How to Use It?
To use the calculator, simply enter the resistance and capacitance values, and the calculator will calculate the output pulse width of a 555 timer monostable circuit.
What is a 555 Timer?
The 555-Timer is a popular integrated circuit (IC) that is used in a variety of electronic applications such as clock timing, signal delay, pulse generation, and signal oscillation. It can produce different output waveforms by adding external RC networks.
The IC was first marketed in 1972 by Signetics and used BJTs. Since then, many companies have produced original BJT-based ICs and later low power CMOS-based ICs.
The 555 Timer is named “555” because it typically contains three 5 kilo-ohm resistors internally that form a voltage divider. It also includes two comparators, a flip-flop, a discharge transistor, and an output stage. Here is the block diagram.
The 555-Timer IC works by using three 5 kΩ resistors to divide the supply voltage in three, and two comparators to compare these voltages to the input voltage, then sets or resets a flip-flop accordingly. The flip-flop controls the output stage and a transistor that connects the discharge pin to ground.
By wiring the 555 timer with resistors and capacitors in various ways, you can get it to operate in three different modes: Monostable, Astable, and Bistable.
Monostable Mode
In Monostable mode, the 555-timer IC forms a Resistance-Capacitance circuit with the external resistor and capacitor. The circuit generates a single, short, positive pulse of a specific duration in response to a trigger signal (when a falling edge is detected on pin 2). That’s why the circuit is also known as a one-shot circuit.
In this mode, when the input signal is applied to the trigger pin (pin 2), the output (pin 3) of the IC goes high and the external capacitor begins to charge. When the voltage across the capacitor equals 2/3 of the IC’s supply voltage, the capacitor stops charging and the output signal of the circuit returns to the low state, ready for the next input signal.
The duration of the pulse can be shortened or extended by adjusting the value of the resistor and capacitor, changing the rate that the capacitor charges.
The duration of the pulse (T) can be calculated using the following equation:
Where:
T is the output pulse width (in seconds)
R is the resistance (in ohms) connected to the pin 7 of the IC
C is the capacitance (in farads) connected to the pin 6 of the IC
Example
Let’s say we have a 555 Monostable Circuit with an external resistor value (R) of 10 kΩ and an external capacitor value (C) of 1 μF.
Using the above equation, we can calculate the output pulse width (T) as follows:
Plugging in the values, we get:
Simplifying, we get:
Therefore, in this example, the output pulse width of the circuit would be 11 milliseconds.
