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I have built numerous 555 circuits over the years. I recently had to built a Monostable 555 cct. as per this site : http://www.allaboutcircuits.com/vol_6/chpt_8/4.html

The cct. is used in a motor vehicle. When the remote control is pushed to open the doors, it sends positive 12V to this Monostable cct. to initiate it and a 3 Amp Power Transistor pulses the siren momentarily to indicatse "unlocking" by audible means.

The issue I have is that Rt and Ct used to time the output pulse is not short enough. I replaced these components to the smallest values I could get, but the output is still "too long". Teh siren makes a "chooow" sound instead of "chow". Sounmds too unprofessional or after-market.

Any ideas to get a short pulse (nearly a few micro seconds) out of this old 555 cct. ?

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    \$\begingroup\$ Just as an aside: A sound of just a few microseconds is likely to be below threshold of human perception. In telephony, 10 microseconds is considered the shortest audible sound, while professional sound recording resources put the minimum duration at 20 microseconds. This article might provide insights. \$\endgroup\$
    – Anindo Ghosh
    Commented Dec 3, 2012 at 15:12

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If you have reduced Rt,Ct and the pulse is still too long, consider that the trigger input signal may also be too long. (this circuit is designed to stretch an input pulse, not shorten it!) In the circuit you linked to, that is controlled by C1 and R4 (though their instructions suggest it is controlled by C1 and R3)

Reduce C1 to 10nf and try again. Reducing C1 too far will make the triggering unreliable.

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  • \$\begingroup\$ Correct. Changing to a real one-shot IC instead of using a 555 as a monostable multivibrator would correct this. \$\endgroup\$
    – Scott Seidman
    Commented May 3, 2013 at 14:09

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