HP-01 available

[PANAMATIK]

Thank you for complimenting the work.

Some thoughts about the 9 digit display.

If you have a look at the single 820 Ohm resistor of the display in the images. It has the function of limiting the forward current of the LED segments, giving them the right brightness. It controls a constant current source indirectly. In the new HP-01 it is not used, instead I needed 9 resistors of 220 Ohm (now 0 Ohm brightness is controlled by timing), which control the current passively. Theoretically the original method is the better, but I couldn't find a modern equivalent chip for it. And with help of software it behaves identical.

The original display driver hardware contained a constant current source to achieve identical brightness of the LED segments regardless of which segments are shown. They used a single resistor to regulate the LED current shown at the top of the display in the images. I couldnt find a suitable constant current source, and of course i couldn't develop a custom chip, so I decided to have a single resistor per digit, and software drives the digits in that way, that only one segment per digit will be light at the same time. After 9 multiplex cycles, because the digits have 9 segments including the colon dot, the cycles will be repeated. This leads to a constant current too and each segment has the same brightness.

If the battery voltage decreases at the end of its lifetime the display will gets darker, but when using SR44 cells, they have a constant voltage up to the very end of their lifetime, therefore I do not expect to see this effect. And if it becomes visible it is time to change the batteries.

Bernhard