Rather than continue to trickle this information out slowly, as I have been doing, I'm going to skip to the good stuff and hand over the schematics that made up the final version of STAR.
NOTE: Without some of the intermediate information that I have not yet posted, your success with this design may vary. I cannot stress enough how difficult it is to shepherd a radiation counter through the low temperature and low pressure regions of the atmosphere.
ALSO NOTE: The schematics and breadboard layout reproduced below could contain errors and do not reflect last minute changes I made, so I suggest you do not follow them blindly.
In total, there were six major revisions of the Geiger circuit and then five minor revisions of the feature frozen circuit during the last month of development.
Revision 6.5
Observe the Wellesley College "W" in an empty section of the breadboard. It has no function in the circuit and may be deleted. I had a tradition of placing one somewhere on all my prototype circuits.
The (Almost) Finished Product
For the final prototype, I only soldered the header pins to the Pi Zero that would be connected to my Geiger circuit and other sensors.
I did this to reduce the chance of making an error while hooking everything up for final testing.
I also saw no reason to have unconnected header pins, which are essentially tiny lightning rods, sticking up from the Pi Zero. I felt these might become focal points for atmospheric static charge or even arcing. These could destroy the Pi Zero during flight or force it to reboot at an inconvenient time.
The Pi Zero is extremely sensitive to tiny electrostatic discharges. I killed one by accident during prototyping despite careful handling. Therefore, I was taking no chances.
This version very nearly flew, but I made one last change three days prior to launch. I will detail that drama in a future post.
Features to note:
I did my best to insulate the high voltage end of the Geiger tube (gold cylinder, left) with shrink tubing. The clip connected to the ground end of the Geiger tube (right) is insulated from the sides of the box but not from curious fingers as it would not be dangerous to touch. (To be truthful, the high voltage end of the Geiger tube would not be dangerous to touch, either, at least not with a dry finger, but I was exercising extreme caution.)
The Geiger tube is on top of nylon standoffs, as high as I could get it away from the Geiger circuit and Pi Zero while still getting the lid of the box on.
All the wires you see are silicone jacketed and rated for 600 V. They are not stiff like solid core hookup wire. They are a lot like working with cooked spaghetti.
The blue square mounted on the black nylon standoff is the MS5607 altimeter module from Parallax. STAR was recording barometric pressure and temperature as well as radiation. To get the MS5607 working with the Pi Zero, I had to write my own C code to drive it. That code can be found in my Github repository. (My code was an adaptation of the C code example Parallax provided in the Downloads section. I adapted their code to use the WiringPiSPI library... then wound up writing my own adaptation of WiringPiSPI to accommodate the read/write delays required by the MS5607. That code is also in my Github repository.)
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