I.C. Transistor Tester - Your little
workbench companion .
Originally published in The
Printed Circuit, Newsletter of the Tallahassee Amateur
Radio Society, June 2013, page 14
Edited/Updated December 2023
the Bases… Collectors and Emitters
have a parts bin stuffed full of dusty old transistors needing to be
sorted? Are you experimenting with a circuit and fear you’ve
‘let the magic smoke out” of a transistor? Then, with the
push of a button, this handy circuit can test the pulse of your
transistor in question – even one soldered into a circuit!
"What's a transistor," you
ask? Seriously? [OK, I'm just covering the range of electronics
learners.] Only one of the most important
inventions in history. A transistor is a "solid-state"
component that can be used as an electric switch or an amplifier.
Their physical size can be tiny, effectively allowing the
existence of the very devices you fit in your pocket and trust for your
hours-on-end scrolling fix. Transistors embedded on
modern integrated circuits are so small that their leads are even the
size of several atoms in width! This Transistor Tester
circuit has been designed to test most common "BJT" types. A
BJT, which stands for "Bipolar Junction Transistor," is the
type of transistor in which the current flow between two terminals (the
collector and the emitter) is controlled by the amount of current that
flows through a third terminal (the base). Shown here is this
simple circuit set up on a solderless breadboard. Keep reading to
find out more... .
If you’re constantly
populating breadboards with one project or another, testing your
circuit’s transistors is more of a necessity than a novelty.
You can, of course, take your multimeter to any transistor in
question, and with some finger dexterity, manipulate the leads while
writing down resistances and remembering which pin combination reading
needs to be 6/10th of a volt higher than the other. I
personally, could never quite get the hang of that myself, so why go
through all that trouble?
If do a quick search online
with Amazon, AliExpress, eBay and retailers of the like, you’ll find
more “Transistor Testers” or "Component Testers" than you can count. These versatile
component testers, many sold as kits, have appeared on the market since
the first publishing of this article. Many are based on the
ATmega328p used in the popular Arduino board and are very
inexpensive and certainly worth putting in your kit. [Read more BELOW]
These testers can detect and test many component types
including many types of transistors such as MOSFET, SCR and more, as
well as resistors, capacitors, inductors and diodes. Many
especially pricey transistor meters geared towards engineers can tell
you all kinds of nice things about any particular transistor, at the
very least, their characteristic curves and “hFE” (or β Beta).
But if all you want to know
are the quick and dirty basics; whether the transistor is an “NPN” or
“PNP” type as well as its pin configuration (Base, Collector, Emitter,)
and oh yes, whether you’ve simply just ‘let the smoke out’ (which is an
important ingredient in electronics components,) then this circuit will
fit the bill. It's simple enough to quickly cobble together
on a solderless breadboard and only requires one IC, a 556 (which is
two 555 timer chips.) .
FIRST, before reading on, you should
brush up on transistor basics and some basic use configurations.
Please check out the following video: The
Ultimate Guide To Transistors (BJT Edition)
by Sine Lab on YouTube.
I wanted this tester to have the
fewest components necessary and none of them could actually be a
transistor because you should be testing the reliability of a certain
component using that same component. U1, the "556"
is a dual-package version of the venerable and easy to source “555”
timer/oscillator IC. In fact, two 555's can be substituted
for the 556 if one is hard to find. One part of the 556 (or a
single 555) is used as a square-wave generator, supplying an
alternating DC (direct current) source as a logic pulse to the test
matrix. The “test
matrix” works by using combinational biasing resistors plus a
few voltage dropping diodes to route power through different leads of
the transistor under test so that in various types of operation (or
lack of) the bi-directional red/green LED indicators (also acting as
steering diodes) will reveal different conditions about the whole test
circuit depending on the combinational operation of the matrix based on
the implementation of the transistor under test. .
The only electrical access
to the matrix involves two alternating logic-state I/O outputs from
each '555' section of the 556. When one output is in a “high”
state, or positive potential, the other output is inverted, in
a “Low” state, or at a potential path to ground, together
forming a complete directional current path for the matrix. The LED's
and diodes route the present DC flow and the resistors help bias, or
“switch on" the test transistor if properly applied (and working) to
the circuit. This is a tricky circuit to describe, but in one
part, when the transistor is conducting, it will essentially ‘short
out’ another, and the corresponding LED's will either light or not
light. So we’re using one of the '555' timer/oscillator
sections as the square-wave oscillator, but the '555' only has
one logic output. How do we get the other inverted
output to complement the oscillator to make a closed DC source for the
matrix? Well, that employs a simple
configuration we’ll set with the other '555' section.
