These circuits generate high voltages and can cause dangerous shocks! Do not build these devices unless you are experienced and qualified to work on high voltage devices.
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Also see the World's Smallest Geiger Counter (Area 50)
The difference is subtle; the feedback signal increases the voltage on the base of the 2N4403 to stop the oscillator instead of stealing current from the capacitor on the emitter. The result is much lower power dissipation when there is little or no load on the high voltage. The new circuit draws less than 1/2 mA when operating at 9 volts without a load using a 1:1 600 ohm audio isolation transformer. The 3 volt circuit may be modified in the same way but make sure to switch to a MPSA18 (or a similar very high gain transistor). The 120 volt zeners are also an improvement over trying to grade ordinary diodes; grading is just too much trouble! A 1N5273A is a typical type to try. Remember, this circuit can only supply a few microamperes so an ordinary 10 megohm voltmeter will load the output too much. (500 volts/10 megohms = 50 uA.)
The transformer in the prototype is a small isolation transformer with opposite ends of the primary and secondary connected together to boost the output voltage. Other transformers will also work, including tiny audio interstage transformers, as long as the impedance is relatively high on both windings. If you don't get a high voltage, try reversing one of the winding connections. If the current doesn't cut back with no load, try the techniques mentioned in the note above. The circuit will work without the secondary connection simply by connecting the collector of the MPSA42 directly to the first .02 uF cap. and diode and leaving the secondary winding disconnected. Using the two winding voltage boost is recommended when attempting to run the circuit on a lower supply voltage.
Here is my favorite 9 volt version using only a molded choke (the ones I have in quantity):
Other combinations of zener diodes and gas tubes may be used to get the desired output voltage but the upper limit is about 1000 volts.
The 0.1uF capacitor on the base of the MPSA18 will reduce the current consumption when using Lumex or some other types of gas tubes and some zeners. It is unnecessary in most cases but the right value or no cap at all can push the no-load current really low, depending on the characteristics of the selected breakdown devices. This circuit will operate on lower supply voltages but, at some point, it will not "cut back" and will draw several mA all the time. The transformer version can more easily generate higher voltages when operating from lower battery voltage. Keep in mind that the current consumption of this circuit is very low when operating properly and a 9 volt rectangular battery will last a long time, giving perhaps a month of continuous duty.
The output voltage can vary by up to 20 volts from no load to a light load due to the measurement of the voltage before the voltage doubler. The regulation is much better once a little current is flowing. For much more precise regulation, connect the zeners or gas tubes and resistor from the base of the MPSA18 directly to the output. You will need twice the breakdown voltage, however. I connected four neon lamps, type WL-1MH (similar to NE-38), in series with the 10 megohm from output to base and the resulting voltage was 540 volts and only changed 7 volts when loaded by 300 meghoms. Switching over to 4, 120 volt zeners gave an output voltage of 520 volts that changed less than a volt when loaded! The current drain seemed to improve in both cases, dropping to 200 uA with no load. If you don't mind using twice the number of zeners, this is a nice modification. For fun, I replaced one of the high voltage zeners with several lower voltage zeners until I hit exactly 500 volts. It took 6 zeners to do the job and I suspect it's overkill for most projects. But if you want a precise voltage and have a sufficient pile of parts, why not!
Below is the older design but it works fine, too. It draws a little more current when unloaded, typically 1 mA. A choke should work in place of the transformer, too, with similar limitations. Ground unused gate inputs. The '4093 may be a '4069 hex inverter.
Here is a 700 volt or less power supply for powering a small Geiger tube or other very low current device. The circuit is very efficient when no current is being consumed which is typically the case in a Geiger counter. Geiger tubes draw about 100 uA when they pulse but the pulses are very short and relatively far apart (hopefully). The current drain from the 9 volt battery is less than 1 mA with no load. The circuit will supply a bit more current when operated from 12 volts. The output voltage is set by the string of devices that includes the neon bulbs. Select a combination of neon lamps, varistors, zeners, etc to achieve the desired voltage. Good results may be had by selecting one or two ordinary small-signal silicon diodes with the desired breakdown voltage. The current in the diode will be quite low and no damage will result.
The circuit shows the power supply in a typical Geiger counter circuit but it may be desirable to use a larger resistor from the high voltage to the tube - see the manufacturer's recommendations. A 10 megohm is a safe value for most tubes.
Note: This generator cannot supply much current. An ordinary 10 megohm voltmeter will pull the output voltage quite low; a very high impedance voltmeter is needed to directly measure the output voltage. A 1000 megohm resistor in series with a 10 megohm voltmeter will give a 100 to 1 voltage reduction and will not overload the circuit. Large Geiger tubes may also draw too much current in moderate x-ray fields so a more robust generator may be needed. (See the 3V version below.)
