24 Nicolas CollinsNow is as good a time as any to introduce the 10th Rule of Hacking, if it is not
obvious already:
Rule #10: Every audio connection consists of two parts: the signal and a
ground reference.
In the case of a contact mike the signal comes from the white part of the Piezo
disk, while the ground is the brassy bit; on the plug the tip carries the signal and
the sleeve is the ground. In future chapters I may get a bit sloppy and only refer
to the signal when describing connections -- always assume that a ground
connection must accompany every signal.
- Plug into your amp and check that your new contact mike works -- tapping
the mike should make a solid thunking sound. If there is no sound check the
joints at both ends of the cable -- make sure they are good and there are no
shorts. If there is hum you may have connected the hot and ground wires to the
wrong conductor of the cable -- de-solder and reverse them. If it works, screw
the barrel down onto the plug and test again -- sometimes squeezing the barrel
down over a marginal solder joint will break or short it. A small piece of
electrical tape can be used to isolate the connections if excess wire tends to short
when the barrel is screwed down.
Whoops! Did you forget to slide the barrel onto the wire before you soldered? If
so, desolder the plug, go back to step 8, but don’t feel too stupid -- everybody
makes this mistake.
Rule #11: Don’t drink and solder.
When you are sure you have an electrically functional contact mike, cover the
ceramic side with a piece of electrical tape -- you can trim it around the
circumference with scissors or a knife, or you can wrap the edges over to the
other side of the disk.
Open up and stir your can of Plasti-Dip. As per the instructions on the label,
slowly dip the contact mike end of your cable into the goop until you have
covered the wire past the electrical tape. Slowly withdraw it and hang it up to
dry. Take a break and go away -- this stuff is stinky. You can dip a second layer
after the first one dries thoroughly, which can take up to several hours. More
than two layers tends to muffle the sound.