The circuit diagram of the first multivibrator is shown in the figure 1. Gates D1.1, D1.3, D1.4 are formed an astable multivibrator, the gate D1.2 is used as an inverter. The frequency of the multivibrator can be changed in a wide range by the potentiometer R2. If R2=0, then F=25 kHz, and if R2=3.3k, then F=370 Hz, i.e. the frequency change is more than 60 times.
In some cases it may need a circuit that produces square pulses with high amplitude. It can be achieved by using a TTL IC with open-collector outputs, just like 7401 IC. It has 4 NAND gates with open-collector outputs. To perform correctly, outputs require pull-up resistors. The circuit diagram of the multivibrator based on 7401 IC is shown in the figure 2.
Logic gates D1.1 and D2.1 are used in the oscillating part of the circuit, gates D1.3 and D1.4 are used to generate high voltage levels, almost up to power supply voltage .
"Radio, Televizia, Elektronika" 1978, 3