My Geiger circuit owes much gratitude to Tom Napier's article "Pocket Geiger Unit" from January, 2004.
I received further inspiration from Jeff Keyzer's website, MightyOhm.
After learning what I could about Geiger circuits from their writing, I sat down and listed everything I thought could go wrong when trying to send a Geiger counter up on a weather balloon. The two biggest factors I could think of were temperature and pressure.
Temperature:
The temperature gets pretty cold at high altitude. At the boundary between the Troposphere and the Stratosphere, there is a temperature minimum of around -70 C. Most electronic components are only rated down to -55 C, and lithium ion batteries prefer much warmer temperatures of 10 C and above. I was going to have to choose my electronics components carefully, matching not only the electrical specifications but the extreme environment. My initial order of components for prototyping included:
● Adafruit Perma-Proto Half-sized Breadboard PCB
● Fastron 11PHC-103K-50 10mH inductor
● LMC555CN/NOPB-ND 555 timer
● FQP30N06L power MOSFET
● 0.01uF 500 Volts 10% ceramic capacitor
● 1/2W 15 Ohms 1% metal film resistor
● 1/2W 4.7 MOhms 1% metal film resistor
● 1/2W 10 Ohms trimmer resistor 25 turn 10%
● Fairchild 2N3904BU NPN transistor
● Fairchild FJN3303FTA NPN power transistor
● 1N4937G 600V 1A fast rectifier diode
Plus a traditional assortment of 1% resistors and some 600 V hookup wire (silicone jacketed).
Pressure:
Geiger tubes need high voltage. They don't need much current, but they generally need anywhere from 400 V to 600 V to work properly. On the surface of the Earth, at one atmosphere (760 Torr), a paltry 600 V isn't going to arc very far. At maximum altitude, the pressure reduces to around 10 Torr or less. Let's consult the Paschen curve for air.
The above graph is in log-log format. The fifth horizontal line from the bottom represents 500 V. In a worst case scenario, the balloon might fly high enough to reach 1 Torr of pressure (1.00E+00). On this graph, that is extremely close to the blue line and thus can be expected to arc to any component within 0.1 inches (0.254 cm).
As the standard hole spacing on breadboards and protoboards is 0.1 inches, it was clear that I would have to either separate high voltage components more than one space apart from anything at lower voltage or insulate. Since I was only getting one chance at this, I did a bit of both and really over engineered it. More about that later.
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