TI-H: Power expansion mod done.
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TI-H: Power expansion mod done.
Greets.
I spent most of my free time this past weekend working on calculator
mods. The one I thought might interest people is the power expansion
port I installed on one of my 85's. At Radio Shack I purchased a
package
of two 'Submini Phone Plugs' the kind with the little switch inside
(sorry,
I don't have the part number handy), for $1.50. The plug is a mono
plug
with three solder connectors. It is constructed such that the three
connectors
are arranged like a SPDT switch.
I installed the jack on the lower right side of the calc case. I then
removed
the positive battery terminal from the main board, and ran a wire from
its
contact over to the jack. I installed a new contact into the
receptacle on the
back of the case and ran a wire from it to the jack. Now the front
and back
of the case are wired together and battery power flows through the
jack switch.
I then connected the final connector on the jack to the negative
battery lead.
Now, when I plug in the 2.5mm plug, the main battery is disconnected,
and
the power from the plug is connected.
I considered installing a voltage regulator into the calculator, so I
could
operate on any DC supply from about 6v to ~25v, but it didn't seem
necessary,
and a standard 7805 was kinda big anyway.
The modification was pretty easy, the only disadvantage of having this
installed is that without more extensive modifications to the case,
the
jack sticks out a bit. Adding some material to the outside of the
case
to mount the switch in would make it look better. I may attempt this
when
I do my main calc (this one is practice model :). A smaller non-radio
shack
jack might work better, something similar to the link port jack would
be nice.
If anyone is interested, I'll try and get some pictures of the mod on
my
web page.
One thing that was interesting is that those unregulated wall
transformer
DC supplies (usually consisting of a center tap transformer, two
diodes
and a small filter cap) don't work too well as supplies. When the
calc
draws more power (like when graphing), the LCD goes very dim. This
might have to do with the bad filtering, but I'm not sure. I added a
larger
filter cap, and it didn't seem to help. I'll try a larger regulated
supply
later and see if that helps. Another possibility would be to just use
one
of those battery holders for 4 D size cells. A calc ought to run for
pretty
near forever on those. Also, you then have the advantage of a
portable
supply, but without a bulky battery extender attached to the calc.
I can provide a wiring diagram if anyone is interested.
DK
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