Here is, to my knowledge, the simplest functioning Geiger counter that you can build. This one uses a Russian-made SMB-20 Geiger tube, driven by a high-voltage step-up circuit robbed from an electronic fly swatter. It detects beta particles and gamma rays, emitting a click for every radioactive particle or gamma ray burst it detects. As you can see in the above video, it clicks every few seconds from background radiation, but really comes to life when radiation sources such as uranium glass, thorium lantern mantles, or americium buttons from smoke detectors are brought near. I built this counter to help me identify radioactive elements that I need to fill out my element collection, and it works great! The only real drawbacks of this counter is that it isn’t very loud, and it doesn’t compute and display the amount of radiation it is detecting in counts per minute. That means that you don’t get any actual data points, just a general idea of radioactivity based on the amount of clicks you hear.

While there are various Geiger counter kits available on the net, you can build your own from scratch if you have the right components. Let's get started!

Step 1: Geiger Counters and Radiation: How It All Works

The Geiger counter (or Geiger-Müller counter) is a radiation detector developed by Hans Geiger and Walther Müller in 1928. Today, just about everyone is familiar with the clicking sounds it makes when it detects something, often regarded as the “sound” of radiation. The heart of the device is the Geiger-Müller tube, a metal or glass cylinder filled with inert gasses held under low pressure. Inside the tube are two electrodes, one of which is held at a high voltage potential (usually 400-600 volts) while the other is connected to electrical ground. With the tube in a resting state, no current is able to jump the gap between the two electrodes inside the tube, and so no current flows. However, when a radioactive particle enters the tube, such as a beta particle, the particle ionizes the gas inside the tube, making it conductive and allowing current to jump between the electrodes for a brief instant. This brief current flow triggers the detector portion of the circuit, which emits an audible “click”. More clicks means more radiation. Many Geiger counters also have an ability to count the number of clicks and compute counts per minute, or CPM, and display it on a dial or readout display.

Let's look at the operation of the Geiger counter another way. The key principal of Geiger counter operation is the Geiger tube, and how it sets up a high voltage on one electrode. This high voltage is like a steep mountain slope covered in deep snow, and all it takes is a tiny bit of radiation energy (akin to a skier going down the slope) to set off an avalanche. The ensuing avalanche carries with it much more energy than the particle itself, enough energy to be detected by the rest of the Geiger counter circuit.

Since it's probably been a while since many of us sat in a classroom and learned about radiation, here is a quick refresher.

Matter and the Structure of the Atom

All matter is composed of tiny particles called atoms. Atoms themselves are composed of even smaller particles, namely protons, neutrons, and electrons. Protons and neutrons are clumped together in the center of the atom - this part is called the nucleus. Electrons orbit the nucleus.

Protons are positively charged particles, electrons are negatively charged, and neutrons carry no charge and are therefore neutral, hence their name. In a neutral state, every atom contains an equal number of protons and electrons. Because protons and electrons carry equal but opposite charges, this gives the atom a neutral net charge. However, when the number of protons and electrons in an atom is not equal, the atom becomes a charged particle called an ion. Geiger counters are able to detect ionizing radiation, a form of radiation that has the ability to transform neutral atoms into ions. The three different kinds of ionizing radiation are Alpha particles, Beta particles, and Gamma rays.

Alpha Particles

An alpha particle consists of two neutrons and two protons bonded together, and is the equivalent of the nucleus of a helium atom. The particle is generated when it simply breaks off of an atomic nucleus and goes flying. Because it doesn’t have any negatively charged electrons to cancel out the positive charge of the two protons, an alpha particle is a positively charged particle, called an ion. Alpha particles are a form of ionizing radiation, because they have the ability to steal electrons from their surroundings, and in doing so transforming the atoms they steal from into ions themselves. In high doses, this can cause cellular damage. Alpha particles generated by radioactive decay are slow moving, relatively large in size, and because of their charge cannot pass through other things easily. The particle eventually picks up a few electrons from the environment, and in doing so becomes a legitimate helium atom. This is how almost all of the earth’s helium is produced.

Beta Particles

A beta particle is either an electron or positron. A positron is like an electron, but it carries a positive charge. Beta-minus particles (electrons) are emitted when a neutron decays into a proton, and Beta-plus particles (positrons) are emitted when a proton decays into a neutron.

Gamma Rays

Gamma rays are high energy photons. Gamma rays are located in the electromagnetic spectrum, up beyond visible light and ultraviolet. They have high penetrating power, and their ability to ionize comes from the fact that they can knock electrons off of an atom.

