A Portable, Sensitive, Low-Power Analog Geiger Counter (2025)

I have previously shared a page regarding a basic Geiger counter, but my fascination with these devices is a lifelong journey. As an electronics enthusiast, building and experimenting with radiation detectors has been a constant thread in my life.

My introduction to the world of Geiger-Müller tubes happened around age 14, when I stumbled upon an old German textbook detailing various electronic components. Later, in the wake of the Chernobyl disaster, I acquired a vintage Geiger tube and constructed my very first counter.

Now, several decades later, I am effectively closing this chapter of my hobby by creating a device that is highly sensitive and responsive, yet remains elegantly simple and power-efficient: a portable analog Geiger counter.

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☢️ The Science of Tube Selection

The physical size of the Geiger tube is the primary driver of its performance. There is a fundamental trade-off between sensitivity and linearity:

• Large Tubes: These possess a greater capture area, resulting in more counts per minute (CPM) for any given radiation intensity. This is ideal for measuring background radiation, as it reduces the need for long averaging periods to get a stable reading.
• Small Tubes: These are superior for high-radiation environments. A large tube in a high-flux area would suffer from "dead time" (the recovery period after a count), causing it to miss pulses and lose linearity.

Since my goal was to detect very low levels—specifically background radiation and weak sources—I opted for a large tube. I selected a Chinese J306β (glass version).

Tube Specifications:

• Background Radiation: $\approx 88 \text{ CPM}$
• Radiation Intensity of $1\mu\text{Sv/h}$: $\approx 8 \text{ counts per second}$

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🧭 Analog vs. Digital: The Design Philosophy

I spent considerable time debating whether to use a microcontroller (MCU) with an LCD or stick to "dumb" circuitry with an analog needle.

The Verdict: Because I used a large tube, I could achieve sufficient averaging using a simple analog low-pass filter, which wouldn't be feasible with a small tube. Furthermore, since digital counters are now commodities you can buy cheaply online, building an analog version is the only way to make a home project meaningful.

The Rule of Thumb: Small tubes + Low radiation $\rightarrow$ Digital Large tubes + Low radiation $\rightarrow$ Analog Any tube + High radiation $\rightarrow$ Analog

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📈 Performance and Range

The device features two distinct ranges to balance sensitivity and speed.

By reducing the averaging time in the high range, the needle maintains a consistent level of stability regardless of the intensity. If I encounter a source that pegs the $10\mu\text{Sv/h}$ scale, my priority is to measure it accurately run away immediately.

Additional Features:

• Dual-range switching
• Analog needle output
• Audio "click" speaker (though I find this feature overrated, as the irregular timing makes it hard to judge intensity by ear).

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🔋 Power Optimization Journey

My initial design was based on a 9V battery with a current draw of $5\text{mA}$. After sharing this with a fellow engineer, he noted:

"0.5mA would be better."

This critique sparked a redesign. I realized that 9V batteries are expensive and increasingly scarce. I decided to switch to a single AA cell ($1.5\text{V}$).

Old Power Path: 9V $\rightarrow$ 400V / 9V $\rightarrow$ 5V New Power Path: $1.5\text{V} \rightarrow 400\text{V}$ (Tube) and $1.5\text{V} \rightarrow 5\text{V}$ (Circuitry).

Using efficient switching regulators, I brought the consumption down to $1.1\text{mA}$ at $1.5\text{V}$. The system remains stable until the battery drops to $0.8\text{V}$.

Battery Life Calculation: $\text{Operation Time} > 2000 \text{ hours per AA cell}$

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🛠️ Circuit Architecture

The schematic is divided into two main functional blocks. The left side handles the power conversion, while the right side handles the detection and filtering.

Click the image above for the high-resolution printable version.

The power supply utilizes a small, pre-made step-up module to simplify the $1.5\text{V}$ to $5\text{V}$ conversion, while the $400\text{V}$ rail is generated via a custom boost circuit.

// Conceptual logic for the analog averaging
// Time Constant (tau) = R * C
// Low Range: tau = 4.4s
// High Range: tau = 0.44s