555 timer as a missing pulse detector

Thread Starter

Reshma

Joined Mar 11, 2007
54
Hi Folks!

I have undertaken a project which is part of my curriculum and I have chosen to use a 555 timer as a missing pulse detector. I am trying to keep my circuit as simple as possible.

At the outset, the input for the missing pulse detector will be a signal will varying pulse widths. This can be generated using a 555 timer in the astable mode with a modulating signal applied at the Control pin 5. I will get a pulse-width modulated signal at the output pin 3. (This Circuit is functioning well).

I gave this modulated signal as the input to the missing pulse detector circuit operated in the monostable mode. But I am unable to interpret the output waveform of this circuit. I know that this circuit will detect and missing pulse according to its R/C combination i.e. T = 1.1RC.

Any flaws in my circuit design? Any suggestions you can make to improvise?

I have attached the circuit diagrams that I have used.


 

thingmaker3

Joined May 16, 2005
5,084
As noted by Tony Van Roon himself on the website where you got your missing pulse detector circuit from, the o/p of your monostable is a function of resistance and capacitance, or 11mSec in your case. (You did even note this in your first post.) The frequency of your input pulses is too low, so your missing pulse detector is detecting oodles and scads of missing pulses. Either bump up your input frequency or change your timing component values.

I suggest re-reading the Tony Van Roon website from which your missing pulse detector circuit was copied.
 

Thread Starter

Reshma

Joined Mar 11, 2007
54
Thanks, :) Also, can you explain the role of the pnp-transistor used in the missing pulse detector circuit. I have used BC557 in my circuit.
 

hgmjr

Joined Jan 28, 2005
9,029
I believe its sole mission is to discharge the cap in preparation for the next pulse cycle. This in turn sets up the remaining internal circuitry for the next cycle.

hgmjr
 
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