Experiment 15 - The astable Multivibrator using NOT Gates

An astable multivibrator can also be build using logic gates. Often NOT gates are used.

In the circuit shown, you use the NOT gates of a C-MOS 4069.

Circuit of an astable multivibrator of NOT gates. (Enlarge)

Circuit diagram of an astable multivibrator of NOT gates.

If you connect the battery, the LED begins to flash. The oscillation period t is determined by the capacitor C1 and resistor R2. You calculate t with the formula...

t = R₁C₁ × 2.2

This circuit is often used in test environments because it is simple to set up.

However, there are some disadvantages...

• For practical circuits, the calculated oscillation period deviates sometimes strongly from the real oscillation period.
• For long oscillation periods, the input of the first NOT gate needs to be protected by a resistor. This makes it even more difficult to determine the oscillation period.
• Occasionally the oscillation fails to start.

The next circuit prevents these problems...