555 Timer Online Calculator

Mono Stable 555 Timer Calculator

Formula: \(t=1.1R_{}C_{}\)

mono stable 555 timer circuit

The above figure shows 555 timer circuit diagram for monostable operation. In monostable operation, the 555 timer produces a square wave of pulse width of certain time duration(t) when there is trigger input signal at the trigger pin 2. The resistor R and the capacitor C are used here to control the pulse width of pulse duration(t) and is given by the following formula. \[t=1.1R_{}C_{}\] We can calculate the pulse width duration(t) using the above online 555 timer monostable calculator. For the trigger input we can connect a pulled high momentary push button to the trigger input pin 2. When we press the push button, there will be a square wave signal of width equal to the value obtained above.

Astable Multivibrator 555 Timer Calculator

1) Calculate Frequency, Duty Cycle, Charging and Discharging Time:


astable 555 Timer circuit
2) Calculate R1 from Frequency, R2 and C:


3) Calculate R1 from R2 and duty cycle:



The above figure shows 555 timer circuit diagram for astable operation. Astable mode is also called Free Running Mode and the above circuit is often referred as free running non-sinusoidal oscillator. In astable operation, the threshold input is connected to the trigger input.

Formula Used \(T_{H}=0.639(R_{1}+R_{2})C,\newline T_{L}=0.639R_{2}C,\newline f=\frac{1.44}{(R_{1}+2R_{2})C},\newline DC=\frac{R_{1}+R_{2}}{R_{1}+2R_{2}}\)

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.

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