With not being at work today, only supervising my youngests home schooling, and needing something to relax me and take my mind off other worries, Ive had time to build the RH Electronics Geiger kit.
I will discuss a few issues ive found with it in due course.
The first task was to solder the connections between the main board and the LCD board. To do this, I decided which side to put the pins on, put the connectors together, sandwiched them between the two PCBs, fastened the boards together with the pair of stand-offs supplied, and soldered them in. This meant that I could then remove the screws, and separate the two boards, with the connectors now in perfect alignment. Note that there are four unused pads on the LCD board, and three pads below on the main board - dont connect these together!
Now, with the boards separated and the LCD safely back in the box, the resistors were installed. Here I found the first bugbear - the spacing between the holes is very tight with the size resistors supplied. This wasnt a big problem with the low voltage side, but the HV 10Mohm resistors are even bigger, and had to be installed as you can see in the photo below. Ive 'zig-zagged' the measurement chain resistors to help them fit more neatly and to offset the nearby bare metal leads.
While fitting the resistors another pet hate came to show itself - most of the silk-screen part numbers get covered up by the components! There is a layout diagram available to download, but I still personally dont like it. Also, why the diagonal parts? I can only think that this was done for some aesthetic rather than electrical reason!
Next up were the right-angled header pins, which allow for 5V supply connection and the TTL interface lines, and also the 400/500V selection jumper. Along with those, the IC DIL socket, push buttons, and clock crystal were fitted at this stage.
With the build coming along nicely, time for the ceramic capacitors. Here I found that the pitch of the holes on the PCB was much smaller than the pitch of the preformed leads on most of the supplied capacitors! All but the two 22pF for the crystal required their leads reforming to fit, an annoying and fiddly task that shouldnt have been necessary if the correct pitch parts had been supplied.
And with the ceramics fitted, next went in the electrolytics (note that the polarity symbol on the PCB is tiny!) and the inductor. The inductor needs its leads bending at a right angle so that it can lay flush to the board, as shown.
At this point - i stopped for lunch! (beans and sausages on toast, if anyones interested).
So, after lunch, the semiconductors were installed. Three transistors and three diodes. One of the transistors is a high voltage MPSA44 which drives the inductor to generate a high voltage pulse, which is then multiplied by the diodes. Here also, it was found that the pitch was insufficient for the diodes to be mounted flush, and so these are also positioned vertically.
Almost done! The two LEDs, one red, one blue, but both unlabelled and clear cases! Luckily my multimeters diode test function is capable of lighting LEDs, otherwise I would have had to find out my LED tester! I decided to mount these a little proud of the board. The buzzer and the screw terminal for the G-M tube connection went on next, and then finally the high voltage capacitors.
After a careful inspection of the board and my solder work, I decided I was happy with it and would now insert the microcontroller. Several rounds of check and recheck here to be absolutely certain it went in the right way around, and that no pins got bent under.
Now, there is space and pads for a battery connector, but no pins or socket supplied. So one was found out from my stock of components. Its a bit of a tight fit, such that it couldnt be disconnected with the LCD board fitted.
Here though is my big disappointment with the design - there are ample connections for ground and 5V on the header pins, so why have a 'battery' connection? 5V batteries are not exactly common! There is ample space on the board, and in fact already suitable filtering and bypass capacitors, for a 78L05 regulator to have been built in! That would have made the 'battery' connection a much more practical 7 to 30V battery input! (I wouldnt recommend running one at 30V though, it will get rather warm! But a 9V PP3 or a 12V supply would have been great for the sake of a single extra TO92 package!). I will probably make this a modification once the unit has been thoroughly tested.
Another modification i may make, is to make up a ribbon cable for the LCD. As it is, the unit is small but chunky, which might make fitting it into a suitable case for portable use tricky. Likewise, when in a case, the LEDs will have to be moved off board. The buttons of course can just be paralleled with a pair chassis mounted.
Because the G-M tube connection is by a screw terminal, whose screws are hidden under the LCD board, I have only carried out a basic functionality test so far.
Details of testing and results will be in the next post!
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