The Fnirsi GC-01 is a compact and affordable Geiger counter, great for beginners – but with a few smart upgrades, its performance can be dramatically improved. In this post, I’ll walk you through how I modded my GC-01 with Rad Pro firmware, a SBM-20-1 Geiger tube, and a larger battery. The result: a much more responsive, reliable, and practical radiation detector. Whether you’re into DIY tech or looking to enhance your measurement tools, this mod is worth a look.
Why have a Geiger counter at all?
A Geiger counter is a handy tool for making ionizing radiation visible – in other words, something you can neither see nor smell. Reasons for owning one:
- Measuring radiation in everyday life:
- Many old objects (e.g. clocks with radium dials, uranium glass, smoke detectors) emit small amounts of radiation. A Geiger counter shows what is really active.
- Scientific curiosity / hobby:
- Ideal for technology enthusiasts, collectors and makers to document natural background radiation, found objects or experiments.
- Emergency preparedness:
- In crisis situations (e.g. nuclear power plant accident, conflicts), your own Geiger counter can provide valuable information if other systems fail.
- Travel / outdoor:
- In some regions (e.g. former uranium mining areas, volcanic areas), increased radiation can occur – a portable counter can help here.
Frnisi GC-01 branded or unbranded?
You can also get the meter for a bit less, which is a cheaper replica of the original. However, this also has some drawbacks. The installed radiation measuring tube is inferior, the microcontroller isn’t supported by Rad Pro, there’s no USB data connection, and the battery is inferior. So, it’s not worth saving a few euros.
Here you can see the cheap replica first:
And here is the original from Fnirsi:
>> You can buy the original Fnirsi GC-01 here <<
I first bought the fake version. But then I quickly realized that it wasn’t possible to flash it with Rad Pro. When I finally bought the original, I was shocked when I opened it. The microcontroller isn’t labeled either. I was worried that this chip wouldn’t be compatible with Rad Pro either.
Rad Pro only supports these two MCUs for the GC-01:
CH32F103R8T6 (WCH) or APM32F103RBT6 (Geehy).
The fake one has a CA32F0156L2-Z94.
The Rad Pro Github page states “Warning: Rad Pro is not compatible with devices using unmarked chips.”
There are different firmware versions tailored to each chip. From experience, I know it’s not wise to experiment with this, as you’ll quickly end up with a bricked-up device that’s only useful as a paperweight.
I didn’t give up hope. After some research, I finally found a way to determine the MCU type. More on this below.
Why the Rad Pro firmware?
The Rad Pro firmware is an unofficial but greatly enhanced firmware that overcomes many of the limitations of the original Fnirsi GC-01:
- More functions & setting options:
- e.g. longer integration times, custom alarm thresholds, calibration, RMS display, unit selection (µSv/h, CPM, CPS)
- Accurate display & better statistics:
- The original firmware is often inaccurate or too “jumpy”. Rad Pro offers better smoothing and reliable values.
- Logging & extended display:
- CSV logs can also be created or graphics displayed.
- Active further development by the community:
- Ideal for hobbyists, open source fans and anyone who wants to get more out of their device.
How to install Rad Pro on the GC-01
There’s an installation guide on the Rad Pro Github page. I’ll describe my approach here.
First, we need to remove the four screws on the back. Then, the back panel can be removed. Now we can see which tube is installed. We can now also examine the MCU. If it’s labeled, you just need to remember the type.
However, if it’s not labeled, we’ll need to read the PID and VID values using a PC.
We need to connect the switched-off GC-01 to a Windows PC via USB-C. Then we turn the device on. The display will remain black, the red light at the top will light up, and Windows has recognized a device.
First, we need to open Disk Management in Windows.
There should now be a drive called “Bootloader.“
Now right-click on the associated disk (not on the Bootloader). There we select “Properties.” Now select the last tab, “Events.”
There we select “Configure Device.”
In the information window, scroll all the way down. There you’ll find the four-digit PID and VID values.
You can either Google these or simply ask in chat gpt.
If you have a VID of 0484, you have a CH32F103R8T6 (WCH) MCU, like me.
Now that we know the MCU type, we can install Rad Pro.
To do this, download the latest version here from Github. I used the latest beta version.
Now, using Disk Management in Windows, open the “Bootloader” folder on the drive. We then copy the Rad Pro firmware with the correct MCU and language into it. And that’s it. The window will close automatically, and the GC-01 will boot with the Rad Pro firmware.
