Jameco Electronics Puzzler: Pressure Sensitive Tone Generator

By Forrest M. Mims III
Tone Genrator

What's the missing component?

Amateur electronic musician Joe Rhythm is planning a one-man video concert that he plans to post on YouTube. Controlling his array of instruments requires both hands and he wants to build a pressure-sensitive tone generator that he can control with a free finger or even an elbow or foot. Joe quickly whipped up a simple tone generator using parts from his bench stock. Since there wasn't enough time to order a pressure sensor, he improvised by making one from materials he had on hand. What did he use?

Puzzler Hints:
In the figure above the 555 is connected as a pulse generator having a pulse repetition rate controlled by R1, C1 and the pressure sensor. This means the pressure sensor is a resistor whose resistance is controlled by pressure.

What is the pressure sensor behind the puzzle piece?

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