V-F converter LM324
Here we're going to make a V-F converter. What do you think V and F stand for?
Not so difficult, V for voltage and F for frequency. So a V-F converter is a device to convert a voltage to a frequency.
The schematic shows you that U1:A is an integrating circuit and outputs an integrated voltage. When this output voltage is applied to U1:B (comparator), a frequency corresponding to that voltage can be obtained. This output is sent to the earphone through U1:C.
After wiring up the project, turn power ON and rotate the control volume. The voltage applied to IC1 changes as you turn the control volume, and this voltage change is converted to a frequency change and you can hear it through the earphone.