Simplest Geiger Counter





Introduction: Simplest Geiger Counter

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.


  • 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.



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33 Discussions

Here a more complex version of how it works. LOL.

Well, the project is interesting plus how a low tech beeping type would work with that diagram. I bet modernate type GC have a cpu counter added instead or with the buzzer.

I cannot find any audio transformers with such impedance. Is something like this would be suitable?

1 reply

or this

I think there'd be some benefits from full wave rectifying the output of the transformer and also adding a smoothing cap so you don't get that annoying high pitched whine. Just a thought. Unless it won't work without AC through the Geiger tube.

3 replies

Nice idea, but I'd like to point out that a full wave rectifier wouldn't do anything to help. The ringing has to do with the physical oscillation of the transformer core which produces hearing-range sound.

he is trying to get high voltage hence the voltage doubler. if you're getting an "annoying" high pitched whine then increase the frequency of the LM555. he did everything right.

yeah no, I understand the high voltage, but in the video you hear a high pitched whine mixed in with the Clicks of the Geiger counter working. I'm saying he can filter it out by running DC through the tube, whether or not the Tube will run on DC is a different question. I do understand you need an AC like wave to use stepup transformers correctly, that's why I'm saying full wave rectification after the transformers output stage.

Hmm, nice work and well done indeed :-)

Your results however raise an interesting question re nucleonic interactions based upon these assumptions re smoke detectors, their radiation source purity, glass effect etc,

1. Smoke detectors utilizing Americium (if very pure) only give off Alpha particles
2. Alpha particles don't travel far in air before encountering air molecules which reduces their energy substantially - for a smoke detector something like 1-2 cm so beyond that there won't be a reading. I see when only close, this is a reading.
3. Glass is effectively a brick wall for the energy range of Alpha expected from a typical (retail) Americium source however, Beta particles can pass through.

Given these facts, then these questions arise:-
a. Is it possible Americium isn't pure & can be contaminated with a Beta emitter ?
b. If there's another radiation emitted Eg neutrons then they will pass through the glass but, on the way through encounter Boron which will absorb a neutron & emit an Alpha which is then detected. In so far as neutrons are not charged particles & wouldn't be ionize hence wouldn't be detected directly, it would be of interest to place a Alpha/Beta shield between the Americium & the glass which can allow neutrons through can you try this ?
c. Although b. is unlikely, its more unlikely the Americium has a contaminant which is a very high energy Alpha emitter which allows the Alpha through the glass, this seems possible as the distance to the glass from the Americium needs to be short to get a reading, can you borrow an Alpha/Beta modern detector or take that Americium source for checking - maybe at a local unit - and hopefully with your setup so a graphical comparison can be made ?

Possible regulatory issue arises as its not clear how well the Americium source you have has passed earlier inspection either at import or at time of fabrication and if indeed it has contaminants then you may have stumbled upon a lurking issue re the sources, Eg If they slipped inspection but, had other radiation sources contaminating then it would be worth pinning this down definitively for the safety of all concerned, good luck...

Thanks for reading & again commendations on the work, well done :-)


Mike Massen
Perth, Western Australia

2 replies


You are mostly correct, but you do have a few incorrect assumptions.

1. Americium gives off alphas. This is correct. Although, incorrect as Americium 241 also gives off gamma in the low KeV range.

2. Correct. Alpha is very short distance.

3. Correct. Alpha is blocked by glass, but as stated before, Am-241 is also a gamma emitter. Beta (dependent on energy) may pass through glass.

a. Possible; but unlikely. Likely 59 KeV gamma.

b. Unlikely seeing neutrons. Even with a specifically designed detector (He3), the probability of interaction between a neutron and your capture medium is usually low.

c. As previously stated; Likely the gamma.

The only issue with regulatory comes from the license granted by the NRC (if in the US). The americium is only permitted for use in the detector and immediately becomes a violation if removed. Americium is a licensed source in the US (because it is an alpha emitter).

Well the possibility of neutron detection sure is unlikely but I want to point out that it IS possible. This is because when a neutron interacts with hydrogen (which is present in the glass) it gives off a proton which is easily detectable with a Geiger counter.

Love this. I mocked the circuit up in EveryCircuit just to see what it would yield. Let me know if I put this together wrong. I substitued the tube with a bulb.


1 year ago

Finally an Instructable that really tells how to build something. Most of these are just instructions on how to assemble a box full of parts bought from a vendor complete. This is actually building something from scratch. Good Job and well explained without going into too much electronics detail.

I am old school, 500 volts is not "high voltage". 2000 Volts IS high voltage. LOL

4 replies

Everything is relative... In electric engineering, 300.000V is High Voltage, but 20.000V is only medium voltage... :)

electronics, where common voltages are 1.5 - 3 - 6 - 9 - 12 - 15 -
+/-15V, 100V is considered high voltage, because more than 48V is

Well, back in the day, we needed 350V for our plate supply so it wasnt considered high. 18 V for the filaments was considered low. Now the 1800V for the CRT plate was HIGH....LOL

I completely agree with you on all aspects of this reply. I love how you stated that 500 volts isn't high voltage. I love tesla coils ( especially spark gap because of the high voltage transformers used ). I built one that uses a 6kv trasnformer.

Well, I wasnt trying to be a smartass, just saying that the new version of Hi Voltage wasnt the way I learned it when Tubes were king.. I have been hit by 25,000v and it knocked me across the room and I bit my tongue, but I survived....shaken...LOL

Thanks for sharing. You did an excellent job of describing the circuit as well as the way you presented it in the Intractable. Instead of the "Geiger" tube could you have used a simple neon lamp of the type used by electricians for testing high voltage in the mains and perhaps using 100 Volts to bias the lamp?



1 year ago

Neat stuff.

You forgot yet another reason to build one of these, the annoying habit us Gizmo Makers have to collect Junk, SPECIALLY, junk we don´t know what's for...

Went a link to my Blog: