Hello,

Yes.
If the total period is 20 mS and TL is 9.52 mS. What is TH then?

Bertus

It will be:
TH = T total - TL
TH = 20 - 9.52
TH = 10.48 mS

 

So to be on the road then:
F=1/TH+TL
then TH=1/F+TL
TH = 1/50 Hz + 9.52*10^-3
TH = 0.029 sec ==> 29.5 mS
right ?

You are being extremely sloppy with your math and your equations and it is biting you big time.

The total period is 1/f and is also the sum of the high-time and the low-time. So

1/f = TH + TL

So

TH = (1/f) - TL

 

You are being extremely sloppy with your math and your equations and it is biting you big time.

The total period is 1/f and is also the sum of the high-time and the low-time. So

1/f = TH + TL

So

TH = (1/f) - TL

yes i know what you mean i totally forgot to put subtract instead of the sum
thanks
i'm being dizzy like that all this time
cause week ago my gf broke up with me and i have to solve this for tomorrow homework and like i said i'm kind of new at this

 

All the more reason to slow down, take it step by step, and verify that each step really does follow from the prior step. Also a very strong reason to properly track your units. Most of the mistakes we make screw up the units and let us determine that (1) we messed up, and (2) where we messed up, almost immediately. But that is only true if the units are there to get screwed up in the first place. Throwing numbers at equations and then tacking on the units that you want the final answer to have is a sure way to get lots of wrong answers that you fail to catch.

 

All the more reason to slow down, take it step by step, and verify that each step really does follow from the prior step. Also a very strong reason to properly track your units. Most of the mistakes we make screw up the units and let us determine that (1) we messed up, and (2) where we messed up, almost immediately. But that is only true if the units are there to get screwed up in the first place. Throwing numbers at equations and then tacking on the units that you want the final answer to have is a sure way to get lots of wrong answers that you fail to catch.

you are right bro
thanks

 

hey guys how are you
today the teacher played a game on us and made the homework exam and some other things including my 555 timer homework
so this is what i wrote to solve the equation
the values the teacher want us to determine are (R1) and (duty cycle)
the values are the same in the photo i gave you but this time the frequency is 42 Hz
----------------------------------------------------------------------------------------------------------------------
TL = 0.7 C1 R2
TL = 0.7*(680*10^-9)*20000
TL = 9.52*10^-3 sec ==>9.52 mS
---------------------------
F = 1/T
so T = 1/f
T = 1/42
T = 0.02 sec ==> 20 mS
--------------------------
T(total) = TH + TL
then TH = T(total) - TL
TH = 20 - 9.52
TH = 10.48 mS
------------------------
am i right until now guys cause my friend said it's wrong

 

Hello,

Wich photo says 42Hz?
The calculation for T=1/42 will NOT give you 20 mS.

You where saying 50 Hz yourself.

The TH is indeed 10.48 mS.
Now you can calculate the R needed for this time.
The R2 you want can be found using the total resistor value found and subtract the 20 K of R1.

Bertus

 

Hello,

Wich photo says 42Hz?
The calculation for T=1/42 will NOT give you 20 mS.

You where saying 50 Hz yourself.

The TH is indeed 10.48 mS.
Now you can calculate the R needed for this time.
The R2 you want can be found using the total resistor value found and subtract the 20 K of R1.

Bertus

yes i know
the photo say 50 Hz
but in exam today 42 Hz

 

Hello,

When the frequency is 42 Hz, the total cycle time will be different (longer).

Bertus

 

Hello,

When the frequency is 42 Hz, the total cycle time will be different (longer).

Bertus

how and why?
my calculator gave me 0.02 ?

 

Hello,

The 0.02 is only valid for 50 Hz.
When I calculate 1/42 I get 0.023809524 as result.

Bertus

 

Hello,

The 0.02 is only valid for 50 Hz.
When I calculate 1/42 I get 0.023809524 as result.

Bertus

dont tell me it will be (23.80) in mS

 

Hello,

I have take an other calculator that says 1/42 = 0,0238095238095, so you will have to use that for the further calcutaions.

Bertus

 

Hello,

I have take an other calculator that says 1/42 = 0,0238095238095, so you will have to use that for the further calcutaions.

Bertus

same value (23.80) mS

 

Hello,

I would rather say 23.81 mS as after the 0 is a 9, so rounding up must be done.

Bertus

 

Hello,

I would rather say 23.81 mS as after the 0 is a 9, so rounding up must be done.

Bertus

so where's that put me
my math is wrong or what bro

 

Hello,

Recalculate TH as it is not 10.48 mS anymore.
When you got the new TH value, you can calculate the R needed for this time.
The R2 you want can be found using the total resistor value found and subtract the 20 K of R1.

Bertus

 

Hello,

Recalculate TH as it is not 10.48 mS anymore.
When you got the new TH value, you can calculate the R needed for this time.
The R2 you want can be found using the total resistor value found and subtract the 20 K of R1.

Bertus

but what's the point i got it wrong in today's exam so no use :'(

 

Hello,

By doing it now, you can get used to doing calculations like these.
You will only learn by doing it.
When we give you the complete answer, you will not have gotten the grasp on how to do the calculations.

Bertus

 

Hello,

By doing it now, you can get used to doing calculations like these.
You will only learn by doing it.
When we give you the complete answer, you will not have gotten the grasp on how to do the calculations.

Bertus

yes i know bro you are right
but tell me something
are my steps true don't care about numbers
just the steps