Building a Shielded
Sensor

Copyright 2002 Aubrey Spurlock

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If you are building a compression mode pickup (under the feet of a floating-bridge ie. violin, mandolin, archtop guitar, etc), then use 110 micron film. If you are building a bending mode sensor (similar to PUTW or Acoustic Feather), then use 28 micron film. The easiest method of shielding is to simply fold the film over, with the hot side on the inside of the fold. The ground on the outside creates a shielded element.

Step one: Cut a piece of film that is slightly wider than the footprint of your bridge, plus 1" longer for lead attachment. A violin bridge is typically 1 5/8" long by just under a quarter inch wide. For this cut a piece of film that is 4.25 inches long by 1/4" wide.  The larger dimensions insure that the pressure of the bridge wont crimp the edges and short out your pickup.  You will need to remove some of the ink. This can be done with a cotton swab dipped in Xyelene. Place a straight edge on top of the film and rub the swab against it to remove the ink. The solvent will not entirely dissolve the ink, but will soften it enough that the rubbing will remove it. You can blot the film to remove excess Xylene.

Remove the ink from around the edges of  the film on one side. This is now the hot side. The clear area will create a insulating buffer between the two sides, and only needs to be a millimeter or so wide.

Next, clean the ink from an area that is 1/4" from the end of the film. Do this on each end, but on opposite sides. When the film is folded, the clear areas will both face the same direction. This provides a more secure anchor for the lead attachments and will prevent the ends from shorting out when the film is folded. Your prepared film should look like the drawing to the right.

Now to make your lead attachments. The easiest method is with copper shielding tape with conductive adhesive.  Cut two pieces of tape, each about 3/4 inch long and just slightly narrower than your solder tab.  Solder the hot wire of your shielded cable to the center edge of one piece and the shield wire to the center edge of the other.  Leave the liner on the tape until you are ready to attach it to the film. The liner will help act as a heat sink, but you still need to use a very brief dwell time as the heat will degrade the adhesive.

Peel the liner off of the foil attached to the hot wire and carefully attach it to the correct solder tab. Emboss it with your fingernail.  Make sure the tape is smooth and flat. NO WRINKLES! The extra pressure from your fingernail will improve the adhesion.

Attach the piece of foil with the hot wire attached to the hot side of the film. Then apply a strip of double sided tape to the film. If you use conductive tape, make sure it is slightly narrower than your piezo film. Again, this keeps the sides from shorting out.

Any double-sided adhesive will work. The thinner and lighter the better, especially if you are building a bending mode pickup. If you are very careful, you only need to cover half the length. Covering the entire length is simpler.

Then fold the film over, with the hot side inside and attach the ground connection

Wrapping the entire pickup with a very thin protective film is a good idea to prevent the feet of the bridge from scratching the electrode of the piezo. I use MonoKote, a material designed for flying model airplanes.  Take care that there are no wrinkles in your covering. Slip a piece of shrink tubing over your lead wire and butt it against the pickup. Use extreme caution when heating the shrink tubing. Piezo film cannot stand heat. I hold the pickup between two pieces of wood, with only the wire and shrink tubing exposed. Short bursts with the main blast of hot air kept off of the film should do the trick.

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