555 monostable

I'm building a 555 monostable timer project to provide a range of 20ms-120ms delay time.

Realizing that the 555's pin3 output will remain high while it's pin2 trigger is held low, I searched these message boards and found the following post:

http://www.eleinmec.com/board_message_view.asp?messageID=128

which discusses the addition of a 0.1uF capacitor and another resistor to enable the 555 to time out even when triggered via a switch versus a momentary button.

My question is: Should the two resistors discussed in the aforementioned link be of equal value? Your example uses 33k, however my project's pre-existing resistor between pin 2 and +V is a 10k.

So am I correct to choose 10k for my additional resistor?

Thanks,
Dave

Posted by Anonymous on 17/09/2007, at 05:08 GMT

555 monostable

The resistor values are not critical. They should be similar but need not be equal so 10k should be satisfactory. If you want to be sure try a prototype on breadboard before soldering the circuit.

Posted by John Hewes on 17/09/2007, at 11:52 GMT