555 Timer Basics - Monostable Mode

The 555 timer could easily be the most common chip used in DIY electronics projects because it’s small, inexpensive, and very useful.

It’s considered a timer because it can output pulses of electrical current for exact amounts of time. For example, it could be used to turn an LED off exactly 5 seconds after a button is pressed. It can also make an LED blink on and off, or generate higher frequency pulses that will make sound when connected to a speaker. This is the first article in a series where we’ll look into the three different modes of the 555 timer – monostable, bistable, and astable. Each mode has different characteristics that determine how the 555 timer outputs current. In this tutorial, I’ll discuss the monostable mode, but check out our articles on the astable mode and bistable mode as well.

In monostable mode, the 555 timer outputs a single pulse of current for a certain length of time. This is sometimes referred to as a one-shot pulse. An example of this can be seen with an LED and a push-button. With one press of the button, the LED will light up, then turn off automatically after a predetermined length of time. The time the LED stays on depends on the values of a resistor and capacitor connected to the 555 timer. The time can be calculated from the equation:

Where t is the length of the electrical output in seconds, R is the resistance of the resistor in Ohms, and C is the capacitance of the capacitor in Farads. As you can see from the equation, the length of the electrical output can be increased by using larger resistor or capacitor values. The opposite is also true. You can get a shorter output pulse with smaller resistor or capacitor values.

To observe the monostable mode of the 555 timer, let’s build a simple one-shot timer that will turn off an LED after a certain length of time. Use the diagram below to connect the circuit:

R1: 10K Ohms

R2: 10K Ohms

R3: 470 Ohms

C1: 470 μF

C2: 0.01 μF

In this circuit, after you press the button once, the LED will light up then turn off after about 5 seconds. The values of R1 and C1 determine how long the LED stays on:

you can use different capacitor value and as you increase the capacitance of capacitor the led will glow for more few seconds