The 555 as an Astable Multivibrator
Inside look at a 555 Integrated Circuit
Timer
555 Connected as an Astable Multivibrator
How does it work?
• When the switch is closed, the capacitor
charges from 0 Volts towards VCC via RA
and RB. The 555 output is high (VCC).
• When the capacitor voltage reaches
2/ V , the internal transistor turns on,
3 CC
discharging the capacitor via RB. The 555
output switches low (0 Volts).
How does it work? (continued)
• When the capacitor voltage falls to 1/3VCC,
the internal transistor is turned off, the
555 output switches high, and the
charging process begins again.
Waveforms for Voltages
Capacitor Voltage
Output Voltage
Timing
During the charging phase, the capacitor
voltage can be written as:

vc (t )  Vcc 1  e
 t /( RA  RB )C

During the discharging phase, the
capacitor voltage can be written as:

vc (t )  Vcc e
 t / RBC

Group Activity
• How long does it take to:
a) charge up from 1/3 VCC to 2/3 VCC?
• Hint: Calculate time to reach each voltage first
then subtract
• b) discharge from 2/3 VCC to 1/3 VCC?