Did you ever wonder how to switch on an LED when it’s dark enough? You will learn in this post how you use a light dependend Resistor Circuit to Switch an LED on and off.

I found a damaged construction site warning light on the side of a street. I thought i would make a cool decoration for my living room, but one thing was missing: It didn’t light up anymore.

After some research online i came up with a schematic:

This simple battery driven switches on the LED using a simple NPN- transistor. Depending on the Resistance of the R3 (LDR – Light dependend Resistor, has a high resistance when no light is there) the Transistor gets enough current to switch.

The LDR (R3) I use has around 350 Ohms when bright light shines on it. In darkness it has a resistance of around 100kOhms.

Resistor R4 is the one that adjust the threshold of your circuit. The lower the value the earlier the light will switch on. It would make sense to use a 10kOhm potentiometer instead of a static resistor so you can adjust the threshold level. The Schematic then would look like this:

And that’s it working:

All Schematics made in KiCAD

Read about my other Projects here