Beginner w/555 basic monostable circuit

[KenPh]

Hi guys,<p>I am working with a 555 and using a Forest Mims book to build a simple monostable circuit.<p>I 'believe' its wired correctly, but its not behaving correctly.<p>My setup is as follows:<p>1. +12 volts to pin 8
2. 9.8K resister between pin 8 and 7
3. jumper between pin 7 and pin 6
4. 22uF cap. between 6 and grnd(pos side @ pin 6)
5. nc switch between +12v and pin 2
6. nc switch between +12v and pin 4
7. 1K resister and LED between pin 3 and grnd
8. .05 uF cap between pin 5 and grnd
9. jumper from pin 1 to grnd<p>The above works intermittently (~50% of the time) unless I touch either the conductors on the LED and its resistor OR the conductor of the .05 uF cap on pin 5. Can anyone enlighten me, I am in the process of attempting to learn a bit about electronics and would appreciate some patient guidance.<p>Ken

[russlk]

You need a resistor from pin 2 to ground. The 555 is triggered on the falling edge of the input pulse, but the input will not fall reliably without a pulldown resistor. 10K resistor should work.

[KenPh]

Russ,<p>Thanks, it worked like a charm.<p>Ken