If you want to use the manufacturer-provided firmware again, you can reinstall it at any time.
Why install an SBM-20-1 geiger muller tube?
| Feature | SBM-20-1 | M4011 | J305 |
|---|---|---|---|
| Origin | Russia (original Soviet/Russian) | China (generic clone) | China (ultra-low-cost) |
| Build Quality | High, industrial-grade | Low to medium, inconsistent | Very basic, often unshielded |
| Length | ~81 mm | ~85 mm | ~80 mm |
| Diameter | ~10 mm | ~10 mm | ~10 mm |
| Operating Voltage | 350–400 V (typ. 400 V) | 350–400 V (typ. 400 V) | 350–400 V (may vary by batch) |
| Detection Type | Gamma and high-energy beta | Gamma and some beta | Mostly gamma (very weak beta detection) |
| Sensitivity | ~29–33 CPM/µSv/h | ~20–25 CPM/µSv/h | ~10–20 CPM/µSv/h (not reliable) |
| Dead Time | ~190 µs | ~200–250 µs | Not officially specified |
| Calibration Data | Well documented | Poorly documented | None available |
| Reliability | Excellent, proven for decades | Varies widely | Very poor (many fail within months) |
| Expected Lifespan | 10+ years (if used properly) | Often < 3 years | Often < 1 year |
| Radiation Window | Thin metal, hermetically sealed | Thin metal or mica-like material | Very basic, often metal tube only |
| Pulse Strength | Strong, clean pulses | Weaker and noisy pulses | Weak, inconsistent pulses |
| Price (2025) | ~25–40 EUR/USD | ~5–12 EUR/USD | ~3–6 EUR/USD |
| Typical Use | Professional/DIY detectors | Hobby projects | Toys, click counters, basic education kits |
| Availability | Good (Eastern Europe, surplus markets) | Widely available on AliExpress, eBay | Very common in cheap Chinese kits |
- J305 → Ultra-cheap, unreliable — only good for toys or experiments.
- M4011 → Budget-friendly for hobbyists, moderate quality.
- SBM-20-1 → Best overall for serious detection and calibration.
The tube is also listed under these names: “СБМ-20-1″, “SBM20-1”, “SBM-20 -1”. And as an additional designation: Geiger Muller counter, GM tube.
You can find them online or often on eBay. The price shouldn’t exceed €50. You can also install an SBM-20 tube. However, this is longer, and the retaining clips have to be removed. But I recommend getting the newer version, the SBM-20-1.
How do I replace the Geiger counter tube with an SBM-20-1
Replacing the tube is pretty simple. First, unplug the battery and wait 10 minutes for the capacitors to discharge. Disconnect the monitor. To do this, you first have to unlock it. Then the cable can be easily pulled out. Now, remove the circuit board from the case. To do this, carefully desolder the old tube.
If your SBM-20-1 tube only has one wire on the positive side, you’ll need to solder a wire for the negative side to the other end of the metal tube and then connect it to the negative terminal on the circuit board. Then solder the wire on the positive side to the positive pin. You can also simply solder it to the clamps like I did.
Then simply glue the new tube to the circuit board with some hot glue.
Now you can reassemble everything.
Start the device and open the settings. There, you need to select the correct tube. To do this, go to the “Geiger Tube” menu item and select the SBM-20. The SBM-20 tube differs slightly from the SBM-20-1, but that’s essentially just the design. The sensitivity is almost the same and can therefore be ignored. Otherwise, you can also select a custom value.
Replace the battery with a larger one
The standard battery has a capacity of 1100 mAh. It makes sense to install a larger one. I have measured what would fit in at most. I was able to find a suitable 2000 mAh battery for about €10. The battery should have the following dimensions: 60 mm x 40 mm x 6.5 mm.
I had to replace the connector. To do so, I simply cut the cable in half and soldered it back together.
Of course you can also solder it directly to the battery.
Conclusion
By installing the SBM-20-1 instead of the simple M4011, the Fnirsi GC-01 has become much more precise and reliable. In combination with the Rad Pro operating system, measured values can now not only be tracked live, but also recorded and analyzed cleanly. This turns an inexpensive geiger counter into a serious measuring device – ideal for anyone who wants more than just clicks per minute.
Very nice article. Thank you. Where did you find the battery for the GC 01 radiation detector?
Thanks. I bought the battery on eBay. But you can buy it anywhere, of course. The dimensions just have to be right.