Get the following components:

NPN transistor- BC547 or BC147 or BC548

Take 2 transistors

Resistor- 1K

Resistor- 330 Ohm

Light emitting diode(LED)- Any color

Connecting wires- Use single-core plastic-coated wire of 0.6mm diameter (the standard size)-You can use wire that is used for Computer Networking.

Power supply-6V or 9V

 Components for this experiment are available at buildcircuit.net.

STEPS

Step 1: Put first transistor Q1-BC547 on breadboard as shown in the picture.

Step 2: Put another transistor Q2- BC547 on breadboard as in step 1.

Step 3: Put wires across emitter pin of both transistors and -ve  terminal of battery(lowest/ bottom row of breadboard.)

Step 4: Put a wire across Collector pin of transistor Q1 and Base pin of transistor Q2.

Step 5: Put a resistor 1K across positive terminal of battery(topmost row of breadboard) and Collector pin of transistor Q1.

Step 6:Insert Light dependent resistor(LDR) across positive terminal of battery(topmost row of breadboard) and base terminal of transistor Q1.

Step 7: Put a resistor- 330 Ohm across base pin of transistor Q1 and negative terminal of battery(lowest bottom row of breadboard).

Step 8: Put a resistor 330R across  positive terminal of battery(topmost row of breadboard)and anode terminal of LED(Light emitting diode) & Connect the cathode terminal of LED to Collector pin of transistor Q2.

The circuit is ready for testing. Connect battery terminals and see the output. As you block light falling on LDR(Light dependent resistor), the LED glows.

OUTPUT:

THIS CIRCUIT IS EXTREMELY SENSITIVE TO DARKNESS. EXPERIMENT THE CIRCUIT IN A HIGHLY ILLUMINATED ROOM.

LED GLOWS EVEN IN LESS DARKNESS. Use torch light if the LED glows in less darkness. You can adjust the sensitivity of the circuit using a variable resistor in place of R3-300 Ohm. Try this circuit with other resistances as well, for example, 1K, 10K and 100K, etc.

Components for this experiment are available at buildcircuit.net.

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