34 August 2017
the detector circuit; and the pulse output circuit. I wrote
the sketch to control the high voltage, count the pulses,
and print the counts per minute directly into Excel.
I wanted to generate a stable voltage across the
anode and cathode with low noise and an adjustable
value from about 250V to 400V. When I looked at high
voltage generator circuits for Geiger counters on the Web,
they were almost all based on a boost circuit and
transformer using a 555 timer with a fixed frequency. This
would be okay if I just wanted a fixed voltage, but I didn’t
know what voltage would work best with these Soviet
So, I decided to design my own boost circuit, but take
advantage of the audio transformers used in most of
these. I used to be able to get these small form factor 10:1
transformers from RadioShack (Figure 4). I visited the only
RadioShack left in our town and bought the last three
units. I figured I was going to use an Arduino anyway, so
why not use it to control the pulses through the boost
circuit based on the voltage on the supply. It would act as
a “smart” digital power regulator.
The circuit is shown in Figure 5. The principle is
simple: A short 5V digital pulse from an Arduino pin
turns the transistor on briefly. This sends current
through the inductor. The voltage across the
inductor is clamped by the 5V supply on one side
and on its other side, the low impedance of the on-transistor tied to ground.
When the transistor turns off, the current
through the inductor suddenly stops and the back
EMF voltage generated across the inductance of the
primary winding goes very high, limited by the
maximum collector-emitter breakdown voltage of
the transistor. This is about 50V for the transistor I
This means I get a 50V voltage pulse on the
primary winding, or a 500V pulse on the secondary
windings. The turns ratio for this simple audio
transformer is 10:1. This 500V pulse on the high side
is rectified by the initial diodes and then gets
dumped into a 10 nF capacitor. The capacitor
smooths out the pulses into a more steady value.
Since I am using potentially 500V, I made sure my
diodes and capacitors had a 1 kV rating.
This is basically a peak detector circuit with a little
voltage boost. I added a 10 meg load resistance so there
would be a constant bleed off of voltage from the last
capacitor. The time constant of the 4 x 10 nF capacitor
and 10 meg resistor is 0.4 sec. This means once pulses
stop entering the transistor, the voltage across the GM
tube would drop to nearly zero in about two seconds. This
is a good safety feature!
Each pulse dumps a little charge into the capacitors. If
the transformer pulses dump charge into the capacitors
faster than the 10 meg resistor drains it off, the voltage
increases. I had to send pulses often enough to charge up
the capacitor. I read the voltage across the capacitor with
an analog channel. I only send pulse trains out to the
transistor when the voltage on the capacitor is below the
Of course, I couldn’t connect the ADC (
analog-to-digital converter) input pin directly to 500V. I’m limited to
a max of 5V on the ADC pin. I used a 1,000:1 attenuator
using the 10 meg bleed resistor and a 10K resistor in
series. The voltage across the 10K resistor is 0.001 x Vcap.
■ FIGURE 5.
Circuit for the
■ FIGURE 4. RadioShack 273-1380 audio output transformer.