Step 1: Step 1: Collect the parts:
1. LDR (Light Detecting / Dependent Resistor)
2. NPN Transistor (I have used BC 547)
3. 9 volt battery
4. 9v battery lid
5. Four LEDs, 5 mm bright white
6. 1/4 watt, 100 ohm resistor (per LED, so 400-500 ohms for 4-5 leds)
7. 1/4 watt, 10k ohm resistor (per led)
8. small piece of PCB to mount the assembly.
Note: Instead of items 6 & 7, you can straightaway use a single 500 ohm & 50K (or 100k) resistor each, if you are going to use only 4 or 5 leds....
Additional tools like, solder, solder wire, hot glue or its equivalent.
Step 2: Step 2: Assemble the parts
1. Mount the four LEDs on the smail pcb with the positive side outside, as shown.
2. Solder the positive legs together and the negative legs as one (in parallel), as shown.
3. Now, connect the positive leg of the 4 leds to the 100 ohm resistor. Connect the loose end of this 100 ohm resistor to the 10k resistor. the loose / remaining end of the 10k resistor should be connected to the central leg of the NPN Transistor.
4. Connect the negative leg of the led to the npn's collector (left leg of the NPN transistor, with the rounded part facing you), as shown
5. Now connect the LDR to the centre leg and the right leg of the NPN.
Now Check: (See the two photos)
Two wire pairs of the final assembly should get used as the positive and the negative side:
- one pair from the ldr & right leg of npn, which goes to negative of battery (or red wire of the lid)
- other from the positive leg of the led alongwith the 100 ohm resistor. This is the positive and should go to the positive of the 9v battery, through the black lid wire.