then boss at CERN,
accelerator laboratory, was throwing
them out and I
Similar ones do
exist, see Reference
This project fits
a 2.75-inch by 4.6-
inch plastic box
with a compartment for a nine volt
the buzzer and the
on/off switch goes
on a small PC
board with cutouts
to clear the box's
If you cut
notches into the
bases of the front
two pillars and slip
the board into them
only two screws are
needed to lock the
board in place. Two
#2 nuts glued to the bottom of the case act as stand-offs.
The switch and buzzer mount inside the lid. Photo 1 shows
the box layout.
For maximum sensitivity the Geiger tube is mounted
near the top edge of the box. The detachable end panel of
the box should be left off or replaced by thin celluloid, it
stops a significant fraction of beta particles.
A socket removed from a female Cannon-D connector
slips nicely over the anode pin of the G-M tube. The cathode connection on my G-M tube is a little metal strap. A
dab of silicone rubber stops it moving around. Two more
tiny dabs on the glass ends of the G-M tube attach it to the
board. (Its metal walls are a bit thin to glue to.)
Photo 2 and Figure 2 show the board layout. As is my
normal practice, I used perforated prototyping board with
copper strips on one side.
FIGURE 2. The prototype G-M counter was built on a small piece of perf-board whose component
side is shown here. Thick vertical lines are bus wires on top of the board, horizontal lines are copper
traces on the underside. Diamonds show where a wire connects to the copper. Editable schematics
and a PCB layout are available for download on the Nuts & Volts website at www.nutsvolts.com
To cut back on leakage I put two Teflon insulated terminals through the board, one for the tube anode and one
for the rectifier cathode. The HV components are wired
between them. Even so, the tube voltage wilts on hot
It's important to keep the stray capacitance at the hot
end of the inductor as low as possible. I cut away unneeded copper strips under L1 and D2. Wiring the outside end
of the winding to + 9 V reduces the effect of capacitance to
One connection doesn't show on the board layout, a
bus wire on the underside of the board that grounds the
unused copper strips under the G-M tube. This stops the
HV pulses coupling into the input of the beeper circuit.
REFERENCE 1. Detailed G-M tube information.
The (G)Eiger Sanction
by Tom Dahlin, Circuit Cellar #150, January 2003
The standard way to set the working voltage of a G-M
tube is to apply a variable voltage and to plot the counting rate with a radioactive source present.
The plot should show a curve which rises from zero,
levels off for a while then starts to rise again. The center