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Dimensions of the Geiger-Muller Tube Holder on the GGreg20_V3 Geiger Counter PCB

Recorded: May 27, 2026, 11:03 p.m.

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Dimensions of the Geiger-Muller tube holder on the GGreg20_V3 Geiger counter PCB - IoT-devices, LLC - Electronics manufacturer for IoT

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Dimensions of the Geiger-Muller tube holder on the GGreg20_V3 Geiger counter PCB

Post published:03.11.2025
Post category:Tips

While we included the basic dimensions in the datasheet for the GGreg20_V3 Geiger counter module, these are not all the measurements we are occasionally asked about.
For makers who want to purchase the GGreg20_V3 Geiger counter module without a tube (because they already own a compatible tube or plan to source one elsewhere), this small technical note details the dimensions of the tube mounting on our module’s PCB.
As you can imagine, having the option to buy just the module can be very convenient for some. We have always provided this option (on our website, Etsy, and previously on Tindie) — allowing you to select and order only the components you need right now. Over time, we received feedback, gained experience, and discussed this matter with our customers.

Note: The GGreg20_V3 was originally designed for the SBM20 tube, but its Soviet-era stock quickly ran out, so we specifically chose the even better J305 tube as a replacement. This means the standard SBM20 will also fit the module, as it was the tube we originally designed it for.

It turns out that the Chinese market offers J305 tubes with varying dimensions. This is why we want to explain exactly which J305 tube (or similar) will fit the GGreg20_V3.
We provide data for the J305 tube that we currently supply with the GGreg20_V3. We use the data provided to us by our J305 tube supplier from China.

Further on, we include a technical drawing with dimensions and a few photos to clearly illustrate the setup.

We also provide a photo of several J305 tubes from the latest batch, which clearly shows that while J305 tubes have minor size variations, they can be considered identical for our purpose. In several years of working with the J305, we have only encountered two initially defective tubes, as well as one tube that significantly differed in size from the rest of the batch and from the dimensions specified in the datasheet.

We hope this publication will help you understand which tube to choose so that it fits the module, particularly in terms of length.
Good luck!IoT-devices team

Tags: DIY, geiger-counter, GGreg20_V3, J305, technical-note

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Yaml

може сховати лістинг у акордеон?
####### ggreg20_esp8266_esphome.yaml #####
esphome:
name: esphome_node1 # Controller Unique Name
platform: ESP8266 # Platform type you have to select when creating new yaml-config in ESP Home
board: nodemcuv2 # Controller type you have to select when creating new yaml-config in ESP Home

wifi:
ssid: "YourWiFiSSID"
password: "SSIDPassword"

# Enable fallback hotspot (captive portal) in case wifi connection fails
ap:
ssid: "Esphome Node1 Fallback Hotspot"
password: "Cpxg9hRIBU7M"

captive_portal:

# Enable logging
logger:

# Enable Home Assistant API
api:
password: "APIpassword"

ota:
password: "OTApassword"

# Just embedded test D3 (GPIO0) button on every ESP8266 Devboard
# You can press D3 button several times to simulate incoming GGreg pulses
binary_sensor:
- platform: gpio
name: "D3 Input Button"
pin:
number: 0
inverted: True
mode: INPUT_PULLUP

# Here we calc and include to the firmware a power and doze values of ionizing radiation as sensor outputs
sensor:
- platform: pulse_counter
pin: D3
unit_of_measurement: 'mkSv/Hour'
name: 'Ionizing Radiation Power'
count_mode:
rising_edge: DISABLE
falling_edge: INCREMENT
update_interval: 60s
accuracy_decimals: 3
id: my_doze_meter
filters:
- sliding_window_moving_average: # 5-minutes moving average (MA5) here
window_size: 5
send_every: 5
- multiply: 0.0054 # SBM20 tube conversion factor of pulses into mkSv/Hour
- platform: integration
name: "Total Ionizing Radiation Doze"
unit_of_measurement: "mkSv"
sensor: my_doze_meter # link entity id to the pulse_counter values above
icon: "mdi:radioactive"
accuracy_decimals: 5
time_unit: min # integrate values every next minute
filters:
- multiply: 0.00009 # obtained doze (from mkSv/hour into mkSv/minute) conversion factor: 0.0054 / 60 minutes = 0.00009; so pulses * 0.00009 = doze every next minute, mkSv.
####### END of ggreg20_esp8266_esphome.yaml #####

 

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This technical note details the dimensions of the tube mounting on the Printed Circuit Board (PCB) of the GGreg20_V3 Geiger counter module, providing information supplementary to the standard datasheet measurements. This information is specifically intended for makers who wish to acquire the GGreg20_V3 module without an integrated tube, allowing them to source compatible tubes independently. The module was originally designed for the SBM20 tube, but due to supply constraints, the developers substituted it with the J305 tube.

A key purpose of this supplementary documentation is to clarify the compatibility requirements regarding the physical size of the J305 tube, as the dimensions offered by the Chinese market can exhibit variations. Therefore, the authors provide specific dimension data based on the J305 tube currently supplied with the GGreg20_V3, using data provided by the J305 tube supplier from China. To mitigate potential fitting issues for the end-user, the note includes technical drawings and photographs illustrating the setup and several examples of the latest batch of J305 tubes, demonstrating that while minor size variations exist, they are considered functionally identical for the module's purpose. The primary objective is to assist users in selecting a tube that correctly fits the module, particularly concerning the tube's length.