Dora the Datalogger

So quite a while back some PCBs arrived courtesy of Cambridge Circuits for the first revision of Dora, a data logger for use in our custom static fire test rigs. The device itself consists primarily of an STM32 processor, a micro SD card, and analogue front ends for a thermocouple, strain gauge and pressure sensor. We also included a USB port for debugging and some GPIO headers in case we wanted to experiment with extra functionality such as an LCD screen.


With the boards looking very nice, we got straight into component placement. Size wasn’t a constraint in this case so we opted to go with 0805 passives to make life easier for ourselves. Solder paste was applied using a stencil and then components placed with tweezers.


With that done it was ready for Dora to explore the inside of our reflow oven. The reflow process itself is rather straight forward and consists of heating the board up at a controlled rate until the solder paste melts and flows into the gaps between the board and components, hence ‘reflow’.


Having survived we then used the soldering iron to tidy up any bridges or shorts that occurred during the reflow process, and also solder on the connectors and other through hole parts such as the voltage regulators and debug connector. Some 0805 passives also had to be soldered on the bottom side, again this was done manually to save putting the whole board through the oven a second time.


With all everything assembled we gave the board a final look over and a clean.


With the board fully assembled it was time to power it up. The voltage regulation circuit was designed with a 14.1v li-po battery in mind, so we set the bench power supply accordingly and success – the green LEDs are on! We quickly flashed a program to the STM32 that just toggled the two status LEDs and it worked a treat. Looks like everything is functioning so far!


We had a thermocouple at hand and so were able to briefly test the functionality of its analogue front end. The output of our circuit is supposed to present 5mV per degree C, with 0 degrees C at 0V. The multi-meter gives a reading of 150mV which equates to around 30 degrees C. We then used a digital thermometer to confirm that this result was accurate.


So with Dora fully assembled and some initial testing complete were ready to start developing the firmware. I look forward to letting you know how we get on, and once again a huge thank you to Cambridge Circuits!

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