1. The 555 as an Astable Multivibrator

2. Inside look at a 555 Integrated Circuit Timer

3. 555 Connected as an Astable Multivibrator

4. How does it work? When the switch is closed, the capacitor charges from 0 Volts towards V CC via R A and R B. The 555 output is high (V CC ). When the capacitor voltage reaches 2 / 3 V CC, the internal transistor turns on, discharging the capacitor via R B. The 555 output switches low (0 Volts).

5. How does it work? (continued) When the capacitor voltage falls to 1 / 3 V CC, the internal transistor is turned off, the 555 output switches high, and the charging process begins again.

6. Waveforms for Voltages Capacitor Voltage Output Voltage

7. Timing During the charging phase, the capacitor voltage can be written as: During the discharging phase, the capacitor voltage can be written as:

8. Group Activity How long does it take to: a) charge up from 1 / 3 V CC to 2 / 3 V CC ? Hint: Calculate time to reach each voltage first then subtract b) discharge from 2 / 3 V CC to 1 / 3 V CC ?