F-V converter II LM324
In the project 308, you used the sound input to the earphone as a signal source. This project has an oscillator. The oscillating frequency changes as you turn the control volume. Listen to the oscillating sound from the earphone while comparing the oscillating frequency and the brightness of the LED, so that you realize the way the F-V converter works.
After wiring the project, turn the control volume fully counterclockwise and turn power ON.
Note the pitch (frequency) of the sound and the brightness of the LED.
Now, slowly turn the control volume clockwise. As you see, the LED gets brighter as the sound goes higher. You can tell that the frequency (F) is converted to a voltage (V) that drives the LED. U2:A is a square-wave oscillator. When the output pulse from U2:B rises, Q1 pulses ON, triggering the monostable multivibrator U2.
As a result, a pulse is output from U2 and lights the LED. The output pulse from U2 has a fixed width, but the LED becomes brighter with increasing trigger frequency, and darker with decreasing trigger frequency.