Project uses vintage soviet Geiger tube (STS-5) to measure intensity of natural background radiation. High voltage (approximately 400V) required by the tube is produced by flyback converter, built with popular MC34063A chip. Pulses generated by the tube are counted by the MC6840 chip, which also generates system "tick" interrupts. I also plan to use one of the timers from MC6840 chip to drive small speaker to produce radiation alarms.
On PCB there is a MOS6551 UART chip with MAX481 transceiver, responsible for RS485 communication. I also added 8255 which serves as general purpose I/O. Some of the lines are wired to DIN-5 socket, to which I plan connect some temperature/humidity/pressure sensor.
Device is also equipped with Compact Flash card. I was able to compile FatFS library with CC65. It takes up a lot of space inside EPROM chip, but it is fully capable to work under such ancient hardware, allowing device to log measurements on fairly modern storage medium.
Of course device needed some reliable source of time to generate timestamps for acquired data. I used popular retro RTC chip - M6242B.