The UNO R4 Minima is not in this diagram since it only uses the standard ‘classic’ pins to the shield.
The only pins not used are 0 and 1 since they are dedicated to serial and A4 and A5 because they are connected to I2C, so the only spare is A3.
The Chinese SD module was really cheap, but its design is screwed up and does not protect the SD card from the 5V Arduino. Hence the diodes on the SD inputs. The pull-ups are useful but the fourth (not shown) was not connected to the SS pin, so I had to add one,
Power is controlled by a p-channel MOSFET which has less than 1 ohm resistance when on. Turn-on occurs by a momentary up movement of the 3-way switch, normally used to select sites. When the Arduino boots, power is sustained by D4, which later turns off the power under program control.
The keyboard is a 4x3 matrix which is polled.
The monochrome OLED graphics display did not come with instructions, but does work like the one in the Scientech meter.
The only components not directly soldered onto, or plugged into, the shield are the SD module and the battery. The SD plug is on the end of a pigtail and goes onto the SD module . These two are under a mounting plate,