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:
The circuit below shows the 555 timer IC connected in the astable mode
The output frequency is given by the following equation
fo=1.44(R2+2R1)C1The HIGH and LOW times of each pulse are given by
th=0.69(R1+R2)C1tl=0.69R1C1Notes:
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
VH=23VCC=0.66VCCVL=13VCC=0.33VCCTo derive the frequency equation of the 555 timer oscillator, we will make use of the universal time constant formula for the RC circuit.
change=(f−s)(1−e−tRC)For th (the period when output is high), the capacitor is charged from VL to VH through the resistors R1 + R2 from VCC. Thus
For tl (the period when output is low), the capacitor is discharged from VH to VL through the resistor R2 to 0V. DISCHARGE pin is switched to 0V by the transistor switch. Thus