Fake Geiger Counter (with Fake Radioactive Source)

This instructable describes a fake radioactivity detector well known as Geiger Counter, would could be used in an escape game or other entertainment purpose.

1 x round display analogic voltmeter

2 x ESP32-Lolin32-Lite (or eq)

1 x passive piezo buzzer

2 x micro USB-B cable

2 x 5V-powerbank

hardware for the housing (plastic box, pvc tube, painting spray, switch...)

I was trying to simulate a Geiger-Müller counter to use in an escape game I had created.

After searching on the net, no affordable solution seeming available, I decided to meet the challenge by myself.

My goal was to create a reliable, as simple as possible and cheap device.

Different technologies seem to be possibly employed to achieve this purpose:

• Use of RFID. Easy to implement, but range is insufficient, or the cost of a long range device was too high for my wishes.
• Use of GPS: not very accurate and reliable, specially indoors.
• Use of ultrasounds was my first attempt - It did work effectively, I used a ultrasound power generator at 40 kHz with a piezoelectric tweeter (used as a pest repeller) as a source, and a transducer taken from a bat detector, coupled to an arduino to generate pseudo-random ticks in relation with the strength of captured signal, as a detector . It worked well, but it needed an external circuitry to amplify the ultrasound signal from the sensor which is weak (some millivolts), and so needs an extra circuit. Furthermore, the ultrasound generator needs some power, and that reduces the autonomy in the case of batteries are used as a power source. In addition, pets don't appreciate very much this kind of sounds.
• Use of Bluetooth signal. As there are many very cheap microcontrollers with WiFi / Bluetooth capabilities on the market, it seemed a good choice in terms of cost and simplicity of implementation. 

For this project I used 2 ESP-32 modules (Lolin32 clones in my case, but many types of ESP32 will be OK), making use of their BLE (Bluetooth Low Energy) onboard transmitter. One is used as a IBeacon and the other as a beacon scanner. You can buy two of them at chinese websellers for three dollars (or euros). I have found mines on AliExpress here

The first module is used as a simulated radioactive source, emitting a signal periodically. We don't make use in this application of the data included in the signal, we are only using its name to identify the source (otherwise the Geiger counter would detect any BT emitter). The second one is used as a simulated Geiger counter, generating a typical "click" sound repeating in a pseudo-random pattern at a frequency related to the signal intensity, so to the proximity of the "source".

I have added an analogic display as an average indicator of the radiation amount, to have a more convincing device.

The schematics is extremely simple:

The ESP32 used as the source does not require any external component, except the power source (I used a power bank connected on the micro-USB connector)

The ESP32 used as the Geiger counter requires a passive buzzer or sonic piezo used to generate the "clicks" - I used a piezo from a smoke detector- connected on GPIO 27,  and a galvanometer (voltmeter) connected on GPIO 26 configured as an analogic output (PWM) . I had a 300V one in my drawers, I removed the internal serial resistor and soldered one of a proper value in order to obtain the full range deviation for max value (255). For the piezo you can use an active one (no need in this case to adjust the frequency), but I found the sound was rather weak with the ones I had in my stock. You can use an amplifier board or a transistor connected on the 5V line to strengthen it (but if you tune frequency in the code to obtain the exact resonance of the piezo, the sound is strong enough, even with a 3,3V square wave).

About the value of the resistor to solder in place of the old one in the display:

In theory, when the value is 255 (duty cycle of 100%), the voltage on digital output should be equal to Vcc, (here: 3,3V). So, if you got a 10V display, for example, the new resistor value should be R'= (3.3/10)*R.

However, the tests show that the value was too high to obtain the full range deviation. The reason is that there is a timing conflict in the software between BLE scan (which takes some time) and ticker used to generate the sound clicks, leading to the fact that the pulses on PWM output are periodically interrupted . So the simplest (and dirty) solution was to use a resistor of a lower value (average 1/3 of theorical value). As it worked well for my purpose, I didn't search another way to do it. Obviously, it could be achieved by the use of an external pulse generator for the "clicks", but I wanted to keep the electronics as simple as possible. If you find a software way to do so, don't hesitate to improve the sketch !

When changing the value of the internal resistor of the display, take advantage of this operation to replace the display screen by a new one inspired by a real GM counter and printed on an adhesive paper cut at the same shape and applied on the old one (see pictures above)

The sketches to upload in the ESP32s can be found here. 

1) Upload the Radiosource sketch (in fact, an Ibeacon program) on the first ESP32. After uploading, you can use a smartphone application to detect the active BLE beacons around you (BLE scanner app like this one) and check that the beacon is working right. It should appear with the name "Radioactive source" (by clicking on the detected beacon you can view its name and other data)

2) Geiger Counter sketch: Compile and upload the sketch in the second ESP32.

Power the source ESP32, (eventually testing it is really emitting BT signal on your smartphone), then power the detector ESP32. When you are in the immediate proximity of the source, the display should be at the maximum deviation, together with a quasi non-stop crepitation sound, while if you move away the click fequency decrease progressively (with a latency of 10 to 15 sec), to a minimum of 1 tick every 5 or 10 sec to simulate the natural radioactivity of the environment (no deviation of the analogic display), when no signal at all is present (source off or too far away).

Both modules can be powered either with a 5V source on micro-USB connector, or a LiPo battery on the JSP connector. In my case, I use 2 cheap powerbank units connected on the micro-USB.

You can find here a video showing the detection range of the modules. As you can see, it easily reaches 25 meters, even with obstacles on the way.

The picture at the top shows you what my counter looks like, inspired from a real old-looking one I found on the net. The housing is made from a bento plastic food box and pvc pipes glued and painted.

And if you want to really frighten your friends, you could even encapsulate the emitter module in a metal block looking shape, achieving to convince them that it is a real radioactive material...

Enjoy !