HP-67 low power indicator circuit?

[John Garza (3665)]

Could someone please point me to a schematic for the low power indicator circuit for an HP-67?

Much obliged.

-j

[teenix]

It is built into the Cathode Driver chip. I don't think there is much you can do with it.

Tony Duell's circuit here, page 6

cheers

Tony

[John Garza (3665)]

Ah... no wonder I could not find the circuit. I was curious how the 67 did it as I'm thinking about building a low power indicator in a TI-58C. To TI's credit, they finally got around to a continuous memory model. However, the only sure sign of low power is a garbled display and by then it's too late. Defeats the whole purpose of continuous memory. I think I'll use a TL431 with a voltage divider to make it sense 3V or so. The 58 seems to work fine all the way down to 2.45V. A small LED in the corner of the display a la 67 will be the indicator.

[teenix]

Strange it wouldn't have one, to match HP models.

Maybe the low power indicator on the budget was glowing ;-)

[Steve Simpkin]

(07-30-2023 10:15 PM)John Garza (3665) Wrote:  Ah... no wonder I could not find the circuit. I was curious how the 67 did it as I'm thinking about building a low power indicator in a TI-58C. To TI's credit, they finally got around to a continuous memory model. However, the only sure sign of low power is a garbled display and by then it's too late. Defeats the whole purpose of continuous memory. I think I'll use a TL431 with a voltage divider to make it sense 3V or so. The 58 seems to work fine all the way down to 2.45V. A small LED in the corner of the display a la 67 will be the indicator.

Yes even the manual states that turning off your TI-58C immediately when it is acting erratically *may* (or may not) save your constant memory contents. Also note that the calculator may be damaged if you connect the charger without the battery pack installed. They probably did not need to copy that *feature* from the HP Woodstock/Spice models.
   

I also found it interesting that TI chose to use an off-the-shelf CMOS RAM chip from RCA, Toshiba or NEC in the TI-58C instead of making a CMOS version of their existing memory chip used in the TI-58/59. To use this commercial RAM chip they had to make an additional chip to interface to it. Because of this there is quite a difference in the circuitry between a 58 and a 58C.

http://www.datamath.org/Sci/WEDGE/TI-58C.htm