Building the TPAD


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Materials

– Glass- 165mm x 130mm x 1.1mm soda lime
– 5 Piezos- 35mm OD x 25mm ID brass backed Murata 7BB-35-3
– Superglue (cyanoacrylate)- fast drying, low viscosity (non-gel) works best
– Superglue remover (optional)
– Stranded wire
– Solder
– Solder flux
– Sandpaper (fine grit) or scotch pad
– Cleaner- glass cleaner, alcohol
– Tape- electrical or masking
– Cue tips (optional)

Tools

– Soldering iron
– Ruler
– Heavy duty scissors
– Wire stripper

1.    Cut piezos to size

Measure and mark an 8 mm strip down the center of the piezos with a pencil.With a heavy duty scissors, cut the piezo along the lines. Note- it is very important that the center portion not become cracked or bent. With a right handed scissors, the right side of the cut piezo will curl down and crack. This is fine for the scraps, but the center piece should remain flat. A good practice is to keep the brass side flat down on the bottom scissor blade.  Cut both sides and then square off the edge. Cut all 5 piezos in this manner.

2.    Mount piezos to glass

Thoroughly clean the glass surface and the cut piezos. With a fine grit sandpaper (~300 works), roughen all of the exposed brass of the piezos. This will help the glue adhere to the back, and the solder to adhere to the front. Place a strip of tape (we used electrical tape, but masking tape would work as well) near the edge of the glass, and with a straight edge, cut it so that its edge is 11mm from the edge of the glass. This will be the edge line for all of the piezos.

One at a time, apply a thin coating of super glue to the brass side of the piezo and spread it as evenly as possible. Not much glue is needed, but it is important that the entire surface come into contact. Place the edge along the line. Repeat for the rest of the piezos, using cue tips to clean excess super glue. It helps to apply pressure to the piezos as they dry to remove any bubbles in the interface. You can see the adherence from the back side. It should be almost completely uniform, without bubbles or pulled away areas. Be careful to not get glue on the top side of the piezo. If you do, it can be removed with a superglue (cyanoacrylate) remover. After 10 minutes, the super glue should be set, however it is recommended that you wait at least an hour before you start soldering.

3.    Solder on leads

First clean the surface of each piezo using rubbing alcohol or some other mild solvent. Then apply two spots of solder flux to each piezo- one to the brass and one to the silvered part of the piezos.
Beyond the length of the TPad, you will want to leave yourself 10-20cm of extra wire. We prepared the wires by stripping away sections of insulation where the solder joints will go. Add spots of solder flux to the piezos where you will make the joints.

Solder each piezo one to the next with common grounds and common positive electrodes in electrical parallel. A good method is to melt solder onto the wire first.  Then placing the wire against the piezo, re-melt the solder by pushing the wire down onto the piezo. It is important not to hold the iron against the piezo for more than a few seconds. Above a certain temperature- called the Curie temperature- piezoceramics lose their piezoelectric polarization.

Finally, spread a thin line of super glue down the length of the piezo wires so as to glue them down securely to the TPad. If this is not done, the TPad is likely to emit a high pitched noise from the vibration of the wires.


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