CHAPTER 16 ■ TRANSISTOR SWITCHES■ Note In a schematic, the letter ‘Q’ is used to label transistors because the letter ‘T’ usually labels
transformers. Transformers existed long before transistors were invented.
The direction of the arrow in the schematic symbol for a PNP transistor indicates that it is PNP. The
arrowhead inside the symbol’s circle points toward the base pin (marked with a ‘B’). The arrow indicates
the direction electricity must flow. Therefore, the emitter (marked with an ‘E’) must connect to positive
power. The electricity then flows from the emitter, through the base and collector (marked with a ‘C’), and
eventually out to negative power.
Resistor R1 and LED1 should be familiar to you. They form a simple LED indicator circuit. No matter
how much of the battery’s current is made available by the transistor (Q1), the resistor (R1) limits the
amount of current LED1 actually receives.
Resistor R2 also provides a current-limiting function. It prevents the transistor (Q1) from having too
much current flow through it. Not only does R2 prevent too much current flowing from the emitter to the
base, but it also controls the maximum current that the LED Indicator circuit can have.
Dimming the LED for Reverse Connections
Most transistors have some gain (3 hFE to 5 hFE) even when connected backwards. We want a circuit that only
lights the LED significantly when the transistor is wired correctly (gain of over 75 hFE). To achieve this, R2 is a very
large resistor value and LED1 is restricted to being a standard LED (not high efficiency or high brightness).
Figure 16-5. Schematic for a PNP transistor test circuit