The 555 timer IC is an 8 pin integrated circuit used in a variety of timer, pulse generation, and oscillator
applications. The 555 can be used to provide time delays or as an oscillator
The 2 common operating modes of the 555 timer IC are:
Monostable mode: In this mode, the 555 functions as a "one-shot" pulse generator. Applications include timers,
missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement,
pulse-width modulation (PWM) and so on.
Astable (free-running) mode: The 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse
generation, logic clocks, tone generation, security alarms, pulse position modulation and so on.
The circuit below shows the 555 timer IC connected in the astable mode
x-value (s) :
y-values (V) :
The output frequency is given by the following equation
Click on the colored nodes in the schematic to show/hide that node waveform.
Select the different R1 and C1 values, press Simulate and observe the change in frequency and waveforms with the
Move the cursor in the chart to obtain the waveform values
In order to analyse the 555 timings, we need to understand a little of the working of the 555 timer internally. There
is a resistor tree with equal resistance values. They set the upper and lower limits of the capacitor
charge/discharge voltage. Thus the voltages are