Simple Dark Sensor Circuit

This circuit shows the application of a transistor as a switch. 

1. Circuit Diagram

2. Breadboard Schematic

3. Parts Needed-
  1. A breadboard
  2. An NPN transistor-eg. BC547, BC557 or 2N222
  3. An LDR
  4. A 220 ohm resistor (Red-Red-Brown) or 330 ohm.
  5. A 220k ohm resistor (Red-Red-Yellow) or 200 ohm.
  6. Any colour LED
  7. A 9V Battery
  8. A 9V Battery clip
All of these components can be found on amazon at a very reasonable rate-
4. YouTube video-

The circuit works on a simple principle, that current always takes the path of least resistance. While using the transistor as a switch when the base of the transistor receives adequate current, it allows current to exit from the collector. When it is dark, the resistance of the LDR becomes high enough to not allow any current to simply bypass the transistor and complete the circuit. So current then encounters an LED in this path and lights it up. When there is enough light,
the transistor is bypassed and no LED is encountered on the way.

Goodluck with your journey in electronics ☺



  1. Thanks for providing such a great information about sensor circuit. I would like to share this article with


Post a Comment

Popular Posts