Making Simple Light-activated relay circuit with PCB

In today’s post, I’m going to share a simple light activated relay circuit. Why should you learn it?

Your daily life is busy. If you need to turn on the electrical device only during the day and then turn it off at night. Sometimes, You may forget it. It wastes time and wastes electricity bills too.

Is your life easier? If it opens and closes by itself. We are not a little wizard. Yes, We can do it by this circuit.

light activated relay circuit

Read below you will get ideas.

You can use it in many ways such as the alarm circuits, light switch controller circuits, the strobe light photography controller, and more. It will control your electric load to be on-off with the light.

How light-activated relay circuit works

The Circuit diagram of Light Relay Switch By BC547 and BC337
The Circuit diagram of Light Relay Switch using BC547 and BC337

As I said, we will catch grasshoppers. It does not need to ride an elephant. Because It is too wasteful.

In this circuit, we choose it because it is simple and works well. That is enough for our necessary.

Let’s see in the circuit diagram. Both transistor Q1 BC547 and Q2 BC337 are connected together in Darlington compound. So they have a higher gain.

When we shine a light on the photocell (LDR1). Its resistance will reduce. The current flows through it to the base of Q1. The Q1 get enough bias current, so it conducts the higher current. They switch on, to turn on the relay, and LED1 grows.

R1 limits the current to LED1 with safe value.
D1 absorbs the current spikes to protects the external relay driver.


We can control the sensitivity of the circuit by adjusting VR1.

If we set its resistance is low, the sensitivity of the circuit is also low. But we set its resistance is high, is high sensitivity, too.

Construction
This circuit is easy. So you can assemble them on perforated PC board. But you want to solder the components on the PCB below.

How to build

This project is easily suitable for a beginner. First, you get all parts lists below.

Parts you will need

  • R1: 100Ω, 0.25W Resistors, tolerance: 5%
  • Q1: BC549, 45V 100mA NPN Transistor
  • Q2: BC337, 45V 800mA NPN Transistor
  • D1: 1N4001, 50V 1A Diodes
  • LED1: any LED as you like
  • LDR1: Photoresistor
  • RY1: Relay with SPDT 10A min switch. Coil Voltage 12V. Coil resistance 150-600 Ohms 
  • PCB, wires and more.

For PCB, you can use a universal PCB. Because it is so faster. However, you can make a normal PCB as the layout below.

pcb-layouts-of-dayLight-sensor-switch Relay Switch By BC547 & BC337
PCB layouts and Components layout of Light Relay Switch circuit

Testing

First of all, Check and check circuit again. Second, try to enter a power supply source in the circuit. Then, adjust the VR1 to sensitivity by you want. Last, shines a flashlight on the photocell. You will hear the relay pulls in. It works.

Keep reading: How to make a Daylight sensor switch project

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Sachin Shetty

Verry useful circuit.

Madhu

Superb circuit

Steve Loke

Hi,

I would like to find a light activated sensor. This sensor will be install at dark area and will trigger the N.C or N.O when detected light by using 12V. Kindly reply me if have any. Thank you.

Best regards,

Steve Loke
J TECH TWO

Steve Loke

Hi,

I would like to find a light activated sensor. This sensor will be install at dark area and will trigger the N.C or N.O when detected light by using 12V. Kindly reply me if have any. Thank you.

Best regards,

Steve Loke
J TECH TWO
[email protected]

Birger Errboe

Light activated relay circuit using BC547 & BC337
Can this circuit be modified, so it provides an On function at 220V output for example in 2-5 sec. when the LED light is turned on and a 220v OF functioning in the same time interval, when the light LED light is closed.
Or maybe you have an other circuit with this ON OFF funktion.

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