Geiger Bot is an iOS application that allows you use your iPhone or iPad as a sophisticated display for an ordinary Geiger counter. It works with most Geiger counters that make an audible “click” for each event or count that is detected.
Here’s how to get your MightyOhm Geiger Counter working with Geiger Bot. These instructions were tested with an iPhone 4, but should work with other iOS devices (iPads and other versions of the iPhone).
Connecting your MightyOhm Geiger Counter to an iOS device
The documentation that comes with Geiger Bot claims that you can collect data by holding your Geiger counter up to the microphone of your iOS device. I didn’t find this to be the case – the number of counts registered was inconsistent and strongly affected by background noise. A direct connection is definitely preferable, so we need devise a way to connect our Geiger counter to our iOS device.
The easiest way to do this is by using the microphone input. Apple uses a 3.5mm 4-way headset connector that is somewhat unusual (you’re not going to find it at Radio Shack). The easiest way to get your hands on the proper connector is to cannibalize a wired headset. Bonus: You get a short length of nice flexible cable for free!
While you could use a genuine Apple headset for this, the quality of the headset isn’t important (since we’re just going to cut it apart anyway). Use the cheapest headset you can find. This one, by Miztech was $3.99 at my local Fry’s Electronics:
The sales rep actually tried to steer me away from buying such an awful headset (I tested it, the audio quality is horrible), but quickly disappeared after I tried to explain what I planned to do with it.
The pinout of the headset connector is well-documented online, but here’s a quick diagram:
We don’t care about the left and right audio outputs. What we’re interested in is the microphone input and the “common” pin, which we will tie to ground.
Cut the headset wire just before it reaches the strain relief for the microphone and strip away about an inch of the insulation to expose the wires inside. Use your soldering iron to tin each of the wires. It takes a few seconds, but the enamel insulation will dissolve into the solder, exposing the copper wires underneath:
Use your DMM or other continuity tester to determine which wire color corresponds to which pin on the headset connector.
The wire colors vary for different headsets. (Apple uses a completely different scheme for their headsets, and combines the microphone wires in a single twisted pair.) In my case, two samples of the same headset had a different pin assignment (no wonder they didn’t work so well!)
The MightyOhm Geiger Counter outputs a short (100 microsecond) pulse on pin 2 of header J6 (marked PULSE) every time an event is detected. This pulse, which is active high, swings all the way from ground to the full battery voltage of the kit, typically around 3V.
The iPhone microphone input expects a much smaller signal, less than 100mV. (I measured that the microphone input starts clipping around 40mVpp @ 1kHz!)
Before we connect the Geiger kit to the microphone input, we need to reduce pulse signal amplitude to levels that are compatible with our iOS device. This simple circuit will do the trick:
C1 is a ceramic (nonpolarized) capacitor and R1 & R2 are common 1/8W carbon resistors.
I built the circuit on a small piece of protoboard and parts I had in the lab.
…and used hot glue to protect the delicate headset wires. I made sure to provide ample strain relief for the headset cable. When plugged into the Geiger kit, the resulting adapter looks sort of like a diving board hanging off the side:
It’s not pretty, but it works!
Configuring Geiger Bot
If you haven’t already, open the Apple App Store and install the Geiger Bot application.
Launch Geiger Bot and you should see the home screen:
Click on the small gear icon to open the settings menu:
Select Geiger Counter
Select Custom GM Tube…
You can leave the Conversion Factor and Offset alone for now. Select I/O Settings…
I experienced lots of missed counts with Auto Adjust enabled, so against the author’s instructions I suggest turning it OFF. RMS Window and Delay Windows didn’t seem to have much of an effect, but I recommend starting with RMS Window = 1 and Delay Windows = 30. The Volume Threshold is the most critical adjustment, and I recommend setting this to 20000.
Scroll down to see more settings:
The other critical setting is Ultrafast Rates, this needs to be turned ON to get reliable counts with my kit (I think this is because the pulse length is short compared to other kits). Leave the rest of the settings at the default values.
Return to the settings page and click Done to return to the Geiger Bot application.
Make sure your Geiger counter is connected to your iOS device. Turn the Geiger counter on. Within a few seconds, you should start seeing counts register on the display:
If you touch the right arrow under the graph a couple times, you can see a display of the pulse signal as intepreted by Geiger Bot. Each vertical peak is a detection event, blue pulses are registered as counts, while black pulses are ignored:
The red line is the volume threshold, and this should be comfortably below the minimum level of the peaks, or you will occasionally miss pulses. If your display looks similar to the one shown, Geiger Bot is working.
That’s it! For more information about Geiger Bot and how to use its many features, consult the Geiger Bot documentation.
Comments or questions? Post in the support forum!