No light is the cause of darkness, we can use light dependent resistors (LDRs) to automatically detect darkness or light intensity. This circuit can be integrated with other electronic circuits or projects to control the light of a small light bulb to automatically light up street lights throughout a city. In this project we will make a simple dark detector circuit.

Components needed to build a dark detector

The dark detection circuit is a very simple circuit that requires very few components, the complete list of components is as follows:



1K Ohm – Resistor

50k Ohm – Resistor

BC547 – BJT

9V battery


working principle

The circuit is based on LDRs, also known as photoresistors. The resistance of the LDR depends on the intensity of the light. The 50k resistor and LDR form a voltage divider circuit. When the light intensity falling on the LDR is high, it will have a high resistance, so the voltage divider will provide a low output to the base pin of the BC547 BJT. The BC547 is an NPN transistor, when the BJT is on, the emitter-base junction is forward biased. The electronic path for our LED is complete, so the LED will light up.

circuit schematic

To make the circuit we connect the battery in parallel to the LED with a 1K ohm resistor, the positive terminal or longer leg of the LED is connected to the end of the resistor and the negative terminal or shorter leg of the LED is connected to our collector BJT. The emitter of the BJT is connected to the negative terminal of the battery and the base of the BJT is connected to the junction connecting the LDR and a 50K ohm resistor. The other end of the 50k resistor is connected to the positive rail of the battery and the other end of the LDR is connected to the negative rail of the battery. To simplify the connections, we wired everything up on a breadboard. The circuit diagram of the dark detector is as follows:

After connecting the components to the breadboard, we tested by blocking the light falling on the LDR and the circuit worked fine. To see it in action, you can check out the video at the bottom of the page.

in conclusion

The dark detector circuit using LDR is a very simple circuit that can have many real world applications.

Leave a Reply

Your email address will not be published.