This version has a KIM 1 keyboard and display circuit. Download SinclairScientific1 from github to make it work. For the most accurate version, see https://oshpark.com/projects/OSfM0RLN

Uses an Arduino Nano or clone.

RKO/RKN are 4.7 K, this is just a path to ground for the ADC.
R1..8 are 1K, smaller resistors makes it brighter

Buttons are standard 6x6x5mm.
You can also use 6x6x4.3mm to make the keys a little harder to push, like the original calculator.
For more accuracy, use blue for numbers and white for everything else.

Uses 3 displays type: 2352B / 2352BS
https://www.aliexpress.com/wholesale?SearchText=0.25"+RED+7+Segment+display+3+Bit

Power Switch is:
https://www.aliexpress.com/wholesale?SearchText=L8.5*W3.5MM+3P+2T+Mini+Toggle+%2F+Slide+3mm

Power comes from a 4 AAA battery holder attached with double sided sticky tape to the back of the keyboard, The width of the holder should be 50mm or less

4 M2 hexagonal standoffs keep it off the table.

Source code:
https://github.com/arduinoenigma/ArduinoNanoSinclairScientificCalculator/tree/master/SinclairScientific1

Hackaday.io page:
https://hackaday.io/project/91895

Tindie Store:
https://www.tindie.com/stores/arduinoenigma/