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Category: Geiger counter

Geiger Counter Audio Circuit Board

Geiger Counter Audio Circuit Board

This post shows the audio circuit portion of the radiation detector in the video, for the simple high voltage power supply portion of the circuit click here

Quick overview:
Q1/Q2 form a one shot that stretches the pulse generated by the Geiger tube output that is coupled by the 1nF 1KV capacitor. The pulse coming from the Geiger tube is very narrow and would not produce a nice sounding click if we just amplified it.

Q3/Q4 form the audio amplifier, the 500 ohm potentiometer connected to the collector of Q4 is for adjusting the volume of the clicks. The 8 Ohm speaker can be any speaker. I used a speaker from a cellular phone ringer and I have used one from an old PC case.

There is an optional meter out that can be connected to an analog meter circuit that will average the counts using a rc circuit (to be discussed later if there is interest).

There is an optional pulse output that will put out a pulse that will pull-down (go from 3V to 0V back to 3V) for every click that can be used for connecting to a micro-controller so that a counts per second display can be used. This output can be utilized if the logic level NMOS and R8 are populated (to be discussed later if there is interest). Also if you do not populate the R8 resistor you can use the fet as a level translator, for instance if you are using a micro-controller that needs 5v logic levels. Connect the fet drain to the micro-controller line with a 5V pull-up resistor and you have an instant level translator.

For a full size image click this image, then click image on the following page:

This is the circuit for the audio board of the Geiger counter in this video

Step by Step Hacking a Disposable Camera Flash Unit to Power a Geiger Tube

Step by Step Hacking a Disposable Camera Flash Unit to Power a Geiger Tube

First off let me stress that working with high voltage is DANGEROUS! Only attempt this if you are very familiar with working with high voltages. Always use the one hand rule when working with the circuit, one hand in your pocket and one hand can only be used to work on the circuit!

Just in… We just received a small supply of these disposable camera flash boards, now available while the stock lasts….
http://www.madscientisthut.com/Shopping/agora.cgi?product=Radiation%20Detection&user4=Camera%20Flash%20PCB

This is a step by step guide to modifying the disposable camera flash unit that powers the Geiger tube in the last post. Geiger tube power supply hack using disposable camera flash unit

Before we get started:

Here is a top view of the unmodified circuit board

Disposable Camera Flash Unit
Disposable Camera Flash Unit

Here is a bottom view of the unmodified circuit board

Disposable Camera Flash Unit
Disposable Camera Flash Unit

Here is the schematic of the board

Short out the bulk storage capacitor before starting, to make sure it is discharged!

Step one, two and three.
Remove the flash activation (discharge switch) wires.
Remove the battery holder.
solder power wires to the circuit board.

Step four.
De-solder and remove the bulk storage capacitor.

Step five.
De-solder the trigger transformer and flash tube wires. (yes I know, I typo-ed the picture annotation 😳 )

Step six.
Remove the flash tube and trigger transformer from the circuit board this can be done by gently squeezing the reflector assembly then pushing it carefully out of the circuit board

Step seven.
Reuse the capacitor from the trigger assembly as a output filter capacitor. Remove it from the board and place it across the traces from the bulk capacitor. This can be done by simply rotating the capacitor across to the nearest hole that is connected to the other leg of where the output capacitor was. (see image)

Step eight.
Remove the on switch (charge switch) arm

Step nine.
Place a 10K potentiometer across the charge switch trace (trace high lighted in blue)

Step ten.
Connect output wires to where the bulk storage capacitor was. (Argh…another typo in the annotation 😥 )

Here is a schematic of what was just accomplished:( 😯 oops another mistake, the switch S1 that is in the schematic is no longer there, it should just be a connection from the potentiometer to the base of the transistor)

Preset the potentiometer to mid point ~5K ohms.

Connect a multimeter to the output wires. Make sure the meter is in a high voltage range, the meter I am using is set in to 1000VDC range.

Next step is to power the unit from ~3.0VDC, the power source is up to the end user. I am using two AA batteries.

Carefully adjust the potentiometer to get an output that will power your Geiger tube. The Russian surplus SBM20 runs fine at 280-500V , so I chose to set my output to ~340VDC because at this level it draws less current from the batteries.

