Description
1. Pinout (TO-92 Package)
When you hold the transistor with the flat side facing you:
- Collector (Left)
- Base (Middle)
- Emitter (Right)
2. Example: Using BC557 as a Switch
In this example, we use the BC557 to turn on an LED. Because it is a PNP transistor, the load is usually placed between the Collector and the Ground.
- Logic: When the Base is HIGH (5V), the “switch” is OFF. When the Base is LOW (0V/Ground), the “switch” is ON.
Components:
- Voltage Source: 5V
- Resistor (R_B): 1k$\Omega$ (connected to the Base to protect it)
- LED + Resistor: 330$\Omega$ (connected to the Collector)
The Circuit:
- Connect the Emitter directly to the +5V rail.
- Connect one end of a 1k$\Omega$ resistor to the Base. The other end goes to your switch or microcontroller pin.
- Connect the Collector to the Anode (long leg) of the LED.
- Connect the Cathode (short leg) of the LED to a 330$\Omega$ resistor, then to Ground.
3. Example: Touch Switch (Darkness Sensor)
Because the BC557 is sensitive, you can create a simple touch-activated circuit.
- Setup: Connect the Emitter to +9V. Connect a small LED and resistor to the Collector.
- Action: If you touch the Base and the Ground rail simultaneously with your finger, your body acts as a high-value resistor. This pulls the Base “low” enough to trigger the transistor and light the LED
