Transistor: exercice
You will need:
• AC adapter, breadboard, wire, and meter.
• LED. Quantity: 1.
• Resistors, various.
• Pushbutton, SPST. Quantity: 1.
• Transistor, 2N2222 or similar. Quantity: 1.
Use your breadboard to set up the circuit shown in Figure 2-85. Be careful to get the transistor the right way around! (See Figure 2-84.) the flat side should face right, if the transistor is packaged in black plastic, or the little tab should face toward the lower left, if the transistor is packaged in metal.
Initially, the LED should be dark. Now press the pushbutton and the LED should glow brightly. Electricity is following two paths here. Look at the schematic in Figure 2-86, which shows the same circuit more clearly. I’ve shown positive at the top and negative at the bottom (the way most schematics do it) because it helps to clarify the function of this particular circuit. If you view the schematic from the side, the similarity with the breadboard layout is easier to see.
Through R1, voltage reaches the top pin (the collector) of the transistor. The transistor only lets a tiny trickle of it pass through, so the LED stays dark. When you press the button, voltage is also applied along a separate path, through R2 to the middle pin (the base) of the transistor. This tells the transistor to open its solid-state switch and allow current to flow out through its third pin (the emitter), and through R3, to the LED.
You can use your meter in volts DC mode to check the voltage at points in the circuit. Keep the negative probe from the meter touching the negative voltage source while you touch the positive probe to the top pin of the transistor, the middle pin, and the bottom pin. When you press the button, you should see the voltage change.
