
Light dependent resistors ( LDR) , also called photoresistors are made of semiconductor material of high resistance. When light hits the device , electrons giving energy photons . This makes the jump to the conduction band and therefore conduct electricity .