Dark/Light Detector using 555 Timer & LDR

Here we have explained a simple Light or Dark Detection using 555 Timer and LDR (Light Dependent Resistor) which has the ability to sense the light and whenever the light is not detected, it triggers the IC and glows an LED connected with the circuit. In place of LED we can also use a buzzer if we want to use it as a morning alarm. The concept of this LDR circuit is quite simple and based on the working of LDR. LDR can be thus used as a photoresistor whose resistance depends upon the quantity on light falling on it

Light or Dark Detection using 555 Timer and LDR

The electronics components required for this projects are as follows:

1. LDR
2. 555 Timer IC
3. Capacitors - 100uF Electrolytic, 0.1uF Ceramic
4. Resistors - 1K, 4.7K, 47K
5. LED
6. 9V Power Supply

The Circuit Diagram of Dark/Light Detector using 555 Timer & LDR is given below:
Dark/Light Detector using 555 Timer & LDR

This is a simple circuit that can be assembled in breadboard or zero PCB Board. The sensitivity of the circuit depends upon the 4.7K resistors. The resistor with slightly lower value can make the circuit more sensitive to particular light level.

Working of the Circuit

Lets first understad the working LDRs are a type resistors made from semiconductor materials to enable them to have their light sensitive properties. There are many types of LDRs but one of the most popular material used is cadmium sulphide (CdS). These LDRs or also known as photo resistors works on the principle of “Photo Conductivity”. Now what this principle says is , whenever light falls on the surface of the LDR (in this case) the conductance of the element increases or in other words the resistance of the LDR falls when the light falls on the surface of the LDR. This property of the decrease in resistance for the LDR is achieved because it is a property of semiconductor material used on the surface.

Here in this dark detecting LED circuit, LDR is configured with 555 timer IC in ASTABLE mode in such a way that 555 ASTABLE generates square wave when the light intensity goes below a certain level.

Video Demonstration: