Light Dependent Motion Sensor Without Arduino

Passive Infra-red Sensor

Passive infra-red measures infra-red light radiations from objects in its field of view. They are commonly used in security alarms and automatic lighting applications. Also known as motion sensor makes it easy to understand that it detects motion. 

Light Dependent Resistor

A light dependent resistor is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.It is abbreviated as LDR

Relay Module

A power relay module is an electrical switch that is operated by an electromagnet. The electromagnet is operated by a separate low power signal from a microcontroller. When activated the electromagnet pulls to either open or close the electric circuit.


  1. PIR sensor
  2. Relay Module
  3. LDR Sensor
  4. STN 2222 Transistor
  5. 9v battery

How will it work without Arduino?

It is not necessary to use a microcontroller board to activate signals for any electronic device. Instead, use a transistor to activate the incoming signals of the PIR sensor. Solder the LDR inside the PIR sensor as shown in figure 1.1.

Figure 1.1
Figure 1.1

Once the soldering is done, follow the schematic below. For ease, the instructions are as follows.

  1. Connect the PIR output pin to the base of the transmitter.
  2. Now connect the emitter pin of the transistor to the signal pin of the relay and the collector pin to the ground.
  3. Make connections of the bulb to relay module and the AC voltage. 
  4. The circuitry is done. Hence Plug in your circuit and you are done. 


Circuit Diagram


The LDR sensor checks for light intensity. When its darks it activates the PIR sensor and when its daytime it deactivates the PIR sensor. This concludes that the motion will be detected in the dark and not not in light. 

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