The circuit presented consists precisely of an oscillator (multivibrator) that takes advantage of the 4 ports of this integrated. In the figure we have the diagram of this oscillator. The potentiometer P1, of 4.7k ohms allows to vary the frequency in the range of approximately 300 Hz to 25 kHz, which gives us a ratio of more than 80 to 1. The capacitor C1 can be changed to other frequency ranges. Note that each of the 4 ports is connected as an inverter, which leads us to a kind of digital amplifier with positive feedback. To isolate the oscillator circuit from the external load, the last port is used as a buffer. In this way, the presence of the load does not change the characteristics of the oscillator, which remains stable. The signal is evidently rectangular and can be used as a clock for digital circuits in the TTL family.