How does the second '555'
section of the 556 invert the logic state of the first section?
A few transistor testing circuits I’ve found out there use
the dual 555 combination, but they’ll use both as astable
multivibrators, except that the first one triggers the second one so
that it yields an opposing logic state; a low for a high. But
in my circuit I decided to try something surprisingly different: . Hack
contemporary spirit of the Maker movement, circuit bending and
"hacker-ism," I’m using the second '555' section of the 556 IC for a
purpose probably not documented in its tech specs, as an ‘inverter’.
Are you in a pinch for just that one more NAND-gate
to complete your digital logic circuit? Some global chip
shortage keeping you from finding even one basic logic IC on your
preferred supplier site? Of course you can’t. So
here, you can take a simple 555 IC (or a few) and create
pretty much any kind of logic configuration from complex gates with
hysteresis to various flip-flops.
Some folks have really
played this unorthodox usage out just to see how far they could take
it. Here’s one site of interest for example: http://www.pmonta.com/555-contest/logic/logic.html
By using a certain wiring configuration, you can essentially
turn this good-ol'-trusty humble timer/oscillator into a “555-gate,”
which is kind of like an AND-gate, but with just one of its inputs
inverted (a “NOT.”) With this basic building block you can
form nearly any other combination. All you need to do is add
upon it and control its logic signals. See the diagram of
how it’s done BELOW.
To make a '555-gate', tie the Threshold
pin to 'High' which is the positive (Vcc) rail. This essentially
disables the 555's internal latch, now use the Reset and Trigger pins
as inputs [See diagram below] and the Output will follow the truth
table as shown in the diagram.
To get my inverter (or NOT
buffer,) I tie the Reset pin of the second 555 to the
positive (Vcc) rail, with the Threshold B (pin 12) tied to Vcc as well
of course, therefore, the Output signal of the will always be
opposite in logic state than the Trigger B (pin 8) input. With
we’ll now have an opposite logic state to act as the return power
source for the Output [A (pin 5)] of the oscillator section and we’ll
have a directionally
alternating flow of DC current to run our transistor test matrix with.
This alternating polarity is needed, as the two different
types of transistors, NPN and PNP, conduct or impede current
according to polarity in their respective ways and thus determine the
illumination state of each indicator LED to reflect the identity and
condition of the transistor under test. .
For a closer inspection of
each part of my circuit, R1 serves to slightly reduce
operating current, especially if you wish to use 10 to 15+ volts for a
power source. The CMOS type 555 IC's, known as 'NE555', can
take up to 15 volts. You should probably just use an N-type 9
volt battery for this circuit, or use a 3 to 5 volt battery source in
conjunction with the LMC555 or TS555 low-power varieties. The
TLC555 can run from 1 to 15 volts. R1 can be omitted for
power source of less than 10 volts, and is not really critical.
The circuit should draw 50 miliamps or less of
current depending on the LED's used and you shouldn’t need more.
D1 is an optional diode to protect your circuit from
accidentally connecting the battery up backwards. This is
also optional and you can expect a source voltage drop of 0.7 volts.
Operation should be reliable
as low as 5 volts. R2 and R3 are for the first 555
section's oscillator timing. With
an astable configuration, R2 controls the time length of
its high logic mark pulse but doesn’t affect the low
space length timing. Increasing R3's resistance will also
increase the high logic mark pulse time length but will also
increase the low space time. Increasing R3 will also
decrease the pulse period duty-cycle. C1 determines the cycle
timing and increasing it will lower its cycling frequency.
With the values set up here, I run this oscillator at about 1
kilohertz with about a 50/50 duty-cycle, enough of a rate so that your
eyes don’t see any noticeable flickering due to persistence of vision.
This operation, in a way, is sort of a pulse width modulation
As you can see,
the 556 IC package has a separate set of controls for each
'555' timer. Pin (5) is the Output A for our oscillator, pin
(9) Output B is the inverted-state version of the pin (5)
Output A, and this composes the source of our alternating DC.
Pin (5) is fed to pin (8), Trigger B, to trigger the second
oscillator section configured as our logic inverter. R4, a
100k ohm resistor, is used to make the Trigger B a high impedance input
which would normally be a path which could drain too much current away
from the matrix section. . Enter
the test matrix, the three resistors, R5, R6 and R7 serve to
supply bias to the test transistor so that in normal expected
operation, the transistor is 'switch on' in full saturation mode, they
also route power through the indicator LED's so that in the right
configuration, the correct indication is given.
