Experiment 17 - The Astable Multivibrator with Symmetric Switching Times

In the last experiments, you build an astable multivibrator. Thereby you surely have noticed the different switching times. Find out here how you build a symmetric astable multivibrator.

If you recall Experiment 15 you realize...

• The capacitor C1 is charged via the resistors R1 and R2.
• The capacitor C1 is discharged only via the resistor R2.

This has the consequence that LED2 is light longer than LED1.

Do you want nevertheless to build a symmetric astable multivibrator where LED2 and LED1 are light equally long, you have to introduce a diode in the circuit...

Circuit diagram symmetric astable multivibrator with 555.

Note, this requires that the resistors R1 and R2 must be equal!

Circuit symmetric astable multivibrator with 555. (Enlarge)

In the next experiment, you go one step further and make the switching times adjustable...