Pitch doubling circuit II LM324
This project produces the same result as in the previous project Pitch doubling circuit. An oscillating output is applied with full-wave rectification so that the frequency (pitch) is doubled. The oscillating block consists of the same circuit, but it uses quite a different method of full-wave rectification.
After you finish wiring, connect the earphone to J1 and turn power ON. You'll hear a beep. Remember this tone.
Next, change the connections of the earphone to J2, and then press S1. Again, you'll hear a beep, one octave higher than the one you heard before. That is, 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 J1, voltage follower U1 A, to U1B. Do you know how full-wave rectification occurs at U1B, Q3, and Q4.
A hint for you: the input signal and output signal of the OP amplifier U1B are opposite in phase, and they are added together (ORed) as they pass through the emitter followers Q4 and Q3. You see, when the output of Q3 is high, Q4 is OFF, and when the output of Q4 is high, Q3 is OFF.
Therefore, only the alternations of high level are kept output as shown in Figure 1.