This is a circuit for a street light. When the natural light is bright, the LED is switched off. When it dims, the LED comes on. It is cool to look at!
It is simple to build. It is also pretty cheap since it uses no ICs. Also, the circuits consumes almost no power when the LED is off so it is pretty efficient.
Step 1: Circuit Diagram
This is the diagram of the circuit we are going to build
Step 2: Collect All the Parts
Collect all the parts required for the circuit as per the diagram in the previous step. All of these should be available from any electronics supply store. Match the colour bands in the registers carefully.
Here are the parts:
Bread board (I have connected the top row for Power and bottom row for Ground using the small red and black wires)
LDR (Light Dependent Register)
BC 548 transistor
Registers - 15K Ohm & 1K Ohm
connecting wire - one small piece
6V battery in a battery box with snapper to connect it to the bread board
Step 3: Insert the Transistor
Insert the transistor on the bread board. It is a general NPN transistor. I have used BC548 which is commonly available.
Note the positions of the Emitter, Base and Collector leads. I have inserted Emitter lead one row ahead of Base and Collector leads to easily identify it later
Step 4: Connect the LDR
Connect the LDR between Base and Emitter leads of the transistor.
Step 5: Connect the 15K Register
Connect the 15K register from base of the transistor to the power line
Step 6: Connect the LED
Connect the negative lead (shorter lead) to collector of the transistor
Step 7: Connect the 1K Register
Connect the 1K Ohm register between positive lead of the LED and the positive power line.
Step 8: Connect Emitter to Ground
Connect Emitter lead of the transistor to ground line using a small connecting wire
Step 9: Connect the Batteries
The circuit is now done.
Connect the positive terminal of the battery to upper line (see red wire) of bread board and negative terminal to lower line (see black wire in the photo)
Step 10: See It in Action!
The LED will be switched off normally but when you darken the room, it will glow. I have used white LED which glows very brightly.
The circuit is also sensitive to amount of darkness. You can see that the LED brightness will increase when darkness in the room increases. Look at the video where I am slowly drawing and opening the curtains.