What it Does
This circuit demonstrates the function of a NOT gate, which inverts the input signal. The LED turns off when the push-button switch is pressed (input HIGH) and turns on when the switch is released (input LOW).
Components Needed
- NPN transistor (BC547) – 1
- Resistor (1kΩ) – 2
- Push-button switch – 1
- Green LED
- Breadboard
- Jumper wires
- Battery (9V) with clip
Step-by-Step Instructions
- Connect the 9V battery to the breadboard using the battery clip.
- Create a positive (Vcc) and negative (GND) rail on the breadboard.
- Insert the NPN transistor into the breadboard. Label its legs as collector, base, and emitter.
- Place a 1KΩ resistor between the collector of the transistor and the anode (longer leg) of the LED.
- Connect the cathode (shorter leg) of the LED to the positive rail.
- Place a 1kΩ resistor between the base of the transistor and one terminal of the push-button switch.
- Connect the other terminal of the push-button switch to the positive rail.
- Connect the emitter of the transistor to the GND rail.
- Verify all connections, then test the circuit by pressing and releasing the push-button switch to observe the LED behavior.
How it Works
The transistor acts as a switch in this circuit. When the push-button switch is not pressed (input LOW), no current flows to the transistor’s base, so the transistor is OFF, and the LED remains ON. When the push-button switch is pressed (input HIGH), current flows to the base of the transistor, turning it ON and allowing current to bypass the LED, which turns it OFF. The resistors limit the current to protect the components.