The SHAFT

special tuning gadget for sightless hams

Vou don't have to see to believe.

exceeding the maximum plate current. 1 am sure that being unable to tune his own gear must be very frustrating. Like most hams, I remembered sever al articles relating to tuning aids for the blind. As luck would have it, a search of my old magazines did not yield much in

Burton H. Syverson K5CW 3401 Garner Lane Piano TX 75023

Recently, a sightless ham friend of mine remarked that he had ruined the final amplifier tubes in his transmitter by formation. The idea of a voltage-controlled oscillator is not exactly new, but it is hard to beat. What articles I found, however, used this idea to tune for maximum or minimum indication

Not finding what I desired, the search for a method of reading discrete values of current or voltage began. The method decided upon involves the comparison of a variable voltage source calibrated in percent of the full scale voltage drop of the moving coil of the meter (Fig. 1) Application of the voltage-controlled oscillator is used to indicate balance t call the unit to be described "The S.H.A.F.T." This stands for the Sightless Ham's A\6 For Tuning.

Safety is always the prime consideration in the design of any electronic gear to be used by blind operators. In order to assure the equipment case was grounded, ac opera tion is used. The connecting cables carry ground to the frame of the equipment whose meter is being monitored. When readings are desired using a multimeter, the shield of the connecting cable is used as an electrostatic shield only, because of the nonme-tallic case. For the operator's safety, use of a multimeter should be restricted to resistance and low voltage measurements

One of the features of this unit is that it can be used with three different meters (i.e., multimeter, plate current meter, or ratiometer). Operation of the unit is quite simple. Referring to the schematic diagram (Fig, 2), it will be noted that zener diode CRT holds the voltage of battery B1 at 3.9 volts I his is applied to a series resistance network so that the voltage drop from the lower end of control R14 to the junction of R13 and R22 is approximately 40

millivolts. This is something less than the full scale voltage developed across the moving coil of most meters. By use of control R14 and switch SW2, any voltage between zero and 40 millivolts may be obtained This voltage is fed into the inverting input of an operational amplifier on IC U1 Control R14 is calibrated from zero to 10% and switch SW2 in 10% steps from 0 through 90. The settings are additive. Voltage from the moving coil ot the meter to be read is fed in jack J1 through 2-conductor shielded cord to switch SW1A which selects control R1, R2, or Ri R1, R2, and R3 are used to adjust the full-scale meter voltage to the 100% setting of the R14 and SW2. The voltage across R10 is fed snto the noninverting input of the same operational amplifier The output of the amplifier is fed into null detector Q1. Q1 causes unijunction oscillator Q2 to increase in frequency whenever the output of the operational amplifier crosses zero volts, The output of the oscillator is connected to a second operational amplifier which serves as an audio amplifier Note, the jumper connected from C2 to terminal 1 of LH may be replaced with a suitable resistor if the constructor feels the audio level is excessive

Due to the dc nature of the unit, the mechanical layout of parts is not critical. The size ot the etched circuit board was chosen to accommodate its being fastened to the back plate of a4'r x x 6" Bud cabinet (Fig 3). Others mav choose a larger cabinet for ease of construction fn my unit,, switches 5W1 and SW2 are rated at 1500 V dc breakdown and need not be insulated However, control R14 is sub-mounted on a

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