What it Does
This circuit demonstrates the function of an OR gate. The LED lights up when either one or both push-button switches are pressed, showing that at least one input is HIGH.
Components Needed
- NPN transistors (BC547) – 2
- Resistor (220Ω) – 1
- Resistors (1kΩ) – 2
- Push-button switches – 2
- 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 first NPN transistor into the breadboard. Label its legs as collector, base, and emitter.
- Insert the second NPN transistor into the breadboard next to the first.
- Connect the collectors of both transistors to the positive rail using jumper wires.
- Place a 220Ω resistor between the emitters of the transistors and the anode (longer leg) of the LED.
- Connect the cathode (shorter leg) of the LED to the GND rail.
- Place a 1kΩ resistor between the base of the first transistor and one terminal of the first push-button switch.
- Connect the other terminal of the first switch to the positive rail.
- Place another 1kΩ resistor between the base of the second transistor and one terminal of the second push-button switch.
- Connect the other terminal of the second switch to the positive rail.
- Connect both emitters of the transistors to the resistor leading to the LED.
- Verify all connections, then test the circuit by pressing each switch individually and both together to light up the LED.
How it Works
In this circuit, each transistor controls one switch. If either push-button switch is pressed, current flows from the collector to the emitter of the corresponding transistor, lighting up the LED. When both switches are pressed, both transistors conduct, and the LED remains lit. The resistors limit the current flow to protect the LED and transistors.