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

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 04/19/05 www.tech-chat.de)


--Winston




--Winston