CHAPTER 16 ■ TRANSISTOR SWITCHESPushing the 2907A’s Button
You can connect most transistors to act as electronically controlled pushbuttons. “Pressing” a PNP
transistor’s base lead with negative power turns on the circuit. Disconnecting the base lead or applying
positive power turns off the circuit. (A thorough example circuit appears later in this chapter).
This may seem a little backwards. Usually something turns on when positive power is applied; NPN
transistors work that way. However, since the LM393 comparator only connects its output signals to negative
power, an appropriate transistor was chosen that turns on when it’s connected to negative power.
So, the LM393 comparator signals to the 2907A transistor to turn on the LEDs and motor by connecting
the transistor’s base lead to negative power. The transistor is the muscle; the comparator is the brain.
Checking the Datasheet
Recall that a datasheet, which lists important facts about a part, is available from the manufacturer or retailer.
The 2907A datasheet indicates that it is capable of sourcing 500 mA of current continuously (as opposed
to short bursts). Wow. That’s a big improvement over 6 mA for the LM393 comparator. The 2907A provides
plenty of electrical flow for the LEDs and the motor, with room to spare.
Just like a chip, it is expected for each part to use a little bit of power for itself. The datasheet indicates
that there is a small, continuous electrical drain for enabling the 2907A transistor. There is also a slight
voltage loss of 0.2 V for the circuit being switched. But these power costs are insignificant on this robot.
Obtaining 2907A Transistors
Table 16-1 lists suppliers of 2907A transistors in the solderless-breadboard–compatible TO-92 package.
Because bipolar transistors are so inexpensive, you can afford to reduce the individual price by purchasing
ten at a time.
Plain 2907 (without the A at the end) transistors are available. They are acceptable replacements for
2907A transistors in the line-following circuit. However, the 2907 transistor has reduced capabilities in
several categories (collector-emitter breakdown voltage, collector cutoff current, and gain), and, as such, is
not always a suitable replacement for the 2907A in all circuits.
Testing Bipolar Transistors with a Multimeter
Depending on your multimeter, there are a couple of methods of testing a transistor. The more informative
testing method requires a multimeter with a transistor test socket. The other testing method is for
multimeters with a diode test function. Later in this chapter, a sample circuit is provided that allows a
transistor to be tested even if your meter has neither a transistor test socket nor a diode test mode.
The following instructions should be applicable to most meters. However, it’s best to follow the specific
instructions provided with your meter.
Table 16-1. Suppliers of 2907A Transistors in TO-92 Package
Supplier Part Number Price Description
Jameco 178520 10 @ $0.09 Various manufacturers
Mouser 512-PN2907ATAR $0.25 Fairchild Semiconductor
Digi-Key PN2907ABU $0.22 Fairchild Semiconductor