Automatic Night Light Sensor Using BC547 Transistors

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1 What is an Automatic Night Light Sensor?

What is an Automatic Night Light Sensor?

An automatic night light sensor is an electronic device that is capable of sensing the light sensitivity in the surrounding area & determines whether it’s day or night. If it’s night, it switches ON the associated circuit & turns OFF in case it’s daytime. So, in this tutorial, we are going to go through a step-by-step procedure on How to make an Automatic Night Light Sensor Using BC547 Transistors.

The main component of this circuit is an LDR or a light-dependent resistor. An LDR is basically a photocell that works on the principle of photoconductivity. This passive component is basically a resistor whose resistance value decreases when the intensity of light decreases.

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5$ PCB Prototype

https://www.youtube.com/watch?v=E442H5BmiTQ

Hardware Components

The following components are required to make Automatic Night Light Sensor Circuit

S.no	Component	Value	Qty
1.	Relay	SPDT/5V	1
2.	LDR	5mm/2mm/	1
3.	Transistor	BC547	2
4.	LED	5mm/3.5V	1
5.	Variable Resistor	10K	1
6.	Diode	1N4007	1
7.	Resistors	100 Ohm, 1K	1
8.	Soldering Iron	45W – 65W	1
9.	Soldering wire with flux	–	1
10.	Veroboard	–	1
11.	DC Battery	9V	1
12.	Battery clips	–	1
13.	Jumper Wires	–	As per need

BC547 Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of BC547

Automatic Night Light Sensor Circuit

Working Explanation

The working of this circuit is pretty simple, during the daytime the resistance of the LDR is pretty high, blocking any meaningful control signal to go through the base of the first BC547 transistor. As soon as night falls, the resistance of the LDR rapidly decreases, causing a control signal to flow through the base of the first transistor.

The collector output of the first transistor serves as the control signal for the second transistor. The subsequent collector output energizes the coil of the 5V SPDT Relay, which makes the external circuit, connected across the COM & NC terminal of the relay. Always use an SPDT relay with a voltage rating comparable to the input power supply (12V/9V/5V).