The hardware:
View of major components.
Geiger Tube (bottom center):
LND-712 was chosen because it is a very sensitive to alpha radiation. This means it provides the highest # of counts for a source with given level of activity. Yes the wiring is jury rigged at the moment. Lead wires could simply be soldered on but I have some ideas on how to improve that. The entire tube body is the cathode so I would like to find a pair of through hole clips which could be attached to a board and the LND-712 could simply clip into place (note the two recessed rings on the tube body). This design has an adjust high voltage supply so it could work with just about any tube. Really the consideration of tube will come down to the entropy source (alpha or beta) and the desired count rate. Much cheaper tubes can be used but it would mean either a higher activity source or using a beta emitter.
Entropy Source (bottom left):
0.9 uCi of Am-241 I liberated from a $4 smoke detector. With the current design and tube this produces ~1,500 to 1,600 cps. Using 2 counts per bit gives a raw output of up to 800 bps. To the right of the LND-712 is a pill bottle I am using as storage container for the entropy source. Am-241 (in this tiny amount) is relatively harmless however one should avoid ingesting it so if you have kids or pets around secure the entropy source when not supervised. I am still brainstorming the final design but I am considering a "cap" which would fit over the end of the LND-712 and hold the entropy source in place.
Microcontroller (right):
I am using a Netduiono 2 Plus development board because it was the MCU I had, and I like the debug/IDE tool (visual studio). It is pretty much overkill for this project. Just about any MCU would work although some might require external SRAM and/or USB connectivity.
Breadboard of HV (upper left):
Close up in next photo
Everything north of row 15 is the HV power supply. Geiger tubes need to be energized to 400V to 700V with the exact voltage depending on the model. The LND-712 has an optimal operating voltage of 500V. The IC in the upper center is the classic 555 Timer which is used to handle PWM needed to step up 5V (or 3.3V) VIN to the HV required for the tube. The blue variable resistor on the right side allows the HV output to be tuned. This brings me to my first unresolved challenge. Accurately measuring the HV output. My meter input impedance is far too low (~10M Ohm) to accurately measure the HV circuit (~1000M Ohm). I used some 10 M Ohm resistors in series to improve the measuring accuracy but really I need one monster resistor (1 G Ohm) but those are expensive. Open to suggestions on measuring the HV output.
The two blue caps (after the HV diode at row 15) store the HV output so it is available to energize the tube between events. The resistor right before the screw terminals is anode resistor to limit current to the tube. The LND-712 recommends 10M Ohm (other tubes will vary).
The circuit after the tube is recommended by LND to cleanup the output. The pulses from output switch the transistor which combined with pull down resistor produce a nice clean VREF on event and ground between events for the MCU.
