PCB Submitted

Since the last post, there has been a change of plans. I am no longer going to make this a crank powered Geiger counter for sake of simplicity and reproducibility. I pulled the crank assembly from a cheap flashlight I bought online and took apart, so it’s not an option if this were ever to go into production. I was initially thinking of making two versions, but one is much easier, and of course, the design is much smaller without the crank to consider.

The PCB is now complete and submitted to BatchPCB for creation, which should be done by the end of the month. I realized that I made one small error; I forgot the 10Ω resistor in between the timer compare output on the MCU and the gate of the MOSFET. I’m not sure why it was needed, but without it the MCU would have serious issues, and it may be an isolated case in how the circuit behaves on the breadboard and with the ATtiny2313, which I think has sustained some damage in my rough testing.

Also, recently, I’ve managed to kill the Geiger tube–the most important and expensive part of the project. I have no idea why, and I’m not sure what could have even caused it. It’s a high voltage device by nature, and the HV circuit can’t put out more than ~475VDC due to the 5 series Zener diodes. It’s possible that too much current flowed through the Geiger tube at one point in time, but otherwise I am not sure. I emailed LND, the manufacturer, and asked if there was any way to test the Geiger tube out of circuit, and I then contacted Electronics Goldmine to see if I could have it replaced. Unfortunately I was on day 30 of the 30 day return period. If only I had discovered this a day or two earlier and taken action I might have been able to get a replacement. So, another Geiger tube is on order, and another $67 is pumped into more electronics I don’t really need.

In other news, my new favorite microcontroller is the ATtiny44A. It costs $1.01 in quantity of 25+ and is one of the most full featured AVRs I’ve seen thus far: 4kB flash, 256B SRAM + EEPROM, 8x 10b ADC, 8b and 16b timers, with 2 compare outputs each, on chip temperature sensor (though I hear it’s inaccurate for general purpose use), speed from 0-20MHz down to 1.8v (4MHz), and it has 11 I/O (not counting /RESET) in the very small SOIC14 package (only about twice as long as the ATtiny13A that I’ve previously used, and the same width).

A couple things I’ve tested on the programming side are the menu system, dual timer compare outputs and dual ADC inputs, the watchdog timer and multiple interrupt service routines, and using the ADC against the 1.1v internal reference and a voltage divider to measure VCC. Now I’ll be able to use the much more accurate watchdog timer for calculating counts per minute (though in testing, the watchdog timer appears to be off several seconds per minute).

Here is a video of the preliminary menu in operation (I had to cut out some features because the ATtiny2313 ran out of SRAM):

Finished the crank assembly

Tonight, I just finished modeling the crank assembly in SolidWorks. It’s fully constrained, so the gears will move at the correct speeds if you move the crank arm in the program. This is the second of three models I’ll need before I can start figuring out how big to make the product enclosure.

Assembly Drawing and Crank