GM-04 Tube tester as portable geiger counter

(PDF manual here)

There seems to be very rare information about this board since it was a prototype,sometimes with buzzer soldered on- in this case without
but its not connected so it even makes no sound when its present, no plans or sourcecode available from the developer :(

modifications made with the help of engineer massimo (italy) ..thank you !!!!

*hint* : the blinking questionmark "?" on initial display means that the measured value in the first 2 minutes is extrapolated due to the short measure time
also the counter seems to be additive when turn over 65535 cpm (~ 220µSv) ,it starts from zero..just add the cpm to 65535,
if radiation decreases over the time, the counter falls below the limit -> it jumps from the higher level to 65535 and decreases from this point

at first move the jumper on the left side for input power selection to bat since we need a battery, i decided for Lipo type, 3,7v/ 800mAh..BAT input testet from 3V-4,5V,
has dc-dc converter, (+) is the lower pin, the DC plug on the top side works from 5V-12V, as you can see a piece of phurcalite (UO2) will keep the tube busy.....

the lcd part is very i put some standing-screws at the board and add some vibration canceling rubbers to lift this side up

when used with battery we can achieve longer running time when lowering power usage of the backround lighting, without mod the board needs 174mA@ 3,7V
bend left pin 20 and solder on 50 or 100 ohm resistor to pin20 on lcd part, more ohm means less power but darker lcd..with 50ohm it needs 105mA

changed the tube limiting resistor R20 from 2Mohm to 5.1Mohm .. more compatible to the max. voltage specifications of the sbm-20 for longer lifetime
(repeat this on R22 for second slot), i had no smd of this value so this little brown bugger will help out this time ;)

in my case there was no buzzer so i get the active buzzer "ekulit RMP-14SP" with internal circuit so it sounds when voltage is applied..soldered on ,the "+" icon must show
to the plug it logo

as mentioned in the manual the buzzer (if there is one ) was not the buzzer needs to be powered...soldered 2x 11kohm resistors from U3 and U5 to R25 so Q2
can drive the buzzer when one of the green leds flashes

on this photo you also see that the U3 and U5 and other parts are sanded to hide the part id´ they should be (without warranty)

U1 PIC18F4550
U2 is TPS2014
U3 & U5 are MC14011
U4 MAX642 U6 is MAX756
U7 is AMS1117-5.0
Q1 is UTC 4N60
Q2 suggest to be a BC817-40 or better BC847-40
D1...D5 are US1J
D6 is SS14
other diodes suggest to be common 1N4148

build transparent box made of PS ,fixed Lipo under the display-board with charger plug to the back, buzzer and buttons elevated with screws and shrink-rubber,the rubber
shrinked first on a test button then fixed with superglue..(since this degas and block contact of the buttons sometimes..flush with isopropyl alcohol from top, then suck out
with a thin piece of paper)

almost finished..need only a power button....

buttons made from peeled and drilled empty capacitors


update: 2nd tube installed, STS-5 (upper one in GT1 slot), seems a bit (2mm) longer, NOS ...made in 1975, mostly the
difference between the tubes is 0,02-0,03 mySV but on the photo they aren´t coincident in opinion...


the manual for the sts-5 tube..can celebrate its 40th birthday in 2 years...

built in a switch and plug to connect a external probe (switch between lower GT2 and external)

this is the SI-15BG alpha+beta+gamma probe,very sensitive, build the probe in a handpiece from sticky roll-on fabric cleaner, resistor with 33Kohm added


the GM-04 and external probe can be attached to this crutch for easy ore hunting