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# 555 timer astable multivibrator

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#### electroboy

##### Member level 5
In the following astable multivibrator circuit,

in some books, it is mentioned that the duty cycle id

d=( R1+R2)/(R1+2*R2)

in some other books, it is mentioned

d= R2/(R1+2*R2)

which is correct? which one should i use if i want to design a circuit of particular duty cycle ?

please tell me whether duty cycle is Ton/(Ton+Toff) or Toff/(Ton+Toff)

duty cycle is Ton/(Ton+Toff)

electroboy

### electroboy

Points: 2

i can't understand still sir.. please explain me ..

electroboy

### electroboy

Points: 2
which is correct? which one should i use if i want to design a circuit of particular duty cycle ?
please tell me whether duty cycle is Ton/(Ton+Toff) or Toff/(Ton+Toff)

I think, it's your choice. That's also the reason, why both expressions for d exist.

In my opinion, you would define high output as "on" and duty cycle as Ton/(Ton+Toff).
In this case, the first expression applies: d=( R1+R2)/(R1+2*R2)

electroboy

### electroboy

Points: 2
please tell me whether duty cycle is Ton/(Ton+Toff) or Toff/(Ton+Toff)

Normally when we say that the duty cycle is 10% we mean then the output is on for 10% of the period time so I think you should use Ton/(Ton+Toff)

Alex

electroboy

Points: 2
Ton/Ton+Toff

electroboy

Points: 2