Contact Mike

index
555 touch sensor
air mike
amplify your toy
box your toy
circuit sniffing
clock tickling
contact mike
electret mike
hack the clock
laying and hacking
laying of hands
light theremin
make a cable
mapping
pickup
piezo driver
resistors
soldering
sudomini
synthesis
tape head

Make your own contact mike for a few pence...

You will need:

• battery powered mini amplifier;
• Piezo-electric disc;
• 8' lightweight shielded cable;
• plugs to match the jacks on the amp;
• plastic insulating electrical tape;
• can of 'plasti-dip';
• small spring clamps;
• molex-style terminal block;
• hand tools, soldering iron and electrical tape;
• very items for experimentation: sparklers, small blowtorch, guitar strings, metal scrap, slinky...

Piezo-disc with wires attached. You can salvage them from broken electronic devices, from toys and alarm clocks to cell phones and computers. If the disc is encased in plastic, carefully remove it. Do not bend or scratch the disc.

Snip off the connecting wires close to any attached circuit boards, keeping the wires as long as possible.

There will be two or three wires connected to the disc. One will always be connected to the metal portion, somewhere near the edge - this is the 'ground' connection. One will connect to the main part of the inner circle - 'hot'. In some cases there'll be a narrow, tongue-like shape differentiated within the crystal, to which the third wire connects - this is unnecessary.

Cut the connecting wires so they protrude about 2" from the disc. Strip aobut 1/2" of insulation from ground and hot wires. Tin the stripped ends (see 'soldering' above).

Shielded cable consists of stranded wire inside insulation, which is then wrapped inside a layer of braided or twisted wire, which is in turn covered by another layer of insulation. A cross-section looks like tree rings or a target. Shielded wire is used to protect audio signals from hum and other electromagnetic interference.

Cut a 5' section of shielded cable, the thinner and more flexible the better. Strip back 1" of the outside insulation. Unbraid the shielding and twist into a single thick strand. Strip 1/2" of the inner insulation and twist the centre conductor into a strand. Keep the two strands separate. Don't melt the insulation.

Twist the disc's hot wire and the inner wire from the shielded cable. Solder together. Twist disc's ground cable and shield from shielded cable. Solder. Wrap both joints separately with tape.

Strip the free end of the shield cable as above - 1" outer insulation, twist shield, 1/2" inner insulation, twist conductor, tin wires.

Unscrew the plug you are using to connect the mike to your amplifier. Pass the barrel back over the shielded wire toward the disc so that the threaded portion faces the freshly tinned end. Unscrewing the barrel of the plug reveals two connector tabs: the shorter one connects to the tip of the plug and the longer one connects to the sleeve.

Solder inner conductor of shielded wire to the tip of the plug and the shield to the sleeve. Sometimes there are holes in connector tabs that you can use, otherwise, tin and hold carefully.

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 disc, whille the ground is the brassy bit; on the plug the tip carries the signal and the sleeve is the ground.

Plug into amp and check that the mike works. If there's a hum you may have connected the hot and ground wires to the wrong conductor of the cable - do them again. If it works, screw barrel onto plug and check again. If necessary tape connections.

Cover the ceramic side of the mike with electrical tape.

Slowly dip contact mike end of your cable into 'plasti-dip' until you have covered the wire passed the electrical tape. Slowly withdraw it and hang it up to dry. Dip a second time once the first has dried (can be a couple of hours).

Use:

• Use double-sided tape or blu-tak to attach mike to things: guitars, violins, drums, pots and pans, wrists and knees, pinball machines.
• Use clamps too.
• 'Terminal strips' can be used. Cut terminal strips into a small section and clamp onto the mike with a spring clamp. Insert thin objects into the terminal openings and hold in place by tightening the screws. Try slinkies, springs, loose guitar strings, toothpicks, satay sticks, broom straws, porcupine quills, cactus needles.
• Many metals make unusual sounds when they heat or cool. Clamp a sparkler in the terminal block, light and listen. Or clamp steel wire and heat with a torch. Try short pieces of wire fixed to discs and 'bowed' with small blowtorches.
• Fill a plastic container with water, drop in the contact mike and put in freezer. Once frozen, remove from freezer and place in hot water.