# How to convert 555 timer as an astable to monostable mode ?

555 timer
I want to use astable 555 timer as a monostable mode, by programming. I know it’s possible by changing device, but I want to use single device. I think logically it’s possible but can’t programmed it properly.

Scholar Asked on March 16, 2017 in

The astable mode is what most people think of when it comes to the 555 timer. Many times when you see a project with flashing LEDs, it’s a 555 timer at work. But it has a lot of other interesting applications too. For example, it can also generate frequencies to produce sound when the output is connected to a speaker. It can even be used as a simple analog to digital converter (ADC).

In astable mode, the 555 timer acts as an oscillator that generates a square wave. The frequency of the wave can be adjusted by changing the values of two resistors and a capacitor connected to the chip. The formulas below will tell you the length of the output’s on and off cycles with different resistors and capacitors:

$t_{on}=0.69\times C1 \times (R1+R2)\\ t_{off}=0.69 \times C1 \times R2 \\ \\ t_{on}: \ Length \ of \ high \ output \ pulse \ in \ seconds \\ t_{off}: \ Length \ of \ low \ output \ pulse \ in \ seconds \\ R1: \ Resistance \ of \ R1 \ in \ Ohms \\ R2: \ Resistance \ of \ R2 \ in \ Ohms \\ C1: \ Capacitance \ of \ C1 \ in \ Farads$With this equation, you can see that increasing the values of either C1 or R2 will increase both the time the output stays on and the time it stays off. Increasing the value of R1 will only lengthen the time the output stays on.

Curious Answered on March 17, 2017.

Pin no 2 of timer 555 is ‘trigger’.        By providing trigger pulse High to LOW, it enters into monostable mode. For programming purposes, keep trigger input of 555 High and make it Low for short duration when you want to make it monostable

Scholar Answered on March 22, 2017.