Building a Geiger counter requires three core components: a gas-filled detector tube, a high-voltage power supply (typically 400V), and a pulse-counting circuit. The project is achievable for someone with intermediate electronics experience, but the high voltage involved demands genuine respect and careful assembly. Here’s how each piece works and fits together.
How a Geiger Tube Detects Radiation
The heart of any Geiger counter is the Geiger-Müller (GM) tube, a sealed metal cylinder filled with an inert gas like neon or helium mixed with a halogen quenching gas. A thin wire runs down the center as the anode, and the tube wall serves as the cathode. When you apply several hundred volts between them, the gas inside becomes extremely sensitive to incoming radiation.
When a gamma ray or beta particle enters the tube, it knocks electrons free from gas molecules. Those freed electrons accelerate toward the central wire under the strong electric field, gaining enough energy to knock more electrons loose from other gas molecules along the way. Those secondary electrons do the same, creating an avalanche effect. Excited atoms in the cascade also release photons that trigger additional avalanches nearby. Within a few microseconds, this chain reaction produces a brief but measurable pulse of current, which is your “click.” The quenching gas stops the discharge so the tube resets and is ready for the next particle.
Choosing a Detector Tube
The most popular tube for DIY builds is the Russian-made SBM-20, widely available from electronics surplus sellers for $10 to $30. It detects beta particles and gamma rays, which covers the most common types of environmental radiation. The SBM-20 has a recommended operating voltage of 400V, with a usable range of 350 to 475V. That 100-volt-plus plateau means you don’t need pinpoint voltage precision for the tube to work reliably.
Other options include the smaller SBM-21 (for compact builds) and the LND-712, a U.S.-made tube with well-documented specifications. Whichever tube you choose, look up its datasheet for the recommended operating voltage, since your entire power supply design depends on hitting that number.
Building the High-Voltage Power Supply
This is the trickiest part of the build. You need to convert a low-voltage source (a 9V battery or 5V USB supply) up to 400V or more. The standard approach uses a boost converter circuit built around a 555 timer IC, an inductor, a switching transistor, and a voltage multiplier or rectifier stage.
The 555 timer generates a square wave that rapidly switches current through an inductor (commonly 10mH). Each time the current cuts off, the inductor’s collapsing magnetic field produces a high-voltage spike. A diode captures these spikes, and a capacitor stores the accumulated charge. Some designs chain multiple stages together in a Cockcroft-Walton voltage multiplier to reach the target voltage.
A feedback loop is essential. Without one, the voltage can climb well past your tube’s safe range or sag under load. A simple voltage divider feeds a sample of the output back to the 555’s control pin or reset pin, shutting off the oscillator when voltage reaches the setpoint and restarting it when voltage drops below. This keeps the output stable around 400V without constant adjustment. Poorly designed feedback is the most common failure point in DIY builds. If your output starts at the right voltage but drops to half within a minute, the feedback circuit isn’t regulating properly.
The Pulse Detection Circuit
Each time the GM tube fires, it produces a brief current pulse. You need to convert that into something useful. The simplest output is an audio click: run the pulse through a coupling capacitor into a small piezo speaker, and you get the classic Geiger counter sound.
For digital counting, an Arduino or similar microcontroller works well. Connect the tube’s output (through a voltage divider and protection resistor to bring the signal down to safe logic levels) to a GPIO pin configured as an interrupt. Each pulse triggers the interrupt, incrementing a counter variable. Sample the count every 10 seconds and multiply by six to estimate counts per minute (CPM), or accumulate over a full 60 seconds for a more accurate reading. A small OLED or LCD display can show CPM in real time.
The conversion from CPM to a dose rate in microsieverts per hour depends on your specific tube. Tube datasheets list a conversion factor. For the SBM-20, roughly 150 CPM corresponds to about 1 microsievert per hour, though this varies with the type and energy of radiation being measured.
Putting It All Together
A typical DIY Geiger counter fits on a single printed circuit board or can be assembled on perfboard. The layout matters more than usual because you’re running 400V in close proximity to low-voltage logic. Keep the high-voltage section physically separated from the microcontroller and display. Use adequate spacing between high-voltage traces (at least 1mm per 100V as a rough guideline) and avoid running them parallel to signal lines.
The GM tube mounts externally or in a dedicated compartment, connected to the high-voltage supply through a high-value load resistor (typically 1 to 10 megaohms). This resistor limits current through the tube and helps shape the output pulse. The tube’s anode connects to the positive high voltage, and the cathode connects to ground through the load resistor. The pulse signal is taken from the cathode side.
Power consumption is low. A GM tube draws almost no current between pulses, and the boost converter only needs to maintain a charge on a small capacitor. A standard 9V battery can run a Geiger counter for dozens of hours.
Safety When Working With 400V
Four hundred volts is enough to cause a serious or fatal shock. The current capacity of a small boost converter is low, which reduces (but does not eliminate) the danger. The real hazard comes from charged capacitors that hold their voltage long after you switch off the power.
Before touching any part of the circuit after it has been powered on, discharge all capacitors at least twice using a grounding rod or a resistor across the terminals. Verify with a voltmeter that the charge is gone. When probing a live circuit, keep one hand in your pocket. This old electrician’s rule prevents current from traveling across your chest through both arms. If you need to touch anything, use the back of your hand first, so an involuntary muscle contraction pulls you away from the contact rather than clamping your hand around it.
Work on a dry, non-conductive surface. Never probe or solder with the circuit powered unless absolutely necessary, and clip your meter’s ground lead so you only need one hand to take measurements.
Testing and Verifying Your Build
Once assembled, turn it on and listen. You should hear random clicks almost immediately, even without a radiation source nearby. Background radiation from the sun, trace uranium in soil, radon gas, and even the potassium in nearby foods produces a constant low-level signal. Normal background readings fall between 5 and 60 CPM depending on your elevation, local geology, and the sensitivity of your tube.
If you’re getting zero counts, check your high-voltage output first. If it’s below your tube’s minimum operating voltage, the gas avalanche can’t sustain. If you’re getting a continuous buzz instead of individual clicks, the voltage may be too high or the quenching circuit isn’t working, causing the tube to fire continuously.
To confirm the counter responds to elevated radiation, you need a check source. Inexpensive check sources (small discs containing a tiny amount of a radioactive isotope) are available online for around $10. These aren’t precision-calibrated instruments, but they produce enough radiation to verify your detector is working and responding proportionally. Some hobbyists use uranium-glazed vintage ceramic (often called Fiestaware), thorium-containing camping lantern mantles, or potassium chloride salt substitute, all of which produce readings noticeably above background.
A properly built Geiger counter won’t match the accuracy of a professionally calibrated instrument. But for detecting the presence of radiation, monitoring relative changes, and understanding how radioactive decay works in practice, a DIY build is both functional and deeply educational.