hi...i wanted to ask that i have been experiencing a problem with the capacitor discharging while using monostable...

as the theory goes the charging of capacitor starts for the -ve edge of the trigger and then when it reaches 2/3 vcc the capacitor completely is charged and till then the output is higher, irrespective of the input.

but i m unable to understand why in my design the capacitor discharging depending on the input signal...it perfectly starts charging the the -ve edge of the signal but is discharged irrespective of its charged value during the +ve edge of the input....

i have tried changing the time constant but this doesn't help....

please reply..

 

Greetings! And welcome to AAC!

I've moved your post into its own thread, where it will draw more responses:
http://forum.allaboutcircuits.com/showthread.php?t=14018

This is a special thread of Bill Marsden for the eBook.

Please refrain from "hijacking" existing threads with tangent or off-topic questions. You can create new threads for new questions using the "New Thread" button on the upper left of the page.

Greetings,
Bertus

 

Show us your schematic, we'll be glad to help.

 

today only i got a thing that when the input duty cycle was more than 25%
then only the monostable was working like it should be...else the output waveforms were like the one i have shown in the diagram....

as u can see...what ever be the T = RC, the capacitor remained charged till the input is low...although when I increased the value of C too high, capacitor missed one input pulse bt operated in the same manner as above..

so please explain why this 25% duty cycle contraint is there and how to over come that..!!

 

Differentiate the trigger with a series C, R pullup to Vdd.

 

ok i'll see that but why is this problem coming at all....??

 

If the trigger pulse persists beyond the RC time constant, then the 555 never times out, and the output stays high until the trigger goes high again.

 

R1 and C1 is needed in this design.

If your logic is normally high, going to low and back to high to trigger the monostable R1 and C1 are not needed, but normally they are required.

In the design shown at 555 Monostable they are also there, for a reason.

Pin 2 must be high when the timer is idle. This is not optional.