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.


MrRedBeard made it!(author)2017-03-06

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.

JimI8 made it!(author)2016-09-25

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

GianlucaG1 made it!(author)2016-11-17

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

JimI8 made it!(author)2016-11-17

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

blackhol1 made it!(author)2016-09-28

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.

JimI8 made it!(author)2016-09-28

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

MikeM124 made it!(author)2016-09-26

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

gigabite1123 made it!(author)2016-10-17


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).

kschmidt2 made it!(author)2016-09-14

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.

JohnC430 made it!(author)2016-10-04

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.

kschmidt2 made it!(author)2016-10-05

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.

JohnC430 made it!(author)2016-10-04

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?

blackhol1 made it!(author)2016-09-28


Edgar made it!(author)2016-09-28

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:

MikeM124 made it!(author)2016-09-26

Further to my last comment, please don't panic, I realise you must have a far better quality geiger tube that most I've seen which either has a very thin glass or a composition which allows some Alpha particles through - this is sort of supported by the fact you need to be very close...

Most geiger tubes I've encoountered years ago weren't so well built, it all depends of course, so please don't worry - though I am curious just how old that smoke detector Americium alpha source & if its passed manufacture/regulatory specs ?


KelemenR made it!(author)2016-09-26

Excelent job! I admire this kind of people, who makes things from junk, McGyver-style, this needs a lot of knowledge and creativity. A whole cake from me... :)

300dt made it!(author)2016-09-25


Nice presentation and nice article!!!!! It is the simplest power supply for a Geiger Counter that I have ever seen!!!

You do however, need to label the power input points for those who are not comfortable with electronic projects. Also label the "ground" points of the schematic.

All the best to you!


hhking made it!(author)2016-09-20


I wanted to know if this is a good start

best regards

hhking made it!(author)2016-09-11

could do we have more details about the component please

hhking made it!(author)2016-09-11

I just bought her

-Russian TUBE Geiger Counter CI-3BG / SI-3BG NEW LOT 1

-20Pcs 0.022Uf 1kV 22nF 1000V 223 Ceramic Disc Capacitors High Voltage V Develope

-DIODE 1N4007 1000V 1A of1 to 20 parts

and for other parts I do not know what to take

tanner_tech made it!(author)2016-09-11

Do you have any broken things that plug into the wall? If you do, then I will tell you where you can find the components in each of them. If not, then I will give you Ebay links to all the parts.

hhking made it!(author)2016-09-11

please give me all parts on ebay


hhking made it!(author)2016-09-12

thank you

hhking made it!(author)2016-09-11

no, sorry I is not broke I prefer to buy on ebay

Tecwyn+Twmffat made it!(author)2016-09-10

Another cookie from me.

grayl made it!(author)2016-09-09

Give'm two. Even I understood this!

_Boltz_ made it!(author)2016-09-09

Give this Buddy a cookie, Great Instructable Buddy.

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