Since the current path is
alternating, each transistor should only conduct on half of each cycle.
When not conducting, or when conducting in an incorrect
manner such as a Collector to Emitter short condition, depending on the
power’s polarity path, certain LED indicators will then light to
correspond with the particular problem. A yellow light
(red+green) from LED D3 will indicate that power is flowing in both
directions through the Collector-Emitter junction, so the transistor
must be shorted. If D2 is conducting both ways causing a
yellow indication light, then this says that there is no current
flowing anywhere, where the transistor should be, more power is free to
flow along D2’s path and perhaps the transistor is in an open
condition, or not connected properly.
Take note on diodes D4 - D7
linked together through LED(s) D2’s path. The LED set of D2's
diode voltage drop properties were not enough in my experiment to
current from completely bypassing, which was giving me an erroneous
reading. Adding the extra small
silicon diodes served, not only to add a voltage drop to the LED pair,
as it was brighter than D3, but since those diodes have a small
drop of 0.6 volts before conducting, this feature serves to
help route power through D2 more so when it’s supposed to, otherwise,
D2’s lower forward voltage would always be attractive to some degree
for current flow and would remain partially lit in both directions
during test. Typically, the forward voltage of an LED is
between 1.8 and 3.3 volts. It varies by the color of the LED.
A red LED typically drops around 1.7 to 2.0 volts, but since
both voltage drop and light frequency increase with band gap, a green
LED will drop around 2 volts, and a blue LED may drop around 3 to 3.3
volts. Why two 1N4148 diodes (D4 - D7) in series and why the
low resistance value for R6? (at only 2.2 ohms, but you can use up to
30 ohms.) Because it worked in my setup. Please note that I
originally had a 330 ohm resistor in place of D4 - D7, so consider
trying that out for yourself to see which method works better.
little tweaks are products of trial and error, and consequently, at
least in my test, when I used two separate 555 IC packages
instead of the 556, I didn’t need to use diodes D4 - D7, just
a 150 ohm limiting resistor in its place. You will have to
try these tweaks in your setup for best results. . Test
constructing this circuit consider what type of socket or test leads to
use for connecting to the transistor under test. ‘Alligator'
clips or precision lead clips are great for testing a transistor
already soldered into a populated circuit board. Because of
the transistor’s low cross-part resistance, the testing matrix may not
even be affected by attached components in a typical BJT transistor
application. You can use an actual transistor socket or even
an IC socket to temporarily mount any loose transistor package.
All you’ll need are three socket holes wired for the most
commonly used arrangement. For most TO-92 type
form-factors, arrange the socket to accept Emitter, Base and Collector [E|C|B] lead pins
respectively (when looking at the flat side of the casing.)
Hunting for a quick answer as to which leads are which on a
more uncommon or vintage transistor? Use a five-pin single
in-line socket wired as [E|B|C|E|B].
Plug the three leads into E, C and B first, then
move down through B,C,E and then C,E,B. If none of those give
you a satisfactory reading, rotate the part so the leads are flipped in
order and try the succession again. This will give you six
combinations. If you cannot find a five-pin socket like this,
use an IC socket and cut or file away what you don’t need.
The DuPont connector sockets used on Arduino development
boards and the like are inexpensive and easy to come by online.
To Work It:
couldn’t be simpler: First Install a 9 volt N-type battery or
whatever source works for you. Push the button S1 to
‘light-up’ the circuit. Don’t worry about hot-swapping, or
plugging and unplugging transistors into the test socket while the
circuit power is on, as (unlike their FET cousins,) BJT's can generally
take the abuse and in that situation you’ll know better if any
oxidation often found on the leads of older parts could be to
blame for any mysterious intermittent function behavior.
When both LED's (D2 and D3)
this signifies that the BJT transistor under test is operating as a
functioning PNP type and that its leads are arranged E-B-C in matching
succession. Two GREEN
LED's signify a functioning NPN type. Those are the best
typical results, but it's possible to see a subtle color variation due
to the test transistor being a little exotic. As long as the
two indicators mostly and apparently agree. However, if you
see something else completely, consider first re-arranging
the lead connection order of the test transistor and re-testing.
Surprisingly, even large power transistors light-up the
circuit just as well.
