This device has a 4-layer PCB, which is 6x4cm in size. I've ordered 5 pieces on Allpcb, which costed 33 USD and 21 USD for shipping to Germany. The price is okay for prototyping, since home-made is no more an option and local PCB manufacturer require 200 Euro for that thing. Actually I have other PCBs to order the same time, so the shipping cost is also acceptable.
For 50 pieces of PCB, the average cost per PCB is only 1.3USD at Allpcb, so it's really a matter of prototype or production.
There are approximately 40 different components and 90 parts on the PCB. The types of the components will be reduced if new version is to design. I've got most of them already in my boxes at home, after long time collection for different projects. However most of them could be ordered at Mouser, 90 percent, which still sells to private customers in Germany. The BOM cost is for the current version still high, if only 1 piece is to build. For 1 piece they cost about 14 Euro for 93% of the BOM at Mouser.
The BOM list can be found in the uploaded files.
Soldering of SMD components at home requires patience. If one is skilled on soldering, then the only difficulty on this board is the transistor, which is quite small and has a thermal pad underneath. I would like to replace this component with a SOT package for new version.
A magnifier is quite helpful. Most resistors and capacitors have 0603 package, which are still ok for hand soldering under magnifier.
The current version has two-side placement due to size limit. However it would be possible to place all components on top side for the new version, leaving only some through hole components.
4Test the power supply
One can measure the 3.3V before soldering the MCU and opamps , or after all components are soldered. There should be no critical problem regarding power supply since it's only LDO.
The MCU STM32F072 has a boot mode called DFU to easily flash the MCU with USB and ST software. To do that one has to bridge R3 with 0 ohm or solder, note that the schematics has wrong value regarding R3 and R4. R3 is DNI, and R4 is 10k.
Then one can download and install the ST software called DfuSe, which has part number "STSW-STM32080"
There is a guide from ST on how to update firmware with DfuSe.
The firmware is a binary file with .bin format.
To simplify the design, the device does not support bi-pole connection, which means negative voltage is absolutely forbidden. For battery one just needs to connect the positive terminal to '+' on board, and negative terminal to '-'. If the power supply is another device, one has to pay attention to the earth potential, the SimpleUI-Logger has the earth connected to USB GND and shielding, and then via PC to protection earth.
There is USB isolator on ebay, which is not cheap but nice to have if one usually deals with power.
I'm not a programmer, but it's neccessary to have one solftware to log the measurement data. So I've struggled to make one with Qt without many nice features. But it does have one which is important for me, which is to store the data in csv format. Then I can visualize the data with other software or script, such has GNU Octave.
The software allows to set the sample rate to fixed 1SPS, 10SPS, 100SPS and 1kSPS.
8Bugs and design failure
It's really a quick design, including hardware, firmware and PC software. And I'm not sure if I would keep on testing it and improving it, since the current prototype is already satisfying for me. I'm not good at programming, so the software is completely open for the ones who would like to fix bugs and implement new features.
But I would keep on improving the hardware, to make it low cost and reliable, since that's what I'm really interested in ;-)