The major gripe I have with my most powerful geiger counter, the Radiation Logger,
is it's extreme lack of portability and robustness. To use it I had to find a 12V supply to plug into mains, or use chunky
lead-acid battery. Not to mention that the counter itself had to placed on a flat surface, without any moving objects nearby,
lest it snag on something and break. To remedy these problems I decided a long time ago to make a Geiger counter which was powered
by standard AA batteries, had a robust casing with some protection of the Geiger tubes, and incorporated bluetooth so I could
monitor it from afar. Upgrading the Radiation logger project with bluetooth was a prelude for this project, demonstrating both
the general idea, and interface with an Android app. This geiger counter is fully compatible with my (at the moment unpopular)
BluGeiger app for Android.
I wanted this counter to be somewhat agnostic to the type of Geiger tube used, as long as the voltage range was within what the
HV module could handle. To put this to the test I made two units, one using the SBM-20, and one using the SBM-19. These tubes have roughly
the same plateau voltage, but different sensitivies. The SBM-20 is smaller, and more robust, lending itself to quicker survey work
in the field. The SBM-19 is large, and thin walled, making it great for ambient radiation detection. In practice I found the SBM-20
very insensitive, almost to the point of being unsuable. I'm unsure if I was unlucky with the tube, or HV supply. The SBM-19 has worked
excellently!
Comparison of SBM-19 and SBM-20 Geiger Muller tubes
Based on past experiences, I knew I didn't want to spend time working on the HV supply. So instead I elected instead to find
a complete module which could run from a sensible voltage in the range 3 - 12V. My choice landed on the IMEX-38-56-1,
which can be found on ebay for a reasonable price. An alternative to this would be to use the microcontroller to generate
PWM, and the built-in ADC to collect feedback. Finding a suitable transformer for this will be somewhat difficult, and
was the main reason I wasn't interested in developing an HV module myself. My experiences with the IMEX-38-56-1 have been
positive, although the output voltage didn't quite reach the desired value for the SBM-20 tube. I may have been unfortunate
with my supply.
The casing for the Geiger counters was designed in OpenSCAD, and source files can be found on thingiverse. Page for the
SBM-20
and page for the
SBM-19
.
I've made a PCB and design files for the electronics board also, which can be downloaded here.
The firmware was created in AVRstudio6, and can be downloaded here.
SBM-20 version of the Geiger counter
To boost the battery voltage up to a usable level for the microcontroller I use a TPS61070DDC from Texas Instruments, which is
a nifty little DC-DC boost converter. It will actually work with voltages down to 0.9V, meaning I can drain every last joule of energy
from the batteries! For bluetooth functionality I used a HC-05 module, which is very common in Arduino projects, and also something
I had decent experiences with in the past. The main control unit is an ATmega328, which has all the hardware modules I desired,
and enough memory to handle the serial protocol. The serial protocol was developed with console use in mind, meaning it should be
entirely human readable. This puts more demand on the embedded devices however, as they need to remember long text strings.
SBM-19 version of the Geiger counter
Closing thoughts
The battery life is not what I was originally hoping for. I had tried to minimize power consumption where possible,
but completely ignored the HC-05 bluetooth module. I measured the current draw for fun while the module was searching
for a unit to pair with, and it was using 175mA at 3.3V! I haven't measured the current draw without a bluetooth module
connected.
Figures on the internet quote
these devices drawing 30-40mA during pairing, and and idle current of 8mA. So some part of my circuit is still drawing
a fair amount of current. This might be something worth investigating if extreme battery life is of concern. For two fully
charged AA batteries, the total lifetime should still be more than 10 hours at 175mA.
A potential alternative to the ATmega and HC-05 would be using a dedicated bluetooth 4.0 chip. These chips usually include a fully featured microcontroller
to handle the radio stack, and would be more than powerful enough to also handle the Geiger counter functions. I explored this route rather
extensively, however I ended on the conclusion that it wasn't feasible sadly. The best candidate I could find in terms of price and sourcability
was the CC2541 from TI, which comes mounted on a HM-10 module. However the only way to program one of this is with IAR workbench, for which there
is no free version, and in addition costs a small fortune to purchase. The other options all involved aquiring BGA type packaged components,
and designing a PCB with an internal antenna. I estimated it would take me a few iterations to get right, so I didn't pursue this route either.
Disclaimer:
I do not take responsibility for any injury, death, hurt ego, or other
forms of personal damage which may result from recreating these
experiments. Projects are merely presented as a source of inspiration,
and should only be conducted by responsible individuals, or under the
supervision of responsible individuals. It is your own life, so proceed
at your own risk! All projects are for noncommercial use only.
This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.