Experiment 10: Transistor Switching

Chapter Experiment 10: Transistor Switching

Make Electronics Book Learning by Discovery
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Make Electronics Book Learning by Discovery

  • Switching Basics and More73Experiment 10: Transistor SwitchingExperiment 10: Transistor SwitchingYou will need:• AC adapter, breadboard, wire, and meter.• LED. Quantity: 1.• Resistors, various.• Pushbutton, SPST. Quantity: 1.• Transistor, 2N2222 or similar. Quantity: 1.A transistor can switch a flow of electricity, just like a relay. But it’s much more sensitive and versatile, as this first ultra-simple experiment will show.We’ll start with the 2N2222 transistor, which is the most widely used semi-conductor of all time (it was introduced by Motorola in 1962 and has been in production ever since).First, you should get acquainted with the transistor. Because Motorola’s pat-ents on the 2N2222 ran out long ago, any company can manufacture their own version of it. Some versions are packaged in a little piece of black plastic; others are enclosed in a little metal “can.” (See Figure 2-83.) Either way, it con-tains a piece of silicon divided into three sections known as the collector, the base, and the emitter. I’ll describe their function in more detail in a moment, but initially you just need to know that in this type of transistor, the collector receives current, the base controls it, and the emitter sends it out.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.) For the three brands I have mentioned in the shopping list, 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.Figure 2-85. The transistor blocks voltage that reaches it through R1. But when pushbutton S1 is pressed, this tells the transistor to allow current to pass through it. Note that transistors are always identified with letter Q in wiring diagrams and schematics. S1: Pushbutton, momentary, OFF (ON) R1: 180Ω R2: 10K R3: 680Ω Q1: 2N2222 or similar D1: LEDS1R2R1Q112vDCD1R3Figure 2-83. A typical transistor is pack-aged either in a little metal can or a molded piece of black plastic. The manufacturer’s data sheet tells you the identities of the three wire leads, relative to the flat side of a black plastic transistor or the tab that sticks out of a metal-can transistor.CollectorBaseEmitterCollectorBaseEmitterFigure 2-84. The 2N2222 transistor may be packaged in either of these formats. Left: RadioShack or Fairchild. Right: STMicro-electronics (note the little tab sticking out at the lower-left side). If you use a different brand, you’ll have to check the manufac-turer’s data sheet. Insert the transistor in your breadboard with the flat side facing right, as seen from above, or the tab point-ing down and to the left, seen from above.