The other two gates in the '4093 may be used to construct a beeper that will drive an ordinary piezo speaker:
The 1N914 (or any similar diode) connects to the output of the last gate above. When a pulse triggers the first gates, the diode becomes reverse biased and the oscillator is allowed to operate for a split second. A '4069 hex inverter will also work but ground the unused inputs.
Here is another version that will supply more current for a larger Geiger tube and runs on two cells (3 volts).
The above circuit was used to construct a home-made Geiger counter employing a 10 inch Geiger tube (LND 78014). The pulses from the tube were converted into 3 volt, fixed-width ( a few hundred microseconds) pulses suitable for averaging using the following circuit (updated 5/5/04):
The pulses may be counted with a digital circuit or may simply be averaged to apply to an analog meter:
The indicated switch is a two-pole, six-position switch. The resistor values in the meter circuit were selected to give the proper reading on one of the two pre-existing meter scales which differ by a factor of two. The older meter required 60 uA to reach full scale even though it was originally a 50 uA movement. In the actual prototype, an additional position and pole were included. The third pole switches in a 1 meg to ground at the input of the op-amp to reduce the signal by two for one more range. the same 19.6k resistor is used for both positions. In a new design, the op-amp and meter circuit would be modified for the chosen meter and desired scales. The op-amp must be one that operates on 3 volts or less and must have a very high input impedance.
The Geiger counter prototype was built into a homemade wooden box:
The tube is mounted with cable clamps over a hole in the bottom of the box covered by a perforated aluminum shield. The holes in the perforated aluminum directly below the tube were drilled to a larger diameter to block less radiation.
The voltage multiplier is mounted on the underside of a Plexiglas subchassis:
The other circuitry is mounted on two pieces of laminate on the top side of the subchassis:
The front panel holds the meter, switches, resistors, speaker and 4.7uF capacitor.
The wiring gets a little messy when the end of the project is in sight! It was discovered that the connection from the capacitor to the op-amp must be well insulated. Bend the op-amp positive input lead up into the air and directly solder the wire. Add an additional low leakage small value film capacitor (0.1 uF is fine) to ground at the op amp plus input since the 4.7uF is some distance away.
The front panel is also made from lexan painted on the outside with metallic hammertone paint.
The performance of this detector is quite good. The background radiation near 0.016 mR/hour give a 2/3 full-scale reading making slight changes in the background easy to see. The extra-long averaging time (44 megohm) gives a very steady reading, albeit slow to respond. This is a sensitive counter and is easily "pegged" with an ordinary test radiation source or even a Coleman lantern mantle.
Techlib reader Dave Mouat modified the circuit as shown below:
In the diagram I've put the transistor numbers as the ones I've
used, although you can use standard 2n3904's etc in these positions without
problems as they're similar spec. The transformer used is a normal audio output
transformer, such as an LT700, and the transistor driving it in this case is a
Darlington pair device, number BD675 (not sure of any equivalents here, but I
imagine its not critical)
The BAT43 diode is just a Scohttky device, and was used simply because of the
low forward bias voltage drop. You'll notice another diode and transistor across
the main drive transistor, this simply derives a voltage high enough to fire the
clicker and LED, in practice its about 12-16volts, quite a useful value if we
decide to run anything else from this supply at a later date. The clicker (drawn
as a speaker) is one of those low impedance devices you see in cordless phones
or kids toys for a ringer, works very well in this position, a nice loud click.
The shutdown transistor (connected to the string of zeners) I've
found needs to be something with a reasonable gain, although the one I used
seems to work fine, regulation would probably be better with one of the MPSA
devices you've used and a higher value load resistor. This is a great way to
reduce power consumption, as in this situation the circuit is firing when
(output voltage/2) falls below the threshold, a better way would be to try and
measure the main output voltage directly, although I've had no luck with this
method. The capacitor which drives the base of the main drive transistor (BD675)
is quite critical, it will work down to 1uF, but don't go above 3.3uF, as the
power consumption went through the roof when I tried this, the LT700 was
obviously becoming saturated.
The badly drawn GM tube that I used was a SBM21, a tiny little tube only 21mm
long (can be seen in the top left of the photograph of the unit), i got mine
from a Russian guy on eBay, unfortunately they're not very sensitive at all,
background counts on this tube are about 4-8 clicks per minute, whereas my
CD-V700 and DRSB-01 both disagree and give something closer to 2-4 times that
value. Still not bad for such a tiny counter.
At one point I tried a small transformer from one of those cheap disposable camera's, this worked and gave reasonable efficiency but whined like crazy, i prefer the off-the-shelf LT700 audio transformer. Also, because I built mine on Vero-board i found that I had to put a 470k-ohm resistor from the base of the first detector transistor down to ground, as the board was conducting and turning it on. I expect there are probably many modifications that could improve the circuit, so you can post it on the site if you like and maybe it will help someone else.
