BUSICOM LE-80A Calculator from the 70's: Texas "Calculator on a Chip" implementation

[jebem]

Dismantling the machine.





Despite being so small, it packs no less than 4 printed circuit assemblies interconnected by the good old wire harness:
- Keyboard
- Processor
- Display
- Power Supply and Timing Clock





As I was in the mood to look a little deeper in the circuit, I hand designed a partial schematics by direct observation.

Despite being a standard design, it doesn't use the conventional driver ICs commonly seen in most implementations with this TMS0105 chip.
They drive the LED segments anodes directly and use 9 x discrete 3796-28 Si NPN bipolar darlington transistors to drive the LED cathodes.

Current consumption using fresh batteries (4x1.5V) is high for such a tiny machine:
- One single digit lit: 140mA
- All digits lit: 190mA



Battery holder showing the corroded contacts.