555 timer in monostable mode does not turn off

Thread Starter

przachar

Joined Feb 8, 2010
16
Its not that the timing is slightly off like you suggested. The led turns off the moment the pushbutton is released. not a few seconds later. Hence the timer is not acting as a timer.
 

Audioguru

Joined Dec 20, 2007
11,248
Then your 1M resistor has a value much less than 1M or your capacitor has a value much less than 10uf or your 555 is defective again.
Maybe your supply voltage is too low. The minimum supply for an ordinary 555 is 4.5V and it should have a supply bypass capacitor.
 

Ron H

Joined Apr 14, 2005
7,063
Its not that the timing is slightly off like you suggested. The led turns off the moment the pushbutton is released. not a few seconds later. Hence the timer is not acting as a timer.
The timing cycle starts when you push the button, not when you release it. Are you depressing the button for way less than 11 seconds?
 

Wendy

Joined Mar 24, 2008
23,421
When I mention the charging cycle in post #13, you should be able to see this on the DVM. Really use your test equipment, don't just look for static voltages.

Use the info on post #13 for the voltages too, I gave them for troubleshooting. You either have a wiring error or a bad (or wrong) component. Right now my money is on the capacitor.

If pin 2 is low then pins 6 and 7 are charging, until they are high. You can see this charge curve with a DVM, and it will stop when they are Vcc.
 
Top