An analog Geiger counter build by Hackaday contributor [ludens] proves that a microcontroller and a colour display are nowhere near compulsory for measuring ionising radiation. Using classic CMOS logic, a stepped-up AA battery supply, and a proper analog dial, the project does everything a modern digital counter can do at background levels, and does it with considerably more character.
Choosing the Right Tube for an Analog Geiger Counter Build
The centrepiece of the design is a Chinese J306β Geiger-Müller tube, selected specifically for its sensitivity to low-level sources. At average terrestrial background radiation, the tube is rated to output 88 counts per minute, or 8 counts per second when exposed to 1 µSv/h. That is a generous count rate by any measure, and it is what makes the purely analog approach workable. When a tube produces that many pulses even at modest dose rates, you have enough signal to smooth out with a simple low-pass filter rather than needing a microcontroller to accumulate and process a sparse stream of events.
The tube itself is a substantial component: 200 mm long and 18 mm in diameter, it sits proud on top of the enclosure like an old broadcast antenna. There is something honest about that. You can see exactly what is doing the detecting, which is more than you can say for a chip buried under a PCB silkscreen.
Power Supply and Circuit Design
Running the whole instrument from a single AA cell is a neat trick that requires a two-stage power supply. The first stage steps the battery voltage up to 5 V to run the CMOS integrated circuits. The second stage continues boosting to 400 V for the tube’s anode, which is the operating voltage a Geiger-Müller tube needs to avalanche-discharge on each ionising particle. Both rails come from that one cell, keeping the build compact and genuinely portable.
CMOS Schmitt triggers handle the pulse shaping and the analog low-pass averaging, replacing what would normally be a microcontroller’s job in a contemporary design. The output drives an old-school analog meter movement, which gives the instrument its signature needle-deflection readout. There are two measurement ranges: 1 µSv/h for sensitive low-level work, and 10 µSv/h for sources that are somewhat more active. [ludens] is candid about why no higher range was included. ‘If anything I find pegs that scale, I prefer to run, instead of measuring exactly how much radiation there is! So I don’t need a higher scale than 10 µSv/h.’
It is a remark that will resonate with anyone who has spent time around radiation measurement. The International Atomic Energy Agency and bodies such as NIST publish guidance on background dose rates, and the global average sits well within the instrument’s lower range, which is precisely the point. Measuring what you are actually likely to encounter day to day, whether it is granite worktops, old luminous watch dials, or a bag of Brazil nuts, is what a sensitive low-range instrument is built for.
The design intentionally sidesteps logging, networking, and data export. Those features have their place, and a digital Geiger counter wired into a home network for continuous background logging is a genuinely useful thing. But [ludens] was not building that. The goal here was a self-contained, battery-powered instrument with an analog readout and enough sensitivity to track the quiet, steady murmur of natural background radiation. The J306β tube, the Schmitt trigger averaging circuit, and the mechanical meter movement are each chosen because they are fit for exactly that purpose, nothing more and nothing less.
If the Cold War aesthetic is what draws you in, the large tube jutting out of the top of the case and the needle swaying gently on its pivot will not disappoint. This is what radiation measurement looked like before firmware updates were a consideration.
