There are quite a few geiger counter instructables. The difference with this one is that I wanted to give it a modern interface, similar to smart phone touch screens, that anyone would feel comfortable using. The reason for this is that I want to use it as an educational tool to teach kids (and interested adults!) about radiation and how it is a natural part of the environment.
Check out the video to see what I mean.
From a technical point of view this project has a few interesting aspects.
- Interfacing a custom circuit with an existing circuit board
- Capacitive touch controls
- 7-segment LED display
- Customising microcontroller software
I don’t consider this project to be too difficult if you can solder, upload Arduino code and are prepared to experiment (which is the fun part anyway!)
It’s only been roughly calibrated. But the circuit is robust enough to be pretty accurate with proper calibration and perhaps some tweaks to the software.
Step 1: Materials and Tools
- This DIY geiger counter. It’s not cheating. There is no point reinventing the wheel! This is a great circuit, already tested and works very well. What is really useful is that it incorporates an ATMega328 microcontroller (it has an Arduino built in). I will be running my own customised code on it.
- A Geiger–Müller tube. This is the sensor that actually detects radioactive particles. These can bought on ebay. I recommend either an SBM-20 or SI-180G. They are especially sensitive to gamma rays and are relatively cheap. In fact, for this project I used one of each in parallel.
- Project box. A nice box to hold everything makes a big difference. I used the biggest size of this. It is the biggest one I could find at a reasonable price that looked alright. It was a wee bit small though and required a bit of creativity to fit everything in!
- 2mm clear acrylic sheet. Often known as perspex this is the “screen” of the geiger counter. The touch switches are hidden underneath.
- Overhead projector transparency sheets. The interface graphics are printed on this and then laminated to the acrylic. I also use these for the method I use to make the circuit board.
- Electronic components
- 5mm LEDs: 9x red, 4x white
- Resistors: 2x 82Ohm, 1x 100Ohm, 5x 220Ohm, 1x 6.8kOhm, 1x 22kOhm, and 4x 10MOhm
- 1x 10kOhm trimpot
- PCB single row headers (male and female)
- 1x Maxim MAX7219 LED display driver
- 2x 4 digit 7 segment LED display (common cathode, 0.36″)
- 1x slide switch. This is the only mechanical switch used to turn the device on. I salvaged mine from an old digital photo frame as it had a nice cap on it.
- 1x 9v battery and holder
- thin single core wire to use as jumpers on the circuit board.
- general purpose wire for connecting geiger tube and battery to circuit
- 2x PCB mount screw terminals (for power)
- fuse holder (to attach to geiger tube terminals)
- dual-layer blank PCB for our circuit board
- single-layer blank PCB to make touch sensors
- 1x 0.1u capacitor
- 1x 24 pin IC socket
- PCB fabricating tools and material. The exact requirements depend on the method you use.
- Milling or cutting tools for the project box.
- Soldering iron
- Fritzing software for PCB design
- Arduino IDE software
- FTDI breakout for reprogramming the ATMega
Step 2: How Capacitive Touch Sensing Works
Capacitive sensing uses the capacitance between the sensor and the human body to detect if a button is activated. (I almost wrote “button is pushed” but this is a proximity sensor. No physical force is required. Pretty cool!)
How it works is there is an input and an output with a resistor and a conductive plate or foil between them. A voltage is applied to the input and the time taken for the output to reach the same voltage is measured. If a finger or other conductive material is close to the plate there is a capacitance which slows down the time taken for the output to change voltage.
The time taken is defined by this formula:
T = R x C
where T is the time, R is resistance and C is capacitance.
A particularly sensitive sensor can detect when a finger is several centimetres from the plate.
This is all well and good but there are issues that need to be taken into account when designing these sensors.
Stray capacitance can be generated by other parts of the circuit which can affect the sensor. This can be dealt with in the following ways:
- Make multiple measurements and average the results. The readings can vary enormously.
- Keep the distance between the input and outputs short.
- A very small capacitor (100pF) between the output and ground can stabilise readings
- Try adding ground layers between the sensor plate and circuit board or between plates. This will reduce the sensitivity of the sensor however.
I experimented with all these but in the end found that simply moving the sensor plates further from the circuit board and adjusting the detection threshold worked best for me.
For more detail: Geiger Counter with Touch Interface!