1. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 1
Transistor Tester
Parts List
 1 – LM555CM
 1 - 10ΩF Capacitor
 1 – 33kΩ Resistor
 4 – 220 Ω Resistor
 2 – 1N4148 Diode
 1 – Red LED (3mm)
 1 – Green LED (3mm)
 9V Power Source

2. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 2
Theory:
With all the circuits I make, I end up with a lot of circuits hanging about.
Not many of my projects are permanently used, their parts being recycled into
new projects. Even fewer achieve the final accolade, "box status", so it is a
measure of how pleased I was with the design that this project was instantly built
into a box with its own interfacing circuit.
So let's see what we need to do to build an intelligent transistor tester. First
off, remember that we think of electricity flowing from the positive terminal to the
negative one, whereas it is actually a flow of electrons the other way round.
Before the true nature of electricity was known scientists made a guess as to the
flow direction, a 50 / 50 chance that they got wrong. So now we are lumbered with
the twin concepts of conventional current or electron flow.
Transistors nowadays, are mainly made using silicon, initially germanium
was used but is seldom found now. By adding certain impurities in minute

3. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 3
amounts you can create N type or P type silicon, the difference is how the
electricity moves through it. If you connect a voltage up to a piece of silicon you
will get electricity flowing through it. With N type silicon electricity conducts by
electrons, negative particles, travelling from the negative to the positive
terminals. Whereas in P type silicon positive particles carry the current going
from the positive to the negative terminals.
Now these positive particles are called holes and don't really exist. Yes
read that again it's right. A hole is the absence of an electron, a vacancy where it
can sit. Any electron filling the vacancy will leave a hole behind it that can be
filled by a different electron. So while the electrons shuffle towards the negative
terminal the hole makes its way to the positive one. It's much easier to think of
the hole as a particle in its own right, rather like considering a bubble rising to the
surface rather than all the surrounding liquid slipping down a bit.
Well having got that concept I can describe a transistor and how it works.
Simply take these two types of silicon and make a sandwich as shown in figure 1.
Looking at this it come as no surprise to learn that we call this an NPN type
transistor. It is possible to make transistors out of the other type of sandwich, the
PNP as we will see later. Now in this figure I have shown an applied voltage
across the sandwich, no current can flow because the electrons can't get over the
P layer in the middle, it is all to do with electric fields stopping them moving
through the P type region.
Now if we take a wire connected to the P type middle region or base of the
transistor and apply a voltage to it you will cause a current to flow from the
emitter to the base. All the free electrons in the bottom part of the sandwich see
all the extra holes in the base and would rather like to combine with them. Like
some other aspects of life, each hole manages to attract several, even hundreds
of electrons to it. One electron combines with the hole but all the others are
running so fast that they end up with enough energy to burst through into the P
layer, flow right across it and end up in the collector region. So a small current
flowing in the base collector region will cause a large current to flow from emitter
to the collector. You can see where the terms emitter and collector come from
now can't you? So why is the middle one called the base? Well that's historic and
comes from how this sandwich is actually made. They start with a base of P type
material and deposit two spots of N type material either side of it. This type of
topology is used because you can only connect conducting wires to the top or
bottom of the silicon, not in the middle.
So in essence a small current flowing cause a larger one to flow and the
ratio of the difference between these two currents is called the "gain" of the
transistor.
FredrickKendrick© ®

4. Fredrick Kendrick Heller Cook Book 3 ET1410 / ET1210 4
December2014toMarch2015