Experiment 20: A Powerful Combination
202 Chapter 4
One Little Detail: The Computer Interface
Old computers used to have a big switch at the back, attached to the heavy
metal box inside the computer, that transformed house current to regulated
voltages that the computer needs. Most modern computers are not designed
this way; you leave the computer plugged in, and you touch a little button on
the box (if it’s a Windows machine) or the keyboard (if it’s a Mac), which sends
a low-voltage pulse to the motherboard.
This is ideal from our point of view, because we don’t have to mess with high
voltages. Don’t even think of opening that metal box with the fan mounted in
it, containing the computer power supply. Just look for the wire (usually con-
taining two conductors, on a Windows machine) that runs from the “power up”
button to the motherboard.
To check that you found the right one, make sure that your computer is un-
plugged, ground yourself (because computers contain CMOS chips that are
sensitive to static electricity) and very carefully snip just one of the two con-
ductors in the wire. Now plug in your computer and try to use the “power up”
button. If nothing happens, you’ve probably cut the right wire. Even if you cut
the wrong wire, it still prevented your computer from booting, which is what
you want, so you can use it anyway. Remember, we are not going to introduce
any voltage to this wire. We’re just going to use the relay as a switch to recon-
nect the conductor that you cut. You should have no problem if you maintain
a cool and calm demeanor, and look for that single wire that starts everything.
Check online for the maintenance manual for your computer if you’re really
concerned about making an error.
After you find the wire and cut just one of its conductors, unplug your com-
puter again, and keep it unplugged during the next steps.
Find where the wire attaches to the motherboard. Usually there’s a small un-
pluggable connector. First, mark it so that you know how to plug it back in the
right way around, and then disconnect it while you follow the next couple of
steps.
Strip insulation from the two ends of the wire that you cut, and solder an ad-
ditional piece of two-conductor wire, as shown in Figure 4-85, with heat-shrink
tube to protect the solder joints. (This is very important!)
Run your new piece of wire to the latching relay, making sure you attach it to
the pair of contacts which close, inside the relay, when it is energized by the
unlocking operation. You don’t want to make the mistake of unlocking your
computer when you think you’re locking it, and vice versa.
Reconnect the connector that you disconnected from your motherboard, plug
in your computer, and try to power it up. If nothing happens, this is probably
good! Now enter the secret combination on your keypad (while holding down
the asterisk button to provide battery power) and listen for the click as the re-
lay latches. Now try the “power up” button again, and everything should work.