What is it?The MightyOhm Geiger Counter is a device for detecting beta and gamma radiation. This kind of radiation is emitted by caesium-134 (134Cs) and caesium-137 (137Cs), i.e. radioactive isotopes of caesium released into the environment by nuclear weapon tests and nuclear accidents, most notably the Chernobyl and Fukushima Daiichi disasters.
Uranium (U) and thorium (Th), two of the most common radioactive elements on earth also produce such radiation. Their so-called decay chains pass through a phase of radon (Rn) which under normal circumstances is gaseous and easily inhaled, and therefore is considered a health hazard. Because radiation is so dangerous, you may want to keep an eye on the environment you live in and invest in a radiation detector like the MightyOhm Geiger Counter reviewed here.
It clicks, beeps and flashesThe detector produces a beep (a ‘click’) and a light flash every time the gas inside its sensor, a Geiger-Müller (G-M) tube, is ionized by passing radiation. To make this process work, a voltage of several hundreds of volts is required to bias the G-M tube. An ionization event results in a short current pulse that can be measured and made audible; these are the famous clicks you have probably heard in movies (e.g. James Bond).
So now we (vaguely) understand how the Geiger counter works and what it is for, let’s build one using the MightyOhm Geiger Counter kit. It's 100% open source and open hardware, meaning that you can study it in detail. It features an LED and a piezo speaker to 'view' and 'hear' the radioactivity levels, respectively. When the beeps start to irritate you can switch them off with the Mute button.
The kit supports several common Geiger-Müller tubes incliding the SI-3BG, SI-1G, and SBM-20; the high-voltage (HV) supply can be adjusted from about 300 VDC to 600 VDC.
MightyOhm Geiger Counter extension optionsHeaders are available for serial communication (3.3 V signal levels), in-circuit serial programming (ICSP) of the AVR microcontroller, and pulse output (to connect the device to something else). The serial port can be used for data logging purposes (9600n81) and outputs once a second a list of comma-separated values containing (CSV) Counts Per Second (CPS), Counts Per Minute (CPM), and equivalent dose in microsieverts per hour (µSv/h). Normal exposure is less than 0.3 µSv/h.
Contents of the box
- Anti-static bag with printed circuit board (PCB) and electronic parts
- Geiger tube
- 2 pcs AAA battery
- Bag with enclosure and mechanical parts, and a pushbutton with a long shaft
- “Join the resistance” sticker
Building the MightyOhm Geiger CounterGo to http://mightyohm.com/geiger to find the assembly and usage instructions. Here you can also find links to the hardware design files in Eagle format and the source code of the software.
Assembling the circuit board is straightforward if you work from low to tall, i.e. first mount the flat resistors and work your way up to the tall capacitors.
The Geiger tube is polarizedBecause the board is suitable for several types of Geiger-Müller tubes, check your tube with the board to find out which support, to mount for the tube, besides J1. For me it was J2. Note that the tube is polarized, so take care to insert the side marked with a ‘+’ into the connector also marked with a ‘+’ (J1).
For S2 you should mount the long-shafted pushbutton if you plan to use the case that is included in the kit.
First power-onBefore trying the counter, it's best to turn VR1 counterclockwise all the way. After inserting the batteries and sliding S1 to ON, I immediately got beeps and flashes. If you don’t get any, you are either in a lead-cladded room or the sensitivity needs adjusting. Turn VR1 to do so. If you don’t get any clicks at all, you probably did something wrong. Check your soldering and component placements and orientations.
Trying out the MightyOhm Geiger CounterTesting the counter is a bit of a problem as you need a source of radiation. There are some more or less common household things that can be used for this, but I didn’t have any of those handy. However, what I did have was a bunch of decommissioned uranium mines not so far away. So, I jumped in the car and after a 45-minute drive through the countryside I was at the spot that was marked on a map that I had found on the Internet.
Looking for nuclear waste around an abandoned uranium mineMy choice of the mine turned out to be a lucky one, as was the timing of my visit because I dropped right in the middle of a meeting between angry locals, mining company representatives and environmentalists. The subject of the get together was a ditch through which contaminated waste water had spilled many years ago and never got cleaned. Not only did these kind people point me to the radioactive ditch, they also furnished some reference values to compare my Geiger-Müller counter against.
It all worked perfectly. I had the counter connected to my laptop computer to log the data. It was easy to follow the stream of nuclear waste. The observed values corresponded to what the environmentalists had told me: around four to five times higher than the background-level radiation. Such levels make it qualify as nuclear waste.
Keep it in a plastic bagThe environmentalists also shared a handy tip: avoid getting radioactive dust on your counter as it will result in incorrect readings, and cleaning the tube is not easy. Therefore, always keep the Geiger-Müller counter inside a plastic (zip, freezer) bag. The plastic bag does not hinder the readings in any way and can be replaced easily.
ConclusionThe MightyOhm Geiger Counter is an easy-to-build kit that quite accurately detects radiation. Its logging capabilities make it a very usable instrument. The Mute option is appreciated when sitting next to the device. Always keep it in a plastic bag to avoid contamination of the tube.
Battery test: the device ran one full week on the two included AAA batteries with buzzer enabled and with a radiation level of about 0.15 µSv/h.