YATPGPSDO - Yet Another TruePosition GPSDO - Part 2: the hardware

The hardware for the GPSDO is very simple. There are two main parts: the power supply and the microcontroller.
All the design was done with KiCAD, design files will be shared in the last part of this series. 

The Power Supply

The power supply is nothing special. The TruePosition board needs 12V, with a peak current of 0.5A, which will decrease somewhat once the oven is warm and the heater switches off. The microcontroller and the display needs 5V with less than 100mA.

This does not need anything extraordinary. I already had nearly everything for the power supply, a 230V/12V transformer, bridge rectifier, capacitors and a 7812 and 7805 regulator. The only component i had to get was the connector/fuse holder/switch to the back.

The microcontroller

The controller part is similarly simple. There is a socket for the STM32 and there isn't much else. Two pushbuttons, four LEDs, three of them connected to the controller. There is also a transistor to switch the LCD backlight and one pot for the LCD contrast voltage. And a couple of headers for connecting all this.

The PCB

The PCB was designed to fit the available space in the instrument case. The transformer and the line voltage parts are on the left, carefully spaced for safety. The power supply is in the middle. Initially I wanted the regulators to go on the backside of the instrument for cooling, so they are both on the edge of the board, but later decided to use a heatsink, so they are connected via wires from their original place. Finally the right side hosts the controller the other small parts and the headers for the display, LEDs, etc. 

Manufacturing was done by JLCPCB, This is the finished board.

Assembly

For such a simple circuit, there wasn't much to get it working. I first populated the power supply part, and made sure that it works as intended. Getting into the case was easy, since the PCB was designed to fit perfectly. There was some fiddly work to get al the wires to the LCD, LEDs and switches, but eventually everything was there. I was considering putting pin headers everywhere, but then decided to simply solder the wires in place. I don't think I ever want to disassemble it and everything is accessible from above as well as from below.

I plugged in the STM32 and tested that it runs and drives the LCD properly. Then finally I connected the power and serial lead to the GPSDO.

It worked without any issues, although the software was far from completed at that time. The BluePill can be programmed in circuit, so I could now concentrate ot the firmware, it was actually easier to do testing in the assembled instrument instead of some breadboard state of completion. I used ST-link V2 to program, but the serial pins are also there on the PCB if someone uses serial programming. 

The next part will describe the software.