Dark Activated Relay Circuit




Automatic Light Activated Relay Circuit is depend with ambient light. easly this device will turn on the relay when the LDR recive ambient light.This project is very useful for us, example for your automatic house lamp, or your gardening project. For the gardening project, you should enlighten the plant with yellow light for the photosynthesis.

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Step 1: Things That We Need

  • 100K Variable Resistor
  • Relay 6v ( depends with your input voltage)
  • LDR or Transistor
  • 4148 Diode (not important, it’s okay if you are not using this)
  • 2 Terminal Blocks
  • PCB (The artwork)
  • Battery 6v-12v

Step 2: Analyzing Circuit

  • You can also replace LDR with the phototransistor like the picture above

If you are still confusing, please comment!

Step 3: How to Make PCB

Step 4: The PCB Artwork

this is using Circuit Wizard software file
Download the software: Circuit Wizard

Step 5: How Does It Work

You can see the different between this 2 pictures.

When the light is high(intensity), nothing happen with the output

But, if the light low(intensity), the output is available

Note: red is +, green is -

Step 6: Application

Use this sensor as a automatic lighting in your garden or for your robot project, maybe as aobstacle sensor

Check out my project blog:


I'm from the same author of this blog

Thanks for Circuit Wizard!



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3 Discussions


Reply 4 years ago on Introduction

i'm using circuit wizard software!

check out this software: