UK Shipping included; overseas buyers see important note at the bottom of the page**

Here's a very rare opportunity to own a classic vintage British-made Geiger counter. The Mini Instruments models 5.10 and 5.40 have been a popular choice in education, industry and research for several decades and we have a small stock of fully refurbished instruments ready for dispatch.

Although they have been around for a few years**** they have proved to be extremely robust and reliable and continue to work as well as they did when they were made and compare favourably with, and even exceed the performance, durability, refinement and ease of use of many modern instruments.



                                        First time buyer?  Please Click HERE


                                           TO CONTACT US PLEASE SEE BELOW FOR MORE INFO


The key feature, still used on current Mini-Monitor 900s, is the large and very easy to read moving coil meter, with a clever logarithmic scale. In a nutshell this means there is no need for a range switch and it can accurately display (in counts per second or cps) both high and low level readings (from normal background, to ‘get the hell out of there’…). 


We are currently upgrading our stocks of Mini Instruments Series 500 Geiger Counters to 9-volt operation. The circuits were originally designed to run on a 12-18 volt supply, which is fine, except that you need a lot of batteries, twelve AA cells to be exact. Not any more, the boffins at anythingradioactive have come up with a rather neat solution, a microchip-controlled power supply module, that uses  a single 9 volt PP3 type battery, OR a single 6 x AA cell battery pack.

As an added bonus we've designed a custom 3D-printed battery holder for both types. And in case you were wondering battery life is hardly affected. An ordinary PP3 lasts for several days of continuous operation and a 6 x AA pack can keep it going for a week or more. Oh yes, and prices stay the same (for now...).

It's going to take us a little while to upgrade our constantly-changing inventory so please be patient and watch out for the red 9-VOLT flags, but if there's a model you are especially interested in, that hasn't been converted yet, let us know and we'll bump it up the list. 

By the way, if you're a Series 500 owner, handy with a screwdriver and soldering iron and you like the sound of 9-volt operation, watch this space as we're working on a DIY conversion kit. More details as soon as we've finished modding our stock.

(N.B. running times approximate, dependent on battery type and probe). 


  • Internal speaker (with mute switch)
  • LED ‘event’ flasher (most models)
  • Audio output socket
  • DC supply socket
  • NEW!* 9-Volt Upgrade on selected all models
  • Detachable probe
  • Adjustable HV supply (approx 250 - 700 volts & 300 - 1500 volts depending on model)
  • Battery Check function
  • Sturdy, compact metal case with carry handle and probe cradle
  • Convenient ‘curly’ probe cable lead
  • Portable operation with long battery life
  • Calibrated prior to dispatch
  • Operating Manual (generic) with calibration info and circuit diagram included
  • Six month no-quibble warranty



As far as performance, reliability and ease of use are concerned the two models are almost identical (Type 5.10 cases may have rounded or square corners). The only significant differences are that the Type 5.10's probe supply is adjustable from around 250 to 750 volts whilst the 5.40 goes from 330 to 1500-plus volts. Additionally, 5.40 models have a handful of extra components, that make them  suitable for use with scintillation type probes.  The result is two highly versatile instruments capable of using a wide range of probes and suitable for a variety of specialist and non-specialist applications. (The operating manual includes full details of how to carry out HV adjustments).

Refurbishment begins with a complete strip down. Where necessary cases are taken back to bare metal, primed and re-sprayed using top quality paint to give them a fresh new look. (Custom colours and finishes on request,for details use Contact Us at the bottom of the page). 

All instruments are fully portable and self-contained, powered by internal batteries (either 12 x standard 1.5 volt AA cells,  and now* 6 x AA cells or a 9-volt PP3 type battery). See item description for specifics. Some models also have external supply sockets so they can be used as bench instruments using a mains adaptor (12 - 18 volts DC). Instruments without captive probe leads have TNC connectors fitted (case and cable), making it a simple matter to exchange probes. An LED indicator/event 'flasher' and audio output socket (standard 3.5mm jack) are fitted to selected models so they can be used with headphones, connected to an amplifier, or a PC, for data logging or analysis.
Prior to shipping every instrument is tested (not a single failure or return so far!), the HV adjusted and the unit pulse-calibrated, according to manufacturer's protocols. A Calibration Check Certificate is provided.



£350.00 inc. UK Shipping

Probe Type:                            Not supplied (TNC socket fitted)
Sensitivities:                            n/a
HT voltage:                             Variable approx 450 to 1300 volts 
Sensitivities:                            n/a
Features:                                 external HT adjustment LED flasher, audio out & DC in sockets 
Applications:                          widely compatible with GM & Scintillation probes
Power Requirements:            6 x 1.5-volt AA cells, 9-volt PP3 & AC adaptor
Condition:                                 A-


5.10 STS-5 9199               
Probe Type:                                 SBM-20 thin wall GM tube rotating beta shield
HT voltage:                                 Variable approx 350 to 700 volts
Sensitivities:                                Beta, Gamma & X-Ray
Applications:                              detection, monitoring, contamination control
Power Requirements.                6x 1.5-volt AA cells. 9-volt PP3 battery
Condition:                                     A


5.1 SBT-11 Pancake Probe  NEW LISTING
£485.00 inc UK Shipping               

Probe Type:                                 SBT-11 mica window pancake probe
HT voltage:                                 Variable approx 500 to 1600 volts
Sensitivities:                                Alpha, Beta, Gamma & X-Ray
Applications:                              detection, monitoring, contamination control
Power Requirements.                6x 1.5-volt AA cells. 9-volt PP3 battery 
Condition:                                     A


5.40 R1288 Scintillation Probe
£525.00 inc UK Shipping

Probe Type:                                Scitillation: Hammatsu PMT, Saint Gobain Nal (TI) crystal
HT voltage:                                 Variable approx 450 - 1500 volts
Sensitivities:                                Gamma & X-Ray
Applications:                              detection, monitoring, contamination control
Power Options                           6x 1.5-volt AA cells, 9-volt PP3 battery or AC adaptor
Condition:                                     A

** Very Important Stuff For Overseas Buyers:
The prices shown are for UK buyers and inclusive of delivery to mainland UK addresses. Non-UK shipping costs will be applied during check out. Please also note that this item may be prohibited or restricted in some countries.

*** 5.40 'bare bones' models are not supplied with probes. The very wide range of HT voltage adjustment (between 450 and 1300 volts) makes them compatible with most probes types, up to and including Scintillation detectors. For added convenience on some models the voltage adjustment pre-set is mounted on the outside of the case. Probes are available, use Contact Us at the bottom of the page if you need more details. 

**** Please note that these units may have been in service for several years so there might be some slight marking (scuffs, small scratches, etc.) on the meter face. There may be other cosmetic blemishes; however these are generally minor in nature. 

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Mini Instruments Type 5.10 and 5.40 Geiger Counters . Now with 9-volt Operation!

  • Product Code: Mini Instruments Type 5.10 and 5.40 Geiger Counters
  • Availability: In Stock
  • £0.00

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