Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!philmtl!philabs!ccnysci!sukenick
From: sukenick@ccnysci.UUCP (George Sