Pitch Switch
Approx construction cost US$32
A novel sound-operated switch with precise frequency response (it can be operated with a penny whistle).
Various types of sound switch exist, including the well-known clap switch, whistle switch, and telephone/doorbell extender. Most sound switches, however, are characterized by their distinct lack of selectivity. At best, they will respond to a spread of frequencies several hundreds of Hertz wide. In effect, this means that almost anyone who can clap or whistle would be able to trigger such a switch.
By comparison, the Pitch Switch described here responds to a narrow passband, or pitch, which has the width of a single tone at all frequencies (to be exact, 55Hz at concert pitch A, or 440Hz). This means that it will "hear" only those sounds that fall within one semitone of a selected frequency. Also, since the Pitch Switch detects frequencies digitally, in theory it will fail to respond to frequencies that fall so much as a single Hertz outside the selected pass-band. This means that it would be particularly difficult for "just anybody" to trigger the switch - it is under the control of the person who holds a specific tin whistle or signal generator.
Besides this, it is exceedingly sensitive, and will trigger at a considerable range. A range of at least 40 meters is achievable with a tin whistle. (This range can also be extended electronically. In the author's most interesting test, a trumpet was blown several times in a cricket stadium in Georgetown, St. Vincent, reliably triggering the Pitch Switch in Cape Town, South Africa, via a normal f.m. radio broadcast. This represents a range of 10,000 kilometers!)
More details on this construction project can be found in the November 2001 issue of EPE Online, the world's first web-delivered electronics and computing hobbyist magazine.
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