Have you have ever wanted to test the radioactivity of your smoke alarm, or have you ever wanted to be extra prepared for that nuclear apocalypse that is always being spoken about? If you have, then this instructable is for you. In this instructable, I will show you how to build a very simple and cheap Geiger counter that can be built with salvaged parts and still get the job done. The video below will compliment this instructable with a verbal explanation of all the steps. Lets get started!

Step 1: How a Geiger Counter Works

To begin, I will explain the basics of how a Geiger counter works. A Geiger counter uses a special tube filled with inert gas at a very low pressure to detect radiation. Inside this tube, there is a cylinder shaped piece of metal that acts as a cathode. Within this cylinder, there is a small metal wire that acts as an anode. When a high voltage is initially put on the anode of the tube, nothing happens, but when a radiation particle strikes within the tube, it causes the inert gas to ionize and break down in a chain reaction. This causes a few micro-amps of current to flow for a few milliseconds. This flow of current can be measured by different electrical circuits. The pictures accompanying this step show a Geiger tube and a diagram of its internal components.

Step 2: An Explanation of the Schematic

Too make this Geiger counter work, there needs to be two parts of this circuit; the high voltage power supply, and the detector circuit. In the diagram above, the high voltage circuit consists of a 555 timer driving oscillator driving a transformer. The 555 timer generates a square wave that, through a resistor, turns on and off a MOSFET transistor. This MOSFET drives a small step up transformer. The output of the transformer is then fed into a voltage doubler where the voltage is boosted to about 500 volts. Then, the voltage is regulated through a series of 4 100v zener diodes to the Geiger tube's recommended 400 volts.

For the detector circuit, the Geiger tube's anode is wires directly to the 400 volt power supply. In between the cathode of the tube and ground, I placed a piezo electric element. This converts the small current flow from the Geiger tube to a audible click.

Step 3: Tools and Materials

To complete this project, you will need a variety of tools and materials.

Tools:

Wire cutters
Wire strippers
Soldering iron
Hot glue gun

Materials: Most of these can be salvaged from old electronic devices.

8:800 ohm transformer(This was the power supply transformer located inside a broken alarm clock)
Geiger tube (This was found on Ebay for around 8 dollars)
555 timer
47k resistors (x2)
22nF capacitor
2.2nF capacitor
1k resistor
Any N-channel MOSFET
Perfboard
1n4007 diode(x2)
100nF 500 volt capacitor
100 volt zener diode (x4)
Piezoelectric element (From an old microwave)
Wire
Solder

Step 4: Soldering the Oscillator and Mosfet

After you have gathered your tools and materials, it is time to do some serious soldering. The first part that you need to solder is the oscillator and mosfet portion of the circuit. To do this, place each component on the perfboard one at a time in an efficient manner. For example, solder the MOSFET near where the transformer will be on the board or place the 47k resistor near pin 7. This should help you use less wire when soldering. After you add each component, solder traces between the corresponding contacts of other components. Use wire if necessary. Finally, trim off excess leads.

Step 5: Soldering the Transformer and Voltage Doubler/Regulator

After the oscillator portion of the circuit is soldered in place, solder the coil of the transformer with the lower resistance in between the mosfet and power. Then solder the other 2 wires to the voltage doubler portion of the schematic on another side of the board. Make sure to place all high voltage components on the opposite side of the board as the oscillator, so it does not cause interference. Then, solder in all the capacitors and zener diodes. When you place the voltage doubler, just think of it as a bridge rectifier with 2 of the diodes replaced with capacitors. After the high voltage power supply is soldered, test it with a voltmeter to see if you have the right voltage. If you have a different Geiger tube than me, look up it's specs to find its specified voltage. Then, add or subtract zener diodes accordingly.

Step 6: Adding the Geiger Tube and Detector Circuit

The final part of this build is soldering on the 2 components that make up the detector circuit of this counter. Start by soldering a wire to each end of the tube. Then, solder the anode to the output of the regulated power supply and the cathode to the piezo element. Finally, solder the piezo to ground. The fact that the detector only uses 2 components is what makes this the simplest Geiger counter. Most at least have to contain a transistor on the detector. It does not need any current limiting resistors because the power supply can barely put out any current anyways.

Step 7: Testing!

Finally, it is time to test the Geiger Counter! To do this, first attach the counter to a power supply, any power supply between 9 and 12 volts will work, even a 9 volt battery. Then, grab a radioactive source to test. I used Americium from a smoke detector. Finally, with pliers, hold the source next to the Geiger tube. You should hear some noticeable clicks on the piezo. To hear and see this, watch the video in the intro. Thank you for reading!

Disclaimer: This project works with high voltage, build and use with caution.