A portable, sensitive, low power, analog Geiger counter

You may already have seen my previous page about a basic Geiger counter. As described there, building and playing with Geiger counters has accompanied me throughout my life as an electronician. I first read about Geiger-Mller counter tubes in an old German book that introduced all sorts of electronic devices, when I was 14 years old or so. When the Chernobyl disaster happened, I bought a small old Geiger tube and built my first counter. Now, quite a few decades older, I'm sort of closing this chapter, by building a highly sensitive and fast-reacting, yet extremely simple and power-efficient analog portable Geiger counter. When it comes to Geiger tubes, size does matter. The larger the tube is, the more counts per minute it will give for a given radiation intensity, simply because of its larger capture area. At normal background radiation levels, a very small Geiger tube might produce only a very few counts per minute, requiring averaging over several minutes to get a meaningful measurement. A large tube will give many more counts per minute, allowing to average it over a much shorter time to get the same quality of measurement. Instead at high radiation levels, smaller tubes are better, because a large tube would give so many counts that it will lose linearity, since too often a count would be missed because the tube is still recovering from the previous count. Since I'm interested only in measuring very low radiation levels, like the background level and weak radiation sources that are at most a few times stronger than the background, I needed a large Geiger tube. I bought a Chinese J306β tube, the glass version, which is rated to give 88 counts per minute at average background radiation, or 8 counts per second at a radiation intensity of 1Sv/h. This tube is nearly 20cm long, and about 18mm in diameter. I took quite some time to decide whether to use a microcontroller to count the pulses and display various processed measurements on an LCD, or choose the classical way and use dumb circuitry and an analog meter. Each method has its own specific advantages: A microcontroller can integrate the counts over a long time, achieving greater accuracy when measuring stable radiation levels, and also measuring the total dose over a long time. Instead an analog meter is far more convenient when scanning an area for radiation sources, because it's so much easier to watch a needle move up, than to read and interpret dancing numbers on a display. It was sort of a tie between the two approaches. The decisive factor, in the end, was that with this large tube I could easily do enough averaging by simply using a basic analog low-pass circuit, a feat that is not really practical when using small tubes. So the rule is: Small tubes used to measure low-level radiation need to be used with digital counters, while large tubes can use analog circuits even at low radiation levels. At high levels, any size of tube can work with analog circuitry. My decision to built an analog Geiger counter was further reaffirmed by the fact that many Chinese digital Geiger counters are widely available. Instead of building my own, it would be more practical and even less expensive to just buy a ready-made one from China. So, to bring any sense into building a Geiger counter at home these days, I went analog. My counter has two ranges. In the low range, full scale is 1Sv/h, while the high range goes up to 10Sv/h. 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 10Sv/h. The averaging time is 4.4 seconds in the low range, and 0.44s in the high range. This change of averaging time combines with the higher count rate obtained at higher radiation intensity, to produce the same degree of needle stability in both ranges. In the high range, the reaction time is almost real-time, allowing very quick scanning of areas, while in the low range it's still fast enough to allow scanning, even if at a slower pace. I included a speaker, so that I can hear a click for each count. But my practical experience is that hearing the clicks is a bit of an overrated feature. When scanning an area, it's hard to clearly notice by ear a moderate increase of the count rate, given the irregular spacing that the clicks of a Geiger counter always have. One really has to look at the meter. Anyway, the clicking sounds cool... While developing the circuit I was in a power-saving mood. I started basing the circuit on the use of a 9V battery, and after some tweaking brought the current consumption down to 5mA. Commenting about this to an electronician friend, he dryly replied that 0.5mA would be better. He was right, of course, so I began working seriously to reduce power consumption. After a few days the consumption was down to 0.21mA at 9V. But then the question popped up: Why use a 9V battery at all? Those batteries are rather expensive for the amount of energy they contain, and are even becoming harder