Digital Electronics - Flip-Flops - Discussion
Discussion Forum : Flip-Flops - General Questions (Q.No. 29)
29.
An RC circuit used in a nonretriggerable 74121 one-shot has an REXT of 49 k
and a CEXT of 0.2 
F. The pulse width (tW) is approximately ________.
and a CEXT of 0.2 F. The pulse width (tW) is approximately ________.6.9 s
s6.9 ms
69 ms
690 ms
Discussion:
11 comments Page 1 of 2.
Sameeksha said:
1 decade ago
How to find the pulse width?
Harish said:
1 decade ago
Pulse width=0.69RC
Substituting values of R&C in the above eqn we'll get 6.9 ms.
Substituting values of R&C in the above eqn we'll get 6.9 ms.
Tamphasana said:
1 decade ago
Thank you Harish.
Sunil said:
1 decade ago
pulse width of 1shot=0.69(RC).
Neetu said:
1 decade ago
Which one is correct ?
Pulse width = 0.69RC.
or
Pulse width = 1.1RC.
Pulse width = 0.69RC.
or
Pulse width = 1.1RC.
Nagaraj said:
1 decade ago
non retriggerable means it is a astable multivibrator.
Uppu said:
1 decade ago
Both are wrong I think because I substituted are and C values in above formulas I didn't get correct answer.
Vasudha said:
1 decade ago
PW = 0.69RC is the right one.
The 74LS121 standard one shot monostable multivibrator or the 74LS123 or the 4538B re-triggerable monostable multivibrator which can produce output pulse widths from as low as 40 nanoseconds up to 28 seconds by using only two external RC timing components with the pulse width given as: T = 0.69RC in seconds.
The 74LS121 standard one shot monostable multivibrator or the 74LS123 or the 4538B re-triggerable monostable multivibrator which can produce output pulse widths from as low as 40 nanoseconds up to 28 seconds by using only two external RC timing components with the pulse width given as: T = 0.69RC in seconds.
Arnab Bhattacharya said:
1 decade ago
As per datasheet of 74121, if Cext>1000pF,
Then, width of the pulse, tW = k*Cext*Rext (k=0.7).
Here Cext = 0.2uF = 0.2*10^6 pF > 1000 pF.
So, tW = 0.7*0.2*10^-6*49*10^3 sec = 6.86 ms ~6.9 ms.
So, option B is correct.
Thank you.
Then, width of the pulse, tW = k*Cext*Rext (k=0.7).
Here Cext = 0.2uF = 0.2*10^6 pF > 1000 pF.
So, tW = 0.7*0.2*10^-6*49*10^3 sec = 6.86 ms ~6.9 ms.
So, option B is correct.
Thank you.
Shahin said:
10 years ago
Correct answer is tw = KxRxC for 74121 k = 0.7.
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