CHAPTER 16 ■ TRANSISTOR SWITCHESCorrecting Problems with the PNP Transistor Test Circuit
If the LED doesn’t light, you can temporarily pull Q1 and connect a jumper between the positive bus and
the top of R1. This is safe because R1 and LED1 form a complete LED indicator circuit by themselves. If it
doesn’t light with a direct connection to power, you’ve probably got the LED in backwards.
After you’ve determined that the LED indicator circuit portion is functioning, you should check that the
bottom of R2 is connected to negative power. If it is, try flipping Q1 around, paying attention to the flat side.
If you insert Q1 backwards or if R2 isn’t connected to negative power, the LED won’t turn on.
Experimenting with the Functioning PNP Transistor Test Circuit
Whatever you do, don’t remove or bypass either R1 or R2. Without the resistors, the amount of current
flowing through either Q1 or LED1 would exceed their maximum rating. They would quickly be destroyed.
With a functioning circuit, try flipping Q1 around to see the LED turn off.
Rather than using a flat jumper between R2 and ground, try a reinforced jumper. After seeing the
LED turn on, you can pull the reinforced jumper from the bottom of R2 and see that that LED turns off.
Connect the jumper to positive power to see that the LED stays off. The base lead of a PNP transistor must be
connected to negative power to turn on the circuit that is attached to its collector.
Gathering Data About the PNP Transistor
When you insert a bipolar transistor with the leads in the proper orientation, the LED lights. Assuming it is
a bipolar transistor, the circuit is now telling you that the transistor is a PNP. You also know which lead is the
emitter, base, or collector, as the LED only lights when the leads match the schematic.
With power applied to a functioning circuit, try measuring the current flowing through R2. If you used
a reinforced jumper at the bottom of R2, simply disconnect the jumper from negative power and instead
connect it to the red test probe of the multimeter. Connect the black test probe to negative power. My meter
indicated only 0.0164 mA (or 16.4 mA) flows through R2.
Try measuring the current flowing through R1. My meter indicated 3.1 mA. These numbers allow the
calculation of the gain of the transistor.
3.1 mA collector current / 0.0164 mA base current = 189 hFE transistor gain
That’s higher than the 171 hFE indicated by my multimeter test for this transistor. However, gain varies
depending on how much current is applied and consumed.
This PNP test circuit will become the circuit actually used in the line-following robot. It will take the
6 mA of negative power from the LM393 and will provide hundreds of mA to the three LED indicators and
motor.
Examining the Schematic for the NPN Transistor Test Circuit
The NPN circuit is an upside down version of the PNP circuit. Compare Figure 16-7 to Figure 16-5. All of the
components and their values stay the same except for Q1.