configuration is a NOR (or not OR) gate — an inverting OR
gate. We do this by wire-OR-ing several inverters together.
Since we rely on the collector pulling down for a low,
but on a resistor for it to rise, we can just string inverters
together while tying their collectors to a common node
so that when any of them are turned on, the output will
go low (see Figure 2). If we want to make that into an OR
gate, we just put a single transistor inverter on the output;
either a PNP or NPN will do.
With our OR gate out of the way, we need to make
an AND gate. We can take advantage of something called
De Morgan’s laws for logic transformation. If we invert the
inputs of a NOR gate (like the one we already made), we
can make an AND gate. You can see that simple circuit in
Figure 3. It takes another transistor for each input to do the
inversion. Similarly, you can invert the inputs to an AND
gate and get a NOR gate, though that’s not what we’re
As for what transistors to use, any garden variety NPN
or PNP will do just fine, as long as the current and voltage
requirements are modest. I typically recommend two
classics: 2N3904 NPN and 2N3906 PNP. They’re not that
fast and not that capable, but often will get the job done.
Note also that the resistor values are not very precise. I
just chose ones that the transistor will be happy with, but
you can change them to make the transistor work harder
so that it will supply more current when pulling down, or
decrease the collector resistor value to pull up harder.
If you’re interested in what can ultimately be done just
with transistors for logic, James Newman built a computer
he calls the Megaprocessor containing 40,000 transistors
and 10,000 LEDs. Check it out at www.bbc.com/news/
Battery Polarity NOT Important?
QWe have wireless microphones at our meeting place. The microphones don’t care which way the 9V battery is inserted. Do you have a circuit hat shows how I can implement this in other
AThat’s a pretty clever thing — making the microphone not care about the polarity of the battery. Fortunately, it’s not that hard to do, but at a small cost both monetarily and otherwise.
Diodes are able to allow current to flow in one direction
and not the other, within certain limits. We don’t care
much about those limits for this particular application, as
long as we use a reasonably good diode that can handle
the necessary current.
When we want to rectify the AC voltage from a
transformer, for example, we can use diodes to do this.
Figure 4 shows a very simple rectifier that will deliver one
half of the sine wave coming out of the transformer which
will be of just one polarity. That’s all
well and good, but we are missing the
chance to get the other half of that
sine wave. There is potential energy
there that could be used to power
In order to extract that, we use a
more clever circuit — a full-wave bridge
rectifier — that can take the voltage
from the positive or negative parts of
the sine wave and deliver them as a
single DC voltage. It will still vary with
the sine wave, but the polarity will
always be the same. We can mostly fix
that with a filter capacitor, but that’s
not what we’re after for this question.
QUESTIONS and ANSWERS
Post comments on this article at www.nutsvolts.com/magazine/article/October2016_QA
n FIGURE 3.
n FIGURE 2.
October 2016 11