You already know that I'm building a theremin. I'm now in my finals period so I have no time to dedicate to the theremin. I panelized another design in the same PCB to fill a 10x10cm square, and that design was a SI-3BG based geiger counter I quickly made it to fill the leftover board area. As this is a pretty simple project it took me less than 2 spare-ish hours to assemble it and code something to make it somewhat work. I copied the PSU design from mightyohm and I don't particularly like it. I'm using a PIC16F1508 as the microcontroller (just because I had it on hand) and it has a configurable logic cell. Maybe in a future revision (if there's any) I could just use the same microcontroller to control the PSU, and get advantage of the CLC to implement some sort of spread-spectrum technique or to use a multi-stage voltage multiplier.The PSU has to step up a single li-ion cell to about 400V. Mightyohm's PSU does this in a single boost stage with fixed duty PWM, and as the boost factor is pretty large it uses a very large inductor. Using that scheme with such a coil makes it beep at audible frequency and that bothers me a bit.
Anyways, I used a PIC16F1508. Timer 1 uses a secondary xtal oscillator at 32,768kHz as clock source. Every 2 seconds the timer overflows. Output pulse from the geiger tube is connected to the external interupt pin. When there's a high to low transition a count is registered. Every 2 seconds the number of counts is stored into a circular buffer and the thing spits the counts per minute value out of the serial port every 2 seconds. At every count it also generates 2ms worth of 4kHz cycles with the PWM module to drive a buzzer and make a "clack" sound and makes a LED flash for the same amount of time.
I also threw in a FT232RL to get USB capability. The whole thing is powered by a single 1/2AA lithium battery also because I had the batteries and some holders on hand. I was too lazy to include a lithium charger so it could be recharged from the USB, and I also was too lazy to implement a mechanism to power the thing via USB. In fact there's no battery monitoring/protection mechanism whatosever and the PIC can't turn off the tube's PSU. I rely on the fact that the power consumption and the currents involved aren't high and on the PIC's BOR which trips at 2,7V. Safe voltage for Li-ions. Again, this was a very quick design and I was lazy.
Here's a picture of the device:
Schematics and board can be found on my theremin post. Code shall be posted when I add some more functionality. Questions might be answered and more information could be given if requested.