On Nov 29, 10:05*am, Winston wrote:
wrote:

(...)

The cell voltage is 1.55v, and it's nearly dead @ 1.2v. *A MOSFET with
Id = 100uA @ Vgs = 1.2v would be fine. *A BSS138 is in that ballpark;
there might be better choices.

Hopefully! *I don't see a gate threshold figure for Id ~100 uA but the
worst case (Id=1 mA) gate threshold for that part is 1.5 V. *http://www.fairchildsemi.com/ds/BS/BSS138.pdf

Id=1mA @ Vgs=1.3V (typical). That's plenty. There are certainly
others, but Fairchild's search function doesn't work for me.


I like an elastomer carbon button driven by a threaded
knob that makes contact between the '+' side of the
button cell and the caliper cell contact instead:

* * * * * * * * * * * *| ---- *Caliper
* * * * * * * * * * * *| * * * *'+' contact
* * * * * * * * * * ----
* * * * * | * * /--------- *-- Single sided
* *.---. *| * * |.--------. * * Flex PCB
* *| * | *| * * || * * * *| * * interposer
* *| * | *| * * || * * * *..
* *| P | *| * * || * * * *||
* *| o | */\/.-.|| * * * *||
* *| w |-\/\/| ||| * * * *||
* *| e | * * | ||| * * * *||
* *| r | * * | | | * * * *|| Caliper
* *| * | * * | | | Button || '-' contact
* *| K | * * | | | * * * *||
* *| n |-\/\/| | | *Cell *||
* *| o | */\/'-' | * * * *||
* *| b | *| * * *| * * * *||
* *| * | *| * * *| * * * *||
* *| * | *| * * *| * * * *||
* *'---' *| * * *| * * * *''
* * * * * | * * *| * * * *|
* * * * * | * * *'--------'

(created by AACircuit v1.28.6 beta 04/19/05www.tech-chat.de)

--Winston

The knob could drive a screw, compressing a leaf spring contact onto
the cell. Or, use a slide mechanism, if you prefer. Viola.

--
Cheers,
James Arthur
--Winston