CHAPTER 16
Transistor Switches
When a chip pin provides current like the positive terminal of a battery, it is said to “source” current. When
a chip pin receives current like the negative terminal of a battery, it is said to “sink” current. These terms are
reminiscent of the water analogy, where water comes out of the source and then goes into the sink.
Many chips have the same limitation as the LM393 comparator as far as their inability to source or sink
significant amounts of current. Not only is maximum current an issue, but also older chips can’t source as
much current as they can sink. So, designers connected LEDs and other circuits “backwards” to these older
chips, to take advantage of the stronger draining. (That dissimilarity has disappeared in newer chips that
have dedicated hardware to provide more current and at equal levels.)
A chip’s low current handling capability isn’t usually a serious issue, because most chips use their
pins only to pass signals to other chips. When a heavier load needs to be serviced, a discrete transistor is
well-suited for the job.
There are many, many kinds of transistors. Their sizes and attributes are tailored for desired
amplification, switching speed, price, noise, power usage, voltage tolerance, and/or current delivery.
Although intermediate and advanced robots use mixed types of transistors, only a couple of general-purpose
transistors dominate beginner robots.
In this chapter, you’ll learn about transistors, specifically the 2907A PNP. You’ll connect transistors to
your LM393 comparator so that the robot can be brilliantly lit by three LEDs on both sides.
Defining Negative Power
Most batteries’ terminals are labeled with a positive (‘+’) and a negative (‘-’). If you connect your multimeter
correctly to the battery terminals, you’ll see a number in voltage mode. If you then switch the probes to the
opposite battery terminals, you’ll see the same number with a minus sign. The battery hasn’t changed, only
the meter’s perspective.
The technically questionable term “negative power” is used in this chapter and throughout the
book because it appropriately reminds the reader of connecting to the negative terminal of a battery.
However, any voltage lower than another is relatively negative. Let’s say you have two test points, the
first has 9 V and the second has 4 V. If you connect something to the 9 V point, then the 4 V point is -5 V
in comparison. You could say the second test point has negative voltage in comparison to the first test
point. Like a battery and a multimeter, if you connect the part in the opposite direction, the voltage will be
opposite.
Electrical power is more than just a difference in voltage. The term “power” is a combination of voltage
and current. As used in this book, the term negative power is used to suggest that there is both a negative
voltage and that sufficient electricity is allowed to flow to that negative voltage point. For example, the
negative terminal on a battery has both a negative voltage and the capability for electrical current to flow to
it, so it could be said to have negative power.