Operation of an Astable Multivibrator (using the 555 timer)

[Amith23]

Hi all,

I just need to know if the operation of my 555 timer is correct. Basically, I have it in the astable configuration. I am given a 1uF capacitor and I need to generate a pulse with a t_H of 1 second and t_L of 0.5 second. Doing the relevant calculations, I end up getting 714285.71 Ohms for both R₁ and R₂. However, during the first pulse my t_H is ~1.57 seconds (LED on)  and t_L is 0.5 (LED off) seconds:

After the first pulse, t_H is ~1 second and t_L is ~0.5 seconds. I have not tried this practically as yet. Is this the normal operation of a 555 timer?

Thanks in advance.

Regards.

[Amith23]

Apparently, the textbook I am studying from (Digital Fundamentals by T.L Floyd) seems to be using the wrong formula.

For t_H the textbook uses:

t_H = 0.7*(R₁ + R₂)*C

The correct formula is suppose to be:

t_H = 1.1*(R₁+R₂)*C