Table below shows the principle of auditory circuit voltage, voltage-controlled audio map in the 400Hz to 1600Hz oscillator is the 2 octave range changes with the control voltage: 0V when in 400Hz, the maximum voltage that full-scale voltage to 1600Hz.
Figure R12 ~ R21 and R11 a total of 10 resistor divider composed of good produce a relatively accurate reference voltage, can generate 11 corresponding reference tone, which is equivalent to voltage meter dial 0 to 10 scale of 10 . The resistance value should be accurate, calibrated, corresponding to the measuring range of 0 ~ 100mV, the voltage meter has a total of eleven 0,10,20,30,40,50,60,70,80,90,100 mV scale.
Transistors VT1, VT2 (both with 3DG6) form a simple square wave audio oscillator, the frequency of the transistor VT3 (3CG4) for voltage control. Operational amplifier A1 (F007) driver transistor VT3. Adjust potentiometer RP1 op amp gain. Sensitivity adjustment potentiometer and frequency potentiometer RP2 RP3 ~ RP12, making the VCO frequency can keep up with changes in the measured voltage, and no matter what the voltmeter scale, when the switch S1 set to "measure" and " more "position, the sound heard is the same.
Circuit is installed, check and correct before they are allowed to adjust, first to set 0 S2, S1 set to "more" position, disconnect the VT2 collector C4, the collector in the VT2 access frequency, and S2 in parallel access mV table. Then adjust the potentiometer RP2, making the VCO oscillation frequency 400Hz, millivolt meter reading is 0V.
S2 were then transferred to the rest of the stalls, and the corresponding adjustment potentiometer RP3 ~ RP12, make mV meter readings were 10,20,30,40,50,60,70,80,90,100 mV, frequency meter readings determined by the voltage meter reading, about 500Hz, 600Hz, 700Hz, 800Hz, 900Hz, 1000Hz, 1100Hz, 1200Hz, 1400Hz, and 1600Hz.
When used to set the left S2, S1 is set on the left, then the operator heard the sound of 400Hz, the apparatus is working. And then set the middle of S1, I heard the voice of the speaker; will be "measured" position, making the speaker’s tone and pitch measurements are basically the same, where the stalls at this time touching S2 Braille, the voltage measured glance value.