Pen-Sized Geiger Counter Runs for Up to a Year on Battery

Geiger counters are fun, practical circuits to design and build. There are many DIY circuits that you can follow. Most of these utilize a simple oscillator like one designed with a 555 timer. While these circuits are functional, they could be improved by being more energy efficient. This Geiger Counter Pen from Tex is very compact and incredibly energy efficient. Its battery life goal is one year!

SBM-20 Geiger–Müller tubes are surprisingly simple devices. They are filled with an inert gas that creates a brief conducting path when hit by ionizing radiation. One downside to these devices is that they need a bias voltage of around 400 volts.

Tex set out to create that bias voltage as efficiently as possible in a relatively small space and power it from two LR44-style batteries. You can often see this cell type in tools like digital calipers. The secret to efficiency is using feedback to determine if the boost circuit's oscillator should run.

In the Geiger Counter Pen's design, a 4x multiplier circuit generates the high voltage, and a comparator monitors it through a high-resistance feedback network. The unique characteristic of this feedback network is the high impedance: over 1 gigaohm!

This clever circuit design results in a 400 volt boost converter that consumes less than 5.5 microamps! But, of course, that consumption changes. First, it depends on the battery's voltage. The worst case occurs as the batteries reach about 1.8 volts. Second, the tube generates more pulses as ionizing radiation hits it, which requires the boost converter to run more often.

An STMicroelectronics STM8 microcontroller controls the segmented LCD, the boost converter, and the speaker for the characteristic clicks of a Geiger counter. Additionally, the microcontroller calculates the time between pulses from the Geiger tube.

As its name implies, all circuit components fit into a board about the size of a pen or pencil. While that achievement is fantastic on its own, even more surprising is the Geiger Counter Pen's runtime: almost one year! Here, one year means 11 months of actual testing! Thanks to some code optimizations, the entire device only consumes about 19 microamps (with normal background radiation!)

Tex is still working on some features and has ideas to reduce power consumption further. There is also a plan to create an enclosure for the pen. But you can download the current schematic and the STM8 C-Code from the Geiger Counter Pen GitHub repository.

Also, check out these project logs for an incredibly detailed look at each component and how they add up to the Geiger Counter Pen.