Making a prefix - Geiger counter for iPhone in 2 hours

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About a year and a half ago, on several network resources, including the Habr, the Do-ra project, an iPhone prefix, began to be promoted, which allows you to measure radiation background and do a lot of tasty things based on information received from a Geiger counter. The news articles of the project mention several multi-million grants allocated for the development of the instrument by the Skolkovo Foundation. Months passed, “Dora” didn’t succeed, buyers were waiting, competitors were not asleep. Is the Dora so complicated how to paint it and how to assemble ten times more sensitive analogue from the parts at hand I will tell those who click on


So let's get started. More recently, I learned about the excellent (and also free!) GeigerBot program that processes pulses from ionizing radiation detectors coming to the iPhone or iPad microphone input and has a nice feature: with a certain combination of settings, a 20 kHz sine signal is reproduced through the headphone output. The combination of settings necessary for this is as follows: in ClickifyLab, all controls should be at maximum, Echo Filter is turned on, and the function of Clickify clicks is turned off. Having made the appropriate settings, I was convinced with the help of a 3.5 mm plug and an oscilloscope that the signal really appears and its peak-to-peak amplitude at maximum volume is about 1.3 volts. At this point, there was no doubt that in an hour this signal would be used aftera small transformation to power the Geiger counter, the pulses from which will be directed to the microphone input.
The counter was taken popular - SBM-20. To supply it, 400 volts of direct current are needed, they can be obtained in a standard way using a transformer having a high transformation ratio and a rectifier. A very high transformation ratio for transformers supplying cold cathode fluorescent lamps in the backlight of monitors. I got a backlight board from already-I don’t remember what, containing the SGE2687-1 transformer (any similar one, hundreds of types will do) with a transformation ratio of about 150. A little bit, but I didn’t have another and the lack of voltage was made up by diode doublers. We take the breadboard and begin to assemble the circuit.



The circuit turned out to be very simple: a transformer, two voltage doublers, a 390 volt varistor as a zener diode and a transistor for increasing the duration of pulses coming from a Geiger counter to digestible values ​​for the iPhone audio ADC. With serviceable parts and proper installation, it will start working immediately, the ratings of most parts can be changed over a wide range without affecting the performance of the entire circuit. Insert the connector into the iPhone and launch the GeigerBot application. We carefully control the voltage at the varistor with a high-resistance (at least 100 MΩ) voltmeter or tester with additional resistance, it should be about 400 volts. We make sure that the type of counter SBM-20 is selected in the GeigerBot settings and observe the number of recorded pulses. With a natural radiation background (0.1-0. 15 μSv \ h) pulses will follow at an average speed of 20-30 per minute. With a large cable length from the connector to the circuit, mutual influence of a relatively powerful output signal with a frequency of 20 kHz on the microphone input is possible, this can manifest itself in the form of a huge speed of pulse recording - several thousand per second. To attenuate this effect, two separate earth wires are used - for the supply and signal circuits. In case of such problems, in the GeigerBot settings it is necessary to increase the threshold for the amplitude of the pulses (Settings - Geiger Counter - Custom GM tube - I / O Settings - Volume threshold set to 10000 or so). this can manifest itself in the form of a huge speed of registration of pulses - several thousand per second. To attenuate this effect, two separate earth wires are used - for the supply and signal circuits. In case of such problems, in the GeigerBot settings it is necessary to increase the threshold for the amplitude of pulses (Settings - Geiger Counter - Custom GM tube - I / O Settings - Volume threshold to set 10000 or so). this can manifest itself in the form of a huge speed of registration of pulses - several thousand per second. To attenuate this effect, two separate earth wires are used - for the supply and signal circuits. In case of such problems, in the GeigerBot settings it is necessary to increase the threshold for the amplitude of pulses (Settings - Geiger Counter - Custom GM tube - I / O Settings - Volume threshold to set 10000 or so).
Here is a short video showing the operation of the device.



The twenty-fifth second shows the reaction of the counter to a salt shaker made in the USA in the forties of the last century and covered with uranium glaze, and the thirty-fifth second shows the shape of the pulses at the microphone input of the iPhone.
That’s all, that’s almost all. To give our detector-prefix a finished look, take a small segment of a suitable tube, put everything we soldered in there, remember to insulate the parts of the circuit from each other and seal it with hot-melt adhesive on the ends. Now that's it, you can go to Pripyat: warned - means armed.

Thanks for attention. Good luck to everyone in technical creativity and good environmental conditions!