CHAPTER 16 ■ TRANSISTOR SWITCHES
Q1’s collector has flipped to the other side of the transistor, but the LED indicator circuit (LED1 and R1)
is still connected to the collector lead. The arrow remains on the emitter of the transistor symbol, but the
arrowhead no longer faces the base. The arrow shows the direction of electrical flow.
R2 is connected to positive power, as is LED1. This is the most significant difference between PNP and
NPN transistors. NPN transistors turn on via positive power to the base, and they connect the attached
circuit to negative power. PNP transistors turn on via negative power to the base, and they connect the
attached circuit to positive power.
Building the NPN Transistor Test Circuit
If you’ve built the PNP transistor test circuit (see Figure 16-6), rotate the breadboard upside down and hold it
near Figure 16-8. It looks almost exactly the same! All the parts are in the same place and orientation, except
the notch in LED1.
Figure 16-7. Schematic for an NPN transistor test circuit