In the circuit that uses a PIC16F84, the audio present at the input will have its level controlled by the DC voltage applied to pin 2 of the MC3340P. The audio level displayed on the output can be changed between zero and an amplitude value four times greater than the signal applied to the input. To achieve a DC voltage, which will be applied to pin 2 of the MC3340P, a PIC16F84 was used which is the "brain: of the circuit, an analog-digital converter DAC0808 and an operational amplifier LF353. The control process is as follows: When presses one of the keys (S1 or S2), which respectively increases or decreases the 8-bit counter of the PIC, the value of the counter is transferred to the DAC0808 which converts it to the digital form of the PIC in an equivalent current. LF353 converts it to an equivalent DC voltage. The converted voltage is applied to pin 2. MC3340P Potentiometer P1 is the scaling of the operational amplifier and should be adjusted, preferably to 5.187 k. P2 is the amplitude control.
