The Original "Transistorized Auditory Gimmick"
(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 print 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.
Circuit Description:
The supply voltage can be from 4 to 15 volts. The negative of the supply is common to ground, as is the cold side of the input.
The transformer secondary goes to a small speaker; if this winding is tapped, the tap is not used.
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 transistor (2N388 used here; 2N35 was a traditional type number). The emitter of the NPN unit is grounded. Its base is the hot input; the cold side of the input is grounded.
Caution! This input going directly to the base means that any input signal exceeding the knee of the base-emitter junction will burn it out. Moreover, the lower range of the VCO is best exploited when the impedance of the source, typically a d'Arsenval meter, is low -- under 10,000 ohms. Thus, care must be taken to assure that any input signal not exceed 0.3 volts.
If this circuit is to "meter" RF circuits, the following conventions should be followed:
The base of the NPN is bypassed to ground by 0.01uF. This base goes through a 2.2mH RF choke to the positive side of a millimeter to be read. The meter side of the choke is also bypassed to ground by 0.01uF. It is assumed that the negative side of the meter is at ground, the cold input. If not, the gimmick may be floating, properly insulating it and its battery, made common only to the meter.
A Typical Application:
(The following circuit is that of a meter board in the MFJ "Artificial Ground". It can be used anywhere that an RF current is to drive a relative indicator.)
A toroidal coil of unknown specifications has a wire carrying RF current running through its center. The bottom end of this coil is grounded. The coil is shunted by a 1K 3-watt resistor (3-watt rating only needed if transmitter power is high).
The top of the coil goes to the anode of a small-signal diode, with the cathode going to the clockwise end of a 10K sensitivity control. The counterclockwise end of this control is grounded. The control is shunted by 0.01uf. The wiper of the control goes through the 100 microamp meter to ground. If only the gimmick is connected between the wiper and ground, smoother operation of the control will be had if a 1.2K resistor, substituting for the meter, goes from the wiper to ground.
The 1955 circuit by Robert Gunderson is crude but very effective when connected across a visual meter whose relative indications are important to note. Only changes in pitch are required in a grid-dip meter or an antenna bridge where a "dip" in the meter reading is all that is to be noted.