1.2.4. The transistor as a switch experiment

Components:

• breadboard
• 5V power supply
• 3mm red LED
• SPST pushbutton switch
• BC547 NPN transistor
• 220Ω, 2.2KΩ resistors

Figure 1‑27 The transistor as a switch

Explanation: We added a pushbutton switch and 2.2KΩ in order to bias the transistor base. It is obvious that when we press the switch, the LED is lit and the transistor acts as a switch. The way we have chosen the resistor value is according to the formula:

The value of the current gain amplification factor hFE can be assumed 10 for switching circuits and I_c is 20mA for the diode to glow. The idea to use a transistor as a switch in a circuit that has a switch seems like nonsense, but we can easily imagine that the bias voltage could be caused by some other digital component.

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