The SMB-20 tube, which we will be using for this build, is a common Russian-made tube. It has a thin metal skin that acts as the negative electrode, while a metal wire running lengthwise through the center of the tube serves as the positive electrode. In order for the tube to detect a radioactive particle or gamma ray, that particle or ray first must penetrate the thin metal skin of the tube. Alpha particles are generally unable to do this, as they are usually stopped by the walls of the tube. Other Geiger tubes that are designed to detect these particles often have a special window, called the Alpha window, that allow these particles to enter the tube. The window is usually made of a very thin layer of mica, and the Geiger tube must be very close to the Alpha source in order to pick up the particles before they are absorbed by the surrounding air. *Sigh* So that's enough about radiation, let's get to building this thing.

Step 2: Gather Your Tools & Materials

Supplies needed:

SMB-20 Geiger Tube (available for around $20 USD on eBay)
High Voltage DC Step-up Circuit, robbed from a cheap electronic fly swatter. This is the specific model that I used:https://www.harborfreight.com/electronic-fly-insec...
Zener Diodes with a combined total value of around 400v (four 100v ones would be ideal)
Resistors with a combined total value of 5 Megohm (I used five 1 Megohm)
Transistor - NPN type, I used 2SC975
Piezo Speaker Element (robbed from a microwave or noisy electronic toy)
1 x AA battery
AA battery holder
On/off switch (I used the SPST momentary switch from the electronic flyswatter)
Scrap pieces of electrical wire
Piece of scrap wood, plastic, or other non-conductive material to use as a substrate to build the circuit on

Tools I used:

"Pencil" soldering iron
Small diameter rosin-core solder for electrical purposes
Hot Glue gun w/ appropriate glue sticks
Wire cutters
Wire strippers
Screwdriver (for demolishing the electronic flyswatter)

While this circuit is built around an SMB-20 tube, which is able to detect beta particles and gamma rays, it can easily be adapted to use a variety of tubes. Just check the particular operating voltage range and other specifications of your particular tube and adjust the values of the components accordingly. Larger tubes are more sensitive than smaller ones, simply because they are larger targets for the particles to hit.

Geiger tubes require high voltages to work, so we are using the DC step-up circuit from an electronic fly swatter to boost the 1.5 volts from the battery up to about 600 volts (originally the fly swatter ran off 3 volts, putting out about 1200v for zapping flies. Run it on higher voltages and you'd have a taser). The SMB-20 likes to be driven at 400V, so we use zener diodes to regulate the voltage to that value. I’m using thirteen 33V zeners, but other combinations would work just as well, such as 4 x 100V zeners, as long as the total of the values of the zeners equals the target voltage, in this case 400.

The resistors are used to limit the current to the tube. The SMB-20 likes an anode (positive side) resistor of about 5M ohm, so I’m using five 1M ohm resistors. Any combination of resistors can be used as long as their values add up to about 5M ohm.

The Piezo speaker element and the transistor comprise the detector portion of the circuit.The Piezo speaker element emits the clicking noises, and the long wires on it allow you to hold it closer to your ear. I've had good luck salvaging them from things like microwaves, alarm clocks, and other things that make annoying beep noises. The one I found has a nice plastic housing around it which helps to amplify the sound coming from it.

The transistor boosts the volume of the clicks. You can build the circuit without a transistor, but the clicks the circuit generates won’t be as loud (by that I mean barely audible). I used a 2SC975 transistor (NPN type), but many other transistors would probably work. The 2SC975 was literally just the first transistor I pulled out of my pile of salvaged components.

In the next step we'll do a tear-down on the electric flyswatter. Don't worry it's easy.

Step 3: Dissasemble the Fly Swatter

Electronic fly swatters may differ slightly in construction, but since we are only after the electronics inside, just tear it apart and pull the guts out lol. The swatter in the pictures above is actually slightly different than the one I built into the counter, as it seems that the manufacturer changed their design.

Start by removing any visible screws or other fasteners holding it together, keeping your eye out for stickers or things like the battery cover that might conceal additional fasteners. If the thing still doesn’t open, it might take some prying with a screwdriver along the seams in the plastic body of the swatter.

Once you get it open, you will have to use a wire cutters to cut the wires off at the fly zapper’s mesh grid. Two black wires (sometimes other colors) originate from the same place on the board, each one leading to one of the outer grids. These are the negative, or "ground" wires for the high-voltage output. Since these wires come from the same place on the circuit board, and we only need one, go ahead and snip one off at the circuit board, setting the scrap wire aside for later use.

There should be one red wire leading to the inner grid, and this is the positive high-voltage output.

The other wires coming from the circuit board go to the battery box, and the one with the spring on the end is the negative connection. Pretty simple.

If you take apart the head of the swatter, perhaps to separate the components for recycling, watch out for possible sharp edges on the metal mesh.

Step 4: Construct the Circuit, and Use It!