Ideally, seeing two ‘greens’
or two ‘reds’ will have you happily sorting your transistor stock in no
time, however, you’re sure to see some of the ‘yellow-ish’ combinations
which may mean trouble. If D2 is DIM
and D3 is YELLOW,
then your transistor has a short-circuit between its Collector and
Emitter junction. If D2 is YELLOW
and D3 is DIM
then because more, or all of the power is being routed through D2, then
the test transistor is either blown (or open) or it’s not sitting in
the socket properly. It gets trickier from here; If D2 is
yellow, not quite green or red, but maybe close, but D3 happens to
be either green or red, then you either have not plugged in an actual
transistor, or most likely, its pin configuration does not match the
socket. Sometimes a perfectly good PNP with show,
what looks like two ‘green’ lights, but not quite. Simply try
each of the six combinations to determine which one gave you the best
looking results, ie: two solid ‘reds’. Also, you may find
that, even when the Emitter and Collector are swapped, you may still
get all solid reds or solid greens. If that's the case, your
best bet is the one where the results are the 'most solid'.
No guarantees on this
circuit in every instance, folks, but it should do a good enough job
without having to fiddle with a multimeter. . Use
of enclosure should you use? Of course, an Altoid’s®
mint tin! I’ve provided an instruction sheet, when printed at
actual size and trimmed should fit inside the lid of a common mint
tin. Please click on the example image below to download
the .PDF. I recommend printing the
label on one of those self-adhesive vinyl sheets used for making your
own decals. With only the 14-pin 556 IC and a few minor
the battery may take up more space than the circuit. With its
sparse workbench footprint and lower current consumption, you can use a
dry-cell or lithium metal battery and probably won’t have to change it
out for a decade! .
word on those Arduino Ali-eBay Every Component Transistor Testers
time I first published this article, a very versatile transistor tester
project based on the Arduino, or rather the ATmega328p microcontroller
IC began to become rather popular. Originally invented by
Markus Frejek and then further developed by Karl-Heinz Kübbeler, this tester featured automatic detection and
parameter testing of many kinds of components. Later,
numerous developers, mostly from China, would increase it's
capabilities and provide kits on AliExpress, eBay, Amazon and others
for very little money, thus flooding the market. You can
check out one quick article here. From what I've read,
the original creators don't receive a dime and millions of these kits
based on their code and methodology have been sold. Sure, I
agree that is an egregious injustice, but the originators did provide
the project as open source and such a device has helped millions enjoy
and progress in the field of electronics. .
I won't post any links here on what to
get and where to find them as this is beyond the scope of this article,
but you can simply search Google for "Component Tester Kit" and you'll
be flooded with options, most ranging from $1.50 to $30 USD.
Many are in kit form and many are finished products built
into decent enclosures. You must use due diligence when
researching one to purchase as there are many shady dealers offering
junk, but any electronics hobbyist should certainly build and use one
of these (along with my transistor tester.) If you're going
to solder one of the kits yourself, just don't spend a lot of money on it.
Specifications and features
will vary by design, but some features may include: a full-color
customizable display, a rotary encoder knob, 7 to 12v operation, even
from a 9 volt battery, auto-shutdown with nearly infinite shelf
life. Also, automatic detection of components including, NPN,
PNP transistors, FET's, diodes, thyristors and SCR's, diodes, Zener
diodes, inductors, capacitors, 2 resistors at once (network), and other
components, automatic pin lead identification and schematic diagram
with pinouts on the display, resistances down to 0.01 ohms, gate
capacitances, ESR and many other component parameters. Many
testers provide frequency generation from 1hz to 2Mhz with 1-99% PWM, a
frequency meter and IR send and receive if an IR device is being
along with various methods of connecting components such as a ZIF
socket and SMD test pads. It's amazing what can be done with
Check out Electronoob's
assessment on these testers:
By the way, give Electronoobs
a bit of subsciption and Patreon love! He's, by far, one of
the most enthusiastic and inventive electronics engineering, YouTube
Makers out there and has inspired many of my own projects, and since
YouTube screwed up their viewer-suggestion algorithm in favor of
mindless junky Shorts content, his channel has been getting the shaft! . Pencils
Down, The Test Is Over
My transistor tester
circuit is a quick and simple weekend project and worth a try on your own solderless
breadboard, even just for academic curiosity, and you can even
you’ve "hacked a 555 for an unintended purpose!" Get out that
old transistors you’ve been hording for so many years and test away! .
The weekend is here, so go and build
73! DE Mike, K4ICY MikeK4ICY@gmail.com