Round Nixie Watch - Part Five

The insides still had to be worked out, although I was working on both the PC boards and the case at the same time, as one has to do with such a design. I had arrived at the logic design of using a MAX6931 20-bit HV driver chip to run the cathodes. The processor chip could now be smaller, since that chip needs only four signals to load it with data. 

I selected a PIC16F819 processor for its SPI port and small 20-pin TSSOP package. It also would work fine with 3V and a 32KHz crystal. Besides, the code was already written for a PIC. With these two small chips the entire clock circuit, minus the power supply, fit between the tube sockets. Crazy small! The round case allowed me to put the tilt sensor in the space below the tube socket board, on its own little board perpendicular to both the tube socket and base boards. 

The shelf that holds the switches and programming connector came about pretty early on with the round case. It was apparent that there was room for a little board there, and I needed somewhere to put switches since I didn’t like the reed switches - they require one to find a magnet. The switch board is soldered to the nixie tube socket pins, and held up by the tilt sensor board underneath it. There were several iterations of the little connector strips that attach the board to each other, as I shuffled parts around to make everything fit. 

The battery holder ended up working out quite well. I wanted to use the socket board to connect to one end of the battery, as that was a freebie. The other end needed something to push on the battery and connect to the negative end. I had first planned on using a spring mounted to the case. Then I found that Keystone made a right-angle spring bracket. It happened to be just the right size for the battery I was using, and its third leg could be cut off to allow it to fit in the round case. I made the cut-off portion bear its pressure against the case side, so that the thin PC board wouldn’t suffer so much strain.