Pitch doubling circuit LM324
In this project, you're going to make a circuit that doubles a sound pitch by manipulating an OP amplifier. The result can be heard from the earphone. Precisely speaking, the circuit applies full-wave rectification to the sine wave input so that the output frequency is doubled, as shown in Figure 1.
It is not that the frequency of the sine wave it self is doubled. After you finish wiring, connect the earphone to J2 and turn power ON. You'll hear a beep. Remember this tone.
Next, change the connections of the earphone to J1, and then press S1. Again, you'll hear a beep, but the tone is different from the one you heard before. In fact, the beep now is one octave higher.
In other words, the frequency, or pitch, has doubled. In this circuit, Q1 and Q2 make up an oscillator. The sine wave output from the oscillator passes through J2 to the OP amplifier U1 A.
The circuit block consisting of U1 A, Q3, R10, and R11 provides full-wave rectification. The voltage follower U1B has high input impedance (AC resistance) to prevent the Q3 output from being affected by the next stage.