The next post will describe the ballast resistor and audio circuit that is in this video:

Geiger tube power supply hack using disposable camera flash unit

Geiger tube power supply hack using disposable camera flash unit

Just in… We just received a small supply of these disposable camera flash boards, now available while the stock lasts….
http://www.madscientisthut.com/Shopping/agora.cgi?product=Radiation%20Detection&user4=Camera%20Flash%20PCB

I bought some surplus Soviet geiger tubes off ebay for about $20 each, and wanted something to do with them. My aim is to build an Arduino shield that includes a GM tube, but for right now I just want to get it working.
The tubes are about 1 cm in diameter, by maybe 15 cm long, and come packed in very old tissue paper in very old boxes.
There are lots of schematics for DIY power supplies on line, and many are really snazzy. But I’m a hacker at heart, and had a couple old salvaged disposable camera flash units I’d saved because I had dangerous ideas regarding the capacitors. Well, I still have dangerous ideas but in the meantime Kirk and I hacked a couple of the boards up to serve as sub-$3 high voltage power supplies.
Here’s a schematic for the boards pre-mod.

schematic for unmodified flash unit
schematic for unmodified flash unit

Here are two shots of the top and bottom with annotations so you can follow what I’m describing.

annotated topside of flash charger board
annotated topside of flash charger board

bottom side of flash charger, annotated
bottom side of flash charger, annotated

For a step by step guide to mod the disposable camera flash click here

First off, you’re going to be working with high voltage. That big electrolytic cap has a *lot* of energy stored in it. The two long leads discharge the flash, and have the full flash voltage across them. Test the board with an AA battery and make sure you can hear it charging when you push the charge button. Carefully short the discharge electrodes and you’ll get a nice flash. Yay. Now cut them off because they’re just dangerous.  Short the big electrolytic cap’s leads to each other with a screwdriver just to make sure it’s discharged and then cut the cap off the board.

Remove the xenon lamp and the pulse transformer. Note where the black wire connecting the xenon lamp connects into the board when you remove it, because the next thing you do is desolder the lead of the remaining cap, that’s closest to the xenon bulb, and shift it over to where the black wire was. The cap is now going from high voltage to ground: it’s doing the same thing the electrolytic cap did, only with vastly less oomph so you don’t fry your geiger tube.

Now you desolder the piece of metal that acted as the charging switch, and where it was, you solder in some wires that go to a potentiometer in series with a power off/on switch.
And finally you add a 1 megohm resistor from the high voltage section, that’ll go to your geiger tube, and between the resistor and the tube, you put, in series, a small high-voltage cap, like a few nanofarads, and a 20:1 or thereabouts resistive divider to ground.

schematic for modifications to the power supply
schematic for modifications to the power supply

You can vary the output voltage both by varying the input voltage, and by varying the potentiometer, which controls the base of the transistor. The pot is nice but the transistor/transformer couple are somewhat tuned and you can quickly get into an area of enormous current draw if you mess about much with that. I’m using a 10K pot but it’s only actually at like 100 ohms.

Geiger tubes like somewhere between 400 and 1200 volts. The ones I have are specified at 900-1200 volts, but are working fine at 800 volts.

Gammascout.com has a nice diagram of sensitivity vs. voltage, that makes it clear why you want to work in the midrange of the tube’s sensitivity, here:
http://www.gammascout.com/geiger-counter-design.html

Right now I’m detecting the geiger counter activity using an oscilloscope. That’s not very realistic for portability, so you’d want to use a one-shot audio amplifier similar to what commercial geiger counters use.
Here’s a neat design using a 555 timer and a transistor to drive a speaker:
http://www.galacticelectronics.com/GeigerCounter.HTML

This page has a couple other suggestions for pure-transistor-based audio outputs:
http://www.techlib.com/science/geiger.html
The version we put into a Victoreen CDV700 survey meter (which originally was supplied with only a meter) is similar to that second one, and fits nicely on a 2x2cm board.

This is the audio section I’ve used in the CDV700:
http://www.cs.utah.edu/~hatch/images/speaker1.gif

Here’s a video of the geiger tube actually running, that I have to post on YouTube because of its size.