# 555 timer as an astable multivibrator?

Discussion in 'Homework Help' started by pfelectronicstech, Oct 11, 2012.

1. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
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I'm trying to calculate the duty cycle of a IC 555 as an astable multivibrator. The Ra is 18K ohms, the Rb is 68K ohms, C is 0.04 and I have come up with 0.56 and that is wrong. Any ideas?

2. ### mlog Member

Feb 11, 2012
276
36
What equation are you using? I'd say you're wrong too.

By the way, it's meaningless to say C=0.04. What are the units?

3. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,393
1,211
Show your work for t1 and t2 ... then calculate the duty cycle

Jan 18, 2012
178
4

5. ### Audioguru New Member

Dec 20, 2007
9,411
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The capacitor charges with 18k + 68k= 86k.
The capacitor discharges with only 68k.
So calculate the duty cycle.

6. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Oh, ok, thanks, I'm coming up with 69.4 now, am I correct, getting closer?

7. ### mlog Member

Feb 11, 2012
276
36
Remember what Audioguru said? Discharge time divided by the sum of the charge and discharge times.

$D = \frac{R_b}{(R_a + R_b) + R_b}$

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8. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Finally, thank you mlog! Thank you, thank you, I just was not thinking about the problem in the right way. I tend VERY often to over think the situation, which is a bad trait. I suppose a lot of Techs over think situations though? Thanks again for the help.

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Page 6 in the datasheet has this equation. Datasheets can be very handy.

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10. ### pfelectronicstech Thread Starter Member

Jan 18, 2012
178
4
Thanks, I did not think to check the datasheet, my nose was stuck in my text books. Thanks again for that.