Introduction: Light in the Dark LED Switching Circuit

About: Hey I'm Rene and i love to work on projects that contain electronics, 3D Prints and Hot Glue. I love building Circuits using Arduino and ESP8266 or ESP32. I'm currently studying mechanical engineering.

Some time ago i found a damaged construction site warning light lying on the side of a road. I thought i would make a cool decoration for my living room, so I brought it home an thought about what to do with it.

My Plan was to make it light up again, depending on if it's day or night. After some online research and some fiddeling around on a breadboard i came up with a schematic.


What you need, one LED, one Light Dependent Resistor (150 Ohms when the light is bright, 100kOhms when it's dark), 3 Resistors or 2 Resistors and a 10k Potentiometer, 1 NPN-Transistor (BC557). Additionally you need a Power Supply or something that can hold a battery.

Step 1: The Schematic

This is the Schematic.

R1 is the simple series resistor to lower the voltage for the LED, i use a 80Ohm Resistor. R2 is a 6.8kOhm Resistor connected to the Base of the Transistor.

R3 is the Light dependent resistor, together with R4 it forms a voltage divider. This switches the transistor on when the light is dark enough.

R4 is the Resistor that sets the threshold when the LED starts lighting.

Instead of using a static resistor you can also use a 10kOhm potentiometer to individually set the threshold (as shown in picture 2).

My Circuit uses a 9V Battery since that was the only i had, therefore the Resistor values a optimized for that.

Step 2: It Lights When It's Dark.

This is the circuit in action. When it's dark enough the LED turn on.

All Schematics made with KiCAD

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