ts.
After making couple of crude mock-ups on component placement, I soldered them down and started a long and painful wiring operation. I made some bad mistakes during the project, and was forced to de-solder parts. Sadly this shows, as connections on final device are ugly and some solder pads detached from board and this caused other problems along the way.
Sadly the first version of a HV-supply did not work out. I used an old 555 timer IC (that tincan on the right) that drew too much current even when just sitting there. Also, I did not have the proper parts for HV, so I added some transistors, resistors and capacitors to the shopping list and went back to drawing board.
While having a pause from inhaling soldering smoke I made some parts for the case. Housing itself came from an old industrial wall switch (those white watertight plastic boxes). Making clean holes and cuts to ABS plastic did not work out as I planned and I wanted to highlight the user panels and the display. Hence the metallic switch plate and plastic display cover. Tools I had on hand were a hacksaw, a cordless drill and some files. Some creativity was needed and my methods probably would not be approved by any school or workplace (like hand-drilling holes to plastic by using screwdrivers and knifes). Considering that the aftermath was surprisingly good; all fingers still in place (just kidding) and even case for device in one piece and decent looking.
The LCD is covered with plexiglass which protects it from hits and scrapes and also gives it a bit more professional look when label is applied under to underside. I designed a simple front label with GIMP and laser printed it to ordinary paper. Another purpose for metal and plastic panels are to maintain semi water- and dust-proof features for the box. After all you do not want any radioactive particles inside the meter, do you?
The case had several versions too, and at this moment it still waits for the 3rd LCD plexpanel as one screw hole is in the wrong place and I tried to glue paper on it unsuccesfully. Lesson of this project is you should do your work properly, or you will need to redo it.
This tube can detect mainly gamma radiation and some beta particles though a thick plastic case may stop all the particles except the very high energy ones. The tube didn’t rattle, and seemed to be in good condition despite one small dent. I do not know if calibration still holds in, as gas inside may have decayed or leaked. This tube may be military specked as it has that diamond marking with 5 inside. If I remember correctly it had something to do with the military. Some other military surplus parts from soviet area have had these markings too.
For the power source I chose a lithium-ion battery cell. The 18650 cell type is readily available as it is used in many laptops, flashlights and so on. I recycled some batteries from an old laptop. For charging I bought a 5V USB-charger board from DX.com. It charges the battery while the device is connected to USB. When using a lithium-ion battery there is no need to discharge it completely before a recharge. It may even damage the device if the battery voltage goes under 3.2V or so. I have an alarm that goes off if the battery level reaches 3.2v, but it does not have a cutoff circuit yet. I have a plan to buy a protected lithium-ion cell that has built-in protection for under- and overvoltage. As the lithium-ion battery only gives voltage between 4.2V to 3.2V, I need something that iccreases it to a steady 5V supply. If the voltage changes, it messes up the contrast oft the display. The voltage of the tube may vary as well, causing some accuracy issues. I bought a small switching mode power supply from DX.com that increases 1-5V to a steady 5.2V. While the USB is connected, the device works with 5V from there, and the current from battery is not drawn. Otherwise the Li-Ion charger would not know when to stop charging. First it starts to charge in a full 500mA current, but when voltage rises it starts to limit the current with a constant 4.2 voltage. When the battery is