555 monostable Operation problem

[shaiqbashir]

Hi guys!

well! im using 555 IC as a monostable multivibrator. What im using values are

R=690kohms
C=10uf

and i have connected trigger pin i.e. pin2 to the ground. Now i think when the circuit will be on, the pin3 should give me a pulse of width around 5-7 sec and then the output on pin3 should become low. BUt here is what im getting:

I get a constant output of around 5V as my Vcc is 5V. and when i connect my trigger pin to Vcc, the output becomes 0, when i connect it to GND, the output becomes 5V. But the thing is that output on pin3 doesnot get low after even a minute. WHat is this problem??

PLease help me as soon as possible!

i shall be thankful to u for this act of kindness!

Good Bye!

 

[audioguru2]

Hi Shaiq,
The datasheet tells how to use a 555.
If the trigger pin is held low then the timing cycle will never end.

View attachment 39313

 

[shaiqbashir]

Thank u very much audioguru!

i got it now.

Hope my monostable will work now!

thanks once again!

 

[audioguru2]

Hi Shaiq,
If the trigger goes to ground for a time period that is too long then add a 10k resistor from pin 2 to the positive supply and a coupling capacitor from the trigger source. Then it makes a short negative pulse no matter how long the trigger source is grounded.