Pulse frequency doubler LM324
This is a pulse frequency multiplier that works with just one transistor. It is called a "pulse frequency doubler" because it has a multiplication factor of 2.
The operational amplifier IC 324 is a square-wave oscillator that generates the AC signals this time. Q1 is a frequency doubler, and its collector output signal is opposite in phase with respect to the base input signal. The two outputs from the base and collector are combined by Da and Db to double the frequency.
When you finish wiring the project, connect the earphone to J1, and turn power on. You'll hear a oscillating sound through the earphone. Remember the tone of that sound.
Now, connect the earphone to J2, and press S1. You will hear a sound higher by an octave, showing you that the frequency is doubled.