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RS232 Geiger Logger

RS-232 Radiation Logger

17.03.10



Geiger Counter connected to PC, displaying counts/min from a pitchblende sample.

The first Geiger counter I built was little more than a toy in terms of practical use, due to it's very limited sensitivity. It also lacked any indication of radiation level beyond your own ability to determine the repetition of pulses, which for my part only allowed me to tell if there was a little radiation, some, or lots. So Stella was so kind to donate a few Czech Geiger tubes from the cold war. They were made quickly some time in the 60's, most likely for a cheap, mass produced Geiger counter, which makes it amazing that some of them still work to this day. The Geiger tubes use an an organic quench gas (methanol or isobutane), and as such require a higher operating voltage. These tubes have a much larger area for ionizing particles to hit than the tiny SBM-21, and thinner walls so fewer will be stopped before they can ionize the chamber. The thinner wall and larger area makes the tube much more suitable for detecting X-rays, which is the primary use of my Geiger counter.

organic gas geiger tube

I had purchased some small 8x2 character LCD displays, and combined with a microcontroller the counter could display all sorts of data based on the counts from the Geiger tube. Not only that but the raw count data could be sent directly over a serial connection to a PC, thus allowing for remote monitoring of radiation levels. The count values could then be analyzed later using and graphed using a spreadsheet program. With this goal in mind I started constructing my next Geiger counter.

schematic  schematic_HV
Nothing new or clever with the circuit design, except for the discrete RS-232 level converter perhaps. I only did that because I didn't have any MAX232 chips at hand, and communications are one-way. Feel free to use a MAX232 instead. The high voltage supply for the Geiger tube is another story. I needed something small, quiet, and efficient to provide a clean, regulated 1100V for the tube. This proved to be the biggest headache with the entire project, and took several weeks to perfect, probably because of my lacking insight in flyback transformer design. The basic schematic was determined fairly quickly, essentially being the same type I used for my first Geiger counter. PCB, firmware, schematics and PC program can be downloaded here.

The transformer was the difficult part, as many would work, but they would give either too little voltage, or draw too much power. I had to play with several different core types and windings until I found a design that worked alright. Quite simply I had to use as many turns as possible. I would first wind several layers of thin wire for the secondary, and then stuff as many primary and feedback windings in as possible. I believe the winding inductance is the most important figure here, so they've been marked on the schematic, not turns. (I didn't keep track of them anyway) The core type I used is unknown, but came from one of those "base-drive-transformers" used in TVs and monitors for the horizontal output transistor. The main drive transistor was also a sore point, and had to be just the right kind in order not to dissipate too much power, or for the output voltage to droop with time. None of this was done empirically, so I can't give any more pointers than that. I found my transformer was very noisy, to remedy this I dipped it in hot wax a few times to seal the windings. This damped the noise almost entirely. If you want to build this project, be sure you can construct a high voltage supply for the tube first, as this is the hardest part of the project.



Application screenshot The firmware for the ATtiny2313 uses the T0 input as a clock source for Timer0, which simply fills up a 16-bit register. Timer1 is used to generate an interrupt every second, which then sums up the value from Timer0 before resetting it. The number of captured events during the last second are now stored. This number is then sent through the USART module to a PC, using a simple serial protocol of three start bytes a 255, then data MSB and LSB, and finally an end byte of 128. After shipping off the count value, the data can then be processed depending on the state the counter is in. If in accumulated mode, the counts are simply summed to indicate the total number of events since entering accumulated mode. If in "cnt/min" or "counts per minute" mode the number of events during the last second are stored in an array consisting of ten variables. The entire array is then summed up, and multiplied with 60 to give a counts/min projection based on the last ten seconds.

 Another fun part of the project was the computer software to go along with it. I've already made several serial port programs in Visual Basic, so this wasn't too difficult. I did bump into one problem though, which delayed this project by three months. The "SerialPort1_DataReceived" function I used to see when new bytes were received would only update once every three seconds, meaning for every three count values I would only get one! This caused me to shelf the project for several months, and it wasn't until I looked at it again this last weekend that I figured it out. In the end I had to use a loop to check if there were still bytes in the read buffer. Just a heads up in case someone else runs into the same problem some day. The received count values are stored in a text file of your choice, and are formatted so they can be imported into any spreadsheet program worth it's salt.




Here I've imported the count values using OpenOffice and graphed them. First several radium watch hands are placed under the tube, then two americium buttons from some smoke detectors, and finally a uraninite (pitchblende) sample. It's hard to see, but even the americium give about ten times background radiation. All samples were measured at four centimeters distance.
Radiation graph

And finally a quick little demo:




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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.