Op Amp Monostable multi vibrator (mild panic)

[gildasd]

This is one of the subjects we might have in a test tomorrow, I disagree with my whole class on the results and method...
Remark: we get extra points if we use realistic parts values (as if we were to breadboard the thing)
This is how it was drawn on the black board and next to it, redrawn in a more "classical" manner.

• The diode D clamps Voltage Vd at -0.8 V
  I need to send a triggering pulse that is bigger to start the circuit.
• The Pulse with "T" is calculated as T=0.693*R*C
  T is given as 2 seconds, I choose 47µF thus R=T/(0.963*C)= 2/(0.693*47*10^-6)=61404 rounded to 62000 +-1%
• Voltage of capacitor (voltage where Vout drops back to -Vcc):/li]
  Vcap= +Vcc*R1/(R1+R2)=15*10000/(10000+10000)=7.5V
  

All of this is then regrouped in the following graph;


They get C=200µF and R=10000... That just seems wrong.

[MatthewEveritt]

I have to agree with your answer.
Assuming, as you seem to have done, the R1 = R2 you need to charge to 1/2, which takes ln(2) time constants (0.693).

C=200µF and R=10000 gives a time constant of 2 seconds, so doesn't work with R1 = R2, but does work with a different ratio, so I can't disagree with their answer either.

[gildasd]

I have to agree with you.

C=200µF and R=10000 gives a time constant of 2 seconds, but assuming, as you seem to have done, the R1 = R2 you need to charge to 1/2, which takes ln(2) time constants (0.693). They seem to have ignored this.

That's what I was thinking, the professor did the demonstration on the astable, why should he do it again and T=RC does not make sense.
C=200µF and R=10000 can work, but it needs more steps to get there.
Thanks!

[MatthewEveritt]

Sorry, bit of a brain fart - their answer does make sense if the ratio of R1 and R2 is set to compensate. I've altered my post to reflect this. R2 = (e-1) * R1 should do the trick.

[gildasd]

Sorry, bit of a brain fart - their answer does make sense if the ratio of R1 and R2 is set to compensate. I've altered my post to reflect this. R2 = (e-1) * R1 should do the trick.

No problem,but it does seem to be the hard way of doing it...

[gildasd]

Crushed the exam... Thanks to all!