Pulse duration: t = 1.1 × R × C
Enter desired pulse width and capacitance, then calculate R. The output triggers a single pulse. Frequency is reciprocal of t.
Mode I – From R₁, R₂, C
tH = 0.693·(R₁+R₂)·C | tL = 0.693·R₂·C
f = 1.44 / ((R₁+2R₂)·C) | DC = (R₁+R₂)/(R₁+2R₂)
Mode II – Find R₁ given f, C, R₂
Mode III – Duty Cycle & R₂, C → R₁, Freq
The above figure shows the 555 timer circuit diagram for astable operation. Astable mode is also called Free Running Mode and the above circuit is often referred to as a free running non-sinusoidal oscillator. In astable operation, the threshold input is connected to the trigger input.
Formula Used
\(T_{L}=0.693R_{2}C\)
\(f=\dfrac{1.44}{(R_{1}+2R_{2})C}\)
\(DC=\dfrac{R_{1}+R_{2}}{R_{1}+2R_{2}}\)
Working Principle
At start up, the dc power Vcc is turned on and when this happens the capacitor C is discharged and holds the trigger voltage less than 1/3rd of Vcc. This will trigger the internal comparator 2 which causes the output to switch from high and the internal discharge transistor Qd to turn off. When the internal discharge transistor Qd turns off, the external capacitor C will start to charge up via the resistors R1 and R2.
When the voltage on capacitor C reaches 2/3rd of Vcc, the output switches low and internal discharge transistor Qd turns on. Now the capacitor C starts discharging via R2 and Qd. When the discharging operation reaches 1/3rd of Vcc the comparator 2 triggers causing the switch back to high and internal discharge transistor Qd is turned off. This cycle of charging and discharging repeats itself and the 555 Timer oscillates creating a square wave at the output pin 3.