to find! It would be better to use something more cost-effective, such as AA cells. But... cells? In plural? Why on earth? The Geiger tube needs 400V anyway, and converting from 1.5V to 400V is no harder than converting from 9V to 400V. And the rest of the circuit works from 5V, and converting from 9V down to 5V, in an efficient switching regulator, is also no different in complexity than converting up, from 1.5 to 5V! So I modified my circuit to run from a single AA cell, and after just a little basic tweaking the consumption was down to 1.1mA at 1.5V, increasing proportionally as the voltage of the battery goes down during its life span. Both voltage converters stay in regulation until the battery falls below 0.8V. This gives a calculated operation time of well over 2000 hours for a single AA cell, and I think that this is good enough to leave it at that! As usual on my website, you can click this schematic to get a high-resolution version that is good for reading and printing. As you can see, the circuit is really simple! But there is quite a bit of out-of-the-box thinking in it, so a thorough description of its operation is in order. Let's start with the left half of it, which is just the power supply, that creates 5V and 400V from the 1.5V cell. Both voltages are regulated, of course. I cheated a little by using a tiny ready-made step-up converter to create the 5V. This module sells for a fraction of one dollar on AliExpress, but I provided its internal circuit here, so that if you want to copy my Geiger counter but without any ready-made modules, you can use the bare parts instead. The converter is a simple, bare-bones, classical boost circuit, using one IC that does almost the entire job. Only the storage inductor, filter capacitor, and the diode are external. In some other such chips the diode is internal too, often implemented as active rectifier! If you buy the ready-made module, be sure to get the 5V version, because it comes in 5V and 3.3V flavors. The 400V converter contains some of my own creativity. Despite its simplicity, it's a quite efficient pulse frequency modulated flyback converter. High voltage sensing is provided by using common switching diodes in reverse, working as 130V Zener diodes (well, purists will claim that any so-called Zener diode above about 7V is actually an avalanche diode, rather than a true Zener diode, but that's another matter). Pressing common diodes into this use is not my own invention. In fact it's quite common in homebrew Geiger counters, because it provides good voltage stabilization at very low current in the feedback path (1A in my design), and without requiring another voltage reference nor an analog comparator nor operational amplifier. Despite the fact that the 1A in the feedback circuit, at 400V, causes a power waste of 400W, which is a considerable part of the total power consumed by this Geiger counter, it's still more power-efficient than some other methods I tried, that give the same voltage stability. On the other hand, there is some room for improvement there, if I ever decide to take power saving to the extreme. The operation of this 400V supply is the following: IC1 is a six-section CMOS Schmitt trigger inverter, one section of which is used as modulated oscillator and MOSFET driver. When the 5V come up after switching on the circuit, C3 pulls pin 13 of IC1 up, keeping it at logic high, and thus keeping the MOSFET off, hopefully until the voltage has reached 5V. Then R2 slowly pulls pin 13 down, eventually crossing the lower trigger point, making pin 12 driving the MOSFET on. A ramp of battery current will flow through the 60-turn primary winding of the autotransformer, while R2 is gradually pulling pin 13 up, discharging C3. When pin 13's voltage crosses the upper trigger point, the MOSFET is turned off, and the energy accumulated in the airgapped ferrite core discharges through D1 into the high-voltage filter capacitor C2. Then the cycle repeats. We have a basic CMOS inverter oscillator, driving a MOSFET, which drives the flyback autotransformer. When the voltage on C2 reaches the breakdown voltage of the three backwards-connected series 1N4148 diodes, roughly 390V, they begin pulling up pin 13, through R1. So it begins to take more time to pull pin 13 down, and less time to pull it up. As a result the pauses between MOSFET conduction pulses become longer, and the pulse duration becomes shorter. If the voltage on C2 is enough so that D2-4 and R1 can keep the voltage at pin 13 above the upper threshold while R2 is trying to pull it down, pulsing ceases completely, until the high voltage comes down a little. The operation mode that results is that after turn-on the circuit pulses continuously, at about 8kHz, until C2 reaches 400V, and then reverts to a pulse rate of just a few tens of Hz, with a pulse duration of around 50s, keeping the voltage in regulation. After every ionization event of the Geiger tube, the pulse rate goes up