Once you have your components, you'll have to solder them together to form the circuit shown in the diagram. I hot-glued everything to a piece of clear plastic I had laying around. This makes for a sturdy and reliable circuit, and also looks pretty good. There is a small chance you could give yourself a bit of a zap from touching parts of this circuit while it is energized, like the connection on the piezo speaker, but you can just cover the connections with hot glue if there is a problem.

Once I finally had all the components I needed to build the circuit, I threw it together in an afternoon. Depending on what values of components you have, you could end up using less components than I did. You could also use a smaller Geiger tube, and make the counter very compact. Geiger counter wristwatch, anyone?

Now you might be wondering, what do I need a Geiger counter for if I don't have anything radioactive to point it at? The counter will click every few seconds just from background radiation, which is composed of cosmic rays and such. But, there are a few radiation sources that you can find to use your counter on:

Americium from smoke detectors

Americium is a man-made (not naturally occurring) element, and is used in ionization-type smoke detectors. These smoke detectors are very common and you probably have a few in your home. It’s actually quite easy to tell if you do, because they all have the words contains radioactive substance Am 241 molded into the plastic. The americium, in the form of americium dioxide, is plated onto a small metal button inside, mounted in a small enclosure known as the ionization chamber. The americium is usually plated over with a thin layer of gold or other corrosion resistant metal. You can open the smoke detector and take the little button out – it’s usually not very hard.

Why radiation in a smoke detector?

Inside the detector’s ionization chamber, there are two metal plates sitting opposite each other. Attached to one of them is the americium button, which is emitting a constant stream of alpha particles that cross a small air gap and are then absorbed by the other plate. The air between the two plates becomes ionized and is therefore somewhat conductive. This allows a small current to flow between the plates, and this current can be sensed by the smoke detector’s circuitry. When smoke particles enter the chamber, they absorb the alpha particles and break the circuit, triggering the alarm.

Yeah, but is it dangerous?

The radiation emitted is relatively benign, but to be safe I recommend the following:

Keep the americium button in a safe place away from kids, preferably in a childproof container of some kind
Never touch the face of the button that the americium is plated on. If you do accidentally touch the face of the button, wash your hands

Uranium glass

Uranium has been used, in oxide form, as an additive to glass. The most typical color of uranium glass is sickly pale yellowish-green, which in the 1920s led to the nickname“vaseline glass” (based on a perceived resemblance to the appearance of petroleum jelly as formulated and commercially sold at that time). You will see it labeled as “Vaseline glass” in flea markets and antique stores, and you can usually ask for it by that name. The amount of uranium in the glass varies from trace levels to about 2% by weight, although some 20th-century pieces were made with up to 25% uranium! Most uranium glass is only very slightly radioactive, and I don’t think it’s at all dangerous to handle.

You can confirm the uranium content of the glass with a blacklight (ultraviolet light), as all uranium glass fluoresces bright green regardless of the color the glass appears under normal light (which can vary widely). The brighter a piece glows under ultraviolet light, the more uranium it contains. While pieces of uranium glass glow under ultraviolet light, they also give off light of their own under any light source that contains ultraviolet (like sunlight). The high energy ultraviolet wavelengths of light strike the uranium atoms, pushing their electrons into a higher energy level. When the uranium atoms return to their normal energy level, they emit light in the visible spectrum.

Why uranium?

The discovery and isolation of radium in uranium ore (pitchblende) by Marie Curie sparked the development of uranium mining to extract the radium, which was used to make glow-in-the-dark paints for clock and aircraft dials. This left a prodigious quantity of uranium as a waste product, since it takes three tons of uranium to extract one gram of radium.

Thorium camping lantern mantles

Thorium is used in camping lantern mantles, in the form of thorium dioxide. When heated for the first time, the polyester part of the mantle burns away, while the thorium dioxide (along with other ingredients) retains the shape of the mantle but becomes a sort of ceramic that glows when heated. Thorium is no longer used for this application, being discontinued by most companies in the mid-'90s, and has been replaced by other elements that aren't radioactive. Thorium was used because it makes mantles that glow very brightly, and that brightness isn't quite matched by the newer, non-radioactive mantles. How do you know if the mantle you have is really radioactive? That's where the Geiger counter comes in. The mantles that I've come across drive the Geiger counter crazy, much more so than uranium glass or americium buttons. It isn't so much that thorium is more radioactive than uranium or americium, but there is much more radioactive material in a lantern mantle than in those other sources. That's why it's really strange to encounter so much radiation in a consumer product. The same safety precautions that apply to the americium buttons apply to the lantern mantles as well.

Thanks for reading, everyone!If you like this instructable, I'm entering it into the "build a tool" contest, and would really appreciate your vote! I'd also love to hear from you if you have comments or questions (or even tips/suggestions/constructive criticism), so don't be afraid to leave those below.

Special thanks to my friend Lucca Rodriguez for making the beautiful circuit diagram for this instructable.

Happy making!