| Damn, those pins are tiny! I won't
tell you how hard it was to solder to them, but you get the idea from how
hot the wires got. Compared to the nice fine traces on the motherboard,
the individual wires from some cat 5 cable look big and clumsy. A
bit of hot melt glue holds the wires from going astray without making the
bond too permanent.
As you can see from the map of the connector
legs, you need to access 5 pins on the connector; 4 sensor inputs and a
+5 volt lead. I soldered in the 5 wires (with some difficulty) and
reassembled the system to check the results. All I got was a constant
reading. After taking the laptop back apart and checking the connections,
I realized that the motherboard might not have all of the necessary components
installed on it to make the gameport work. Since I had no schematic
for the board, this was going to take some guesswork. (Oh boy, I
get to experiment with my new computer....)
Remembering that a game port works by a
558 timer (4 - 555 timers in one package) and that each of the inputs to
the timer requires a capacitor to trigger the resistance reading, I set
out to check whether or not the caps were installed on the motherboard.
This proved to be an exercise in futility. The traces for pins 55
though 58 were buried in one of the layers of the board that I could not
see. There was no way to follow them to check for components on each
path.
Time to trust in my intuition. I
surmised that the lack of caps was the probable reason that I was getting
the bad readings. I grabbed a spare sound card with a gameport output
and searched for a value for the caps. This ended up being .01µF
on the sound card I had. That sounded like a good place to start.
I made up a small circuit board using the
schematic above, and attached it to the top of the motherboard. The
circuit requires a ground, which I picked up from the USB connector.
This had a nice large pad to solder to. The circuit board has 6 wires
out to the motherboard ( +5 v., ground, and the 4 sensor lines) and 8 wires
for the sensors (4 pair - each pair consisting of a +5 v. line and a data
line). This is a lot of wire to be stuffed in such a small area,
but it fit with some gentle persuasion.
Next, the 4 pair of wires were fed outside
the laptop case and the laptop was reassembled. Thermistors were
attached to the wires. I crossed my fingers and booted up the computer.
Success! I got good readings from all 4 sensors. Wonders never
cease.
After calibrating
the thermistors, I got to take the laptop apart again. It was time
to mount the sensors. |
