After a lot of revisions, changes, corrections and doubting, I finally decided to commit to a first board production. We will see if this comes out good. The design is still untested. Just reporting on the progress (which is glacially slow).
And in less than a week please meet the revision 1.1.
This release addresses annoying NFC bug introduced in rev. 1.0. Such, to cleanup schematic I've moved NFC capacitors to a dedicated space where the C18 was turned upside down in order to improve text readability. This led to C18 net changes which were overlooked for the PCB. After zone refill it resulted in tying the GPIO_P0.10/NFC2 to the ground thus making the whole NFC antenna defunct.
From the good, this revision adds highly requested mounting hole to the main board:
Have a nice time!
@NeverDie The cell probably is not so sophisticated as the one installed in the fx-260. Also, from four visible sections only three are working. Sigh.
It seems the calculator can acceptably work as long as it supplied with 1.3V or more. This roughly estimates to 40 lux for this particular PV cell. However, the display is the resource hog - every additional digit causes voltage drop. Usable number of digits - five or less. After reset the calculator draws 3ĀµA. Every "8" adds up 0.1ĀµA thus reaching PV short circuit values very soon. The IC seems completely unregulated - at higher voltages it draws proportionally more, up to 1mA.
It's also possible to gain little bit more power (about 70mV) if bypass polarity protection diode. However, it might be rather better to employ 10ĀµF capacitor to compensate for keypress actions. With 92ĀµF it's also possible to enter longer numbers, but you have to type them fast, then store the number in memory, then quickly clear display and wait until the capacitor charges. Now you can type another number, press operation, and, finally, MR and = to calculate result
@ceech
I really like this design, however I think it has a problem:
According to the datasheet the TPS610986DSET has no feature to disconnect the load at low battery voltages. I think the result will be that e.g. a ESP32 or other power hungry chips will not power up properly. The TPS63031 might therefore be a better fit.
btw.: Do you still sell assembled versions of this design? Would you be willing to create a new version with the TPS63031 and probably also with 10 or 20F capacitors or an board to solder your own capacitors.
Best regards
Tobias
Started designing the enclosure for 3d printing ... I will use PET-G as it's pretty strong and not very flammable or so it seems after some tests. I don't know about electrical insulation properties yet ...This all very depends on what other compounds are used when it's manufactured.
