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Posts from: March '12


Geiger counter + iPad


Two months ago, I purchased a geiger counter kit from MightyOhm. The assembly was easy, and the tool works perfectly at its intended purpose. However, I wanted to export the radiation data to a higher-level device, e.g. an Android phone, thus making the geiger counter purely a "sensor", that the higher-level device uses. This enables you to get exact radioactivity readings (in µSv if you wish). Also, you can accumulate data for a longer time window, enabling you to detect if one place is just slightly more radioactive than another (which is very hard, if not impossible, to do by ear). You can also get GPS-matched logging, graphing and whatnot.

So the first idea was to solder a small bluetooth module to the free pins of the microcontroller. However, I subsequently went into a different direction, prompted by an iOS app I saw on the internet - Geiger Bot. It supports any geiger counter it is able to "hear", either from the built-in mic, or by using the microphone input. So I decided to use the PULSE output header of the geiger counter kit and connect it with an "audio" cable to the higher-level device's audio port. It turns out that the headphone socket of the iPhone/iPad is really a 4-connector TRRS one, so while it accepts standard 3.5mm stereo jacks, it can also take a 4-connector one like the one below:

Image

The other end of the cable is an ordinary RCA ("cinch") connector, since I don't need more than one channel. The figure also shows what the connections should be like.

Then I removed the PULSE header from the geiger kit's PCB and soldered two wires, that lead to the female RCA socket:

Image

From left to right, the PULSE header's pads are V+ (3 V), signal, and ground.

The pulses from the geiger counter are a bit high, they have to be converted to around 0.1V, using a 30:1 voltage divider. The schematic:

Image
(This explanation is courtesy of comment I found somewhere around the Geiger bot's webpage)

I installed the RCA socket into a hole I drilled in the transparent plastic. As I don't own a dremel, the drilling was done in the following way: take a paperclip and unfold it into a "L"-like shape. Then heat the short part to a candle flame, until it glows red. While it's hot, quickly press it to the plastic, melting a small hole in it. The hole is later enlared by putting a small screwdriver inside it and rotating it until it widens the hole enough. The hole will probably still be too small when you're done with that screwdriver, so next you get a larger one, etc. In the end you get a very beautiful, round hole, and nobody would even suspect the medieval method you've used :)

Anyways - the RCA socket is now installed, you need to wire up that resistor divider, and the "audio interface" is ready:

Image

Image

You don't need to build a special connector cable; you can find a TRRS-to-3-RCA cable in most camera stores, as they're usually used for the TV out function of digital cameras.

Then we connect the devices, fire up the Geiger bot, and we have a high-level Geiger counter, with all the featues you could've wanted!
Here's it in action:


Image
Youtube


I'm testing it with the same rock as in the previous video.

Just a quick note, if you're dealing with radioactive sources like the one in the video: it seems that Geiger bot is only optimized for low radiation levels, and the default settings impose a limit of around 50-60 CPS and don't detect higher levels. This is more than enough for background radiation sources, but that stone actually emits around 250 CPS (and the geiger counter can actually supply up to 10 000)! So, you have to go in settings, and select Custom GM Tube..., and then Audio IO settings and decrease the RMS window/delay windows to the minimum. These settings allow it to show the actual radiation of this piece.

Now, the only downside is that I still want to have this on my phone, and I don't have or want an iPhone. So I'm thinking about porting this app to Android (probably somewhat simplified, I'm not promising a complete port!).


Posted in category Hardware -- clock 23 Mar 2012, 13:03, 0 comments

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