a high positive voltage, while the outer metallic cylinder
(the cathode) is at a lower voltage. When a particle like
a gamma particle or beta particle passes through the
thin gas, it leaves a trail of ions and electrons behind it.
The ions accelerate to the lower voltage outer shell (the
If the voltage is high enough and the gas pressure
is just right, the ions collide with more air molecules
and ionize them; they get accelerated and ionize more;
and we get an avalanche and a large current burst. This
avalanche multiplies the original number of ions created
by the gamma particles by a factor of 100 or more. It
self-terminates in a micro second or so.
There are Geiger tubes for sale from scientific
supply houses, but they range in price from $100 to
more than $300. However, when you do a web search
for Geiger tubes, the first items that come up are
vintage Soviet military Geiger tubes on eBay for only
$5-$20. These were used by Soviet soldiers when
investigating potentially hot areas.
I went ahead and purchased two of the STS- 5 Geiger
counter tubes (shown in Figure 2) which had never been
used. I paid $16 for each one.
The specs say the Geiger tube operates between
280V to 390V. The background rate is about 25 counts
per minute, with a max count rate of about 150,000/min.
These units are shipped from the Ukraine. When the
package arrived with the Ukraine return address and
Cyrillic writing, my wife warned me that we were probably
on a Homeland Security list somewhere. When I
mentioned I might be ordering some radioactive samples
to test my Geiger counter, she drew the
line and told me she did not want a “visit”
from Homeland Security. I had to make do
with something else.
The next step was to build the
electronics to power-up the Geiger counter
and count current pulses, record the
counts, and convert to counts per minute
to be plotted directly into Excel.
The High Voltage
Since I am enamored with the power
of the Arduino, my detector was going to
be Arduino based. I didn’t need anything
very sophisticated so decided on an Uno,
which is my workhorse general-purpose
Arduino. I use this microcontroller in the
workshops I teach at our local hackerspace,
Tinkermill. In my three years of teaching
this workshop — with more than 300
participants and all of my playing around,
plus Maxwell as my ESD tester — I have
never damaged an Uno. Until this project.
I blew two Arduinos by accidently touching a digital
pin to the 50V on the low side of the high voltage supply.
I was very careful after this.
The Geiger tube takes about 300V. This voltage level
with its capacitance can be lethal! DO NOT ATTEMPT
THIS CIRCUIT UNLESS YOU ARE EXTREMELY CAREFUL!
As a special precaution, I built the entire circuit inside
a plastic box (Figure 3) which I could seal up, and keep
fingers and cat noses and tails from touching anything that
might be high voltage. This also made the project
transportable and easily handled.
This circuit has three parts: the high voltage generator;
August 2017 33
■ FIGURE 2. Soviet military surplus Geiger tubes and
■ FIGURE 3. The Geiger counter circuit and tube are inside a sealed
plastic box for safety and convenience. Note the battery connection
for portable operation.