The Transistor – Finger Switch

The Transistor is basis of modem day electronics and computing. It is used in electronics for two main application.
1. As an electronic switch – the transistor use small electronic signals to its base pin and then allows electricity to flow between its collector and emitter pins like a switch.
2. As an amplifier – the transistor receives small signals to its base pins and then allows a larger relative amount of electricity to flow between its collector and emitter pins therefore amplifying each small signal.



Diagnosing problems:

  • If any component is getting hot disconnect the battery.
  • Ensure a resistor is used in the circuit or LED will be burned out and damaged
  • Ensure LED positive leg (longest) is connected to positive side of circuit
  • Check battery voltage with multimeter in Volts DC mode
  • Try a different LED in case it has been burned out (ensure you are using a resistor!)
  • Look at breadboard picture bellow and ensure all connection are correct.

Parts List

Part Qty Name
VCC1 1 4.5V – 6V Battery Supply
R1 1 330 Ohm Resistor
R2 1 22K Ohm Resistor
LED1 1 Red LED 5mm
S1 1 6mm Push Button Switch
Q1 1 NPN Transistor BC546

Fritzing Breadboard Layout and Schematic Diagram:

Note: there are multiple variations of transistor which will effect the performance of this circuit. If you are using a BC327, BC546, BC547, BC548 please use the following breadboard layout which has the collector on the left (CBE if the flat side was down when looking from above).
If you are using a 2N222 or similar which has the opposite pin configuration (EBC) you will need to flip the transistor 180 degrees. Please see this link for more details.

Finger SwitchTransistor Finger Switch Schematic

Teaching Points:

  • Ensure LED, Diode, and battery are in correct polarity (direction)
  • Read Transistor part number and follow instructions above


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