Transistorized Auditory Gimmick

Posted on Feb 7, 2014

(This circuit first appeared in `The Braille Technical Press`, edited by Robert W. Gunderson, in 1955. Gunderson, W2JIO, designed this `gimmick` to read transmitter meters. It appeared many times since over a 20-year period, included even in commercial aids and appliances for the blind. Bill Gerrey, et al, has never gotten around to generating a p

Transistorized Auditory Gimmick
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rint schematic diagram of the "gimmick" by itself, although it has been part of Smith-Kettlewell adaptive instruments of various sorts over another 30-year span; its circuit appears in those published designs. ) Key components of this VCO are: A hard-to-find NPN germanium transistor (needed for its beta at very low base bias -even though it is non-linear in this service). The transformer is for push-pull output in transistor radios of Yesteryear, but for the time being, many still exist in surplus (even from Radio Shack, 270-1380). The circuit uses a "blocking oscillator", which requires a center-tapped primary winding - specified impedances from 500 to 25, 000 ohms, (center tapped) are appropriate. The transformer secondary winding drives a small speaker; here again, the rated impedance (3. 2 to 16 ohms) is not critical, even though the speaker may be rated at 8 ohms. The center tap of the primary goes to the emitter of a PNP small-signal transistor (type not critical; a 2N2907 was used here). The full primary winding is shunted by 0. 022uF (which increases the loudness). One end of the primary goes through 0. 022uF to the base of the PNP transistor. The other end of the primary winding goes through an SPST on/off switch to the positive of the battery. The collector of the transistor is grounded, as is the negative of the battery. The base of the PNP transistor goes through 22 K ohms to the collector of an NPN germanium...

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