Multivibrator circuit experiment based on 4069

A typical 4069 multivibrator circuit diagram is shown below.

This circuit consists of two NOT gates, resistors R1, R2 and capacitor C1. This oscillator circuit outputs a rectangular square wave with a duty cycle of about 1:2. The so-called "duty cycle" refers to the ratio of the time the high potential of the rectangular square wave is maintained in one oscillation cycle to the oscillation cycle. If the time occupied by the high potential and the low potential in one cycle is equal, the duty cycle is 1:2 (50%). In the multivibrator, the oscillation frequency is mainly determined by the resistor R2 and the capacitor C1. The role of the resistor R2 is to make the oscillator work more stably. The resistance value of R1 is usually equal to or greater than R2; for circuits with low requirements on oscillator performance, the resistor R1 can be omitted. The oscillation period of the output pulse of this circuit can be calculated according to the formula: T≈2.2R2C1. The oscillation frequency f=1/r. For example, R1=1M, R2=1M, Cs=0.47μF, T=212x1×0.47=1.04s.

The experimental process of the harmonic oscillator:

① Assemble the experimental circuit The experimental circuit is shown in the figure below. Assemble the experimental circuit according to the circuit diagram, the power supply voltage is 3~15V, and after the power is turned on, you can usually hear the piezoelectric ceramics make a sound, and the light-emitting diode is also lit.

②Experience the relationship between the frequency of the multivibrator and R2 and C1. Adjust the resistance of the potentiometer R2, and you will hear the frequency of the sound emitted by the piezoelectric ceramic piece change accordingly. The capacitor C1 is used for experiments with 0.47uF and 0.01uF respectively. When the capacitor C1 is used with 0.47μF, adjust the potentiometer R2 to the maximum. The piezoelectric ceramic piece does not make a sound, but the light-emitting diode will produce a flash about once per second. Gradually reduce the resistance of R2, and you will observe that the flashing frequency of the light-emitting diode gradually increases, and you will hear the piezoelectric ceramic piece making a sound. As the resistance of R2 decreases, the light-emitting diode stops flashing and becomes constantly bright. This does not mean that the multivibrator stops oscillating, but that its oscillation frequency exceeds dozens of times per second. At this time, the flashing of the light-emitting diode is no longer distinguishable by the human eye. This can be proved by the increasingly high sound emitted by the piezoelectric ceramic piece. After selecting a capacitor of 0.01 μ F, repeat the above experiment again, and you can find that the frequency of the sound emitted by the piezoelectric ceramic piece is higher at this time.

③Verify the oscillation period (frequency) formula.

Theoretically, the pulse period generated by this circuit is 2.2R2×C1(s). This formula can be verified by measurement. The specific method is to use a clock to record the number of oscillator pulses within a period of time, and then compare it with the result of theoretical calculation. For example, adjust the oscillator frequency by a few hertz, and then you can see the light-emitting diode flashing; use a stopwatch to record the number of times the light flashes within 100s, and then divide the result by 100 to get the frequency of the pulse generated by the oscillator.

④Change other factors and conduct experiments.

In addition to changing the resistor R2 and the capacitor C1, you can also change the resistance value of the resistor R1 and the power supply voltage, and then conduct an actual measurement experiment of the oscillation frequency.

Experimental summary:

The experimental results show that the pulse frequency output by the oscillator of this circuit is inversely proportional to the size of the resistor R2 and the capacitor C1. The resistor R1 and the power supply voltage can also affect the operating frequency of the oscillator to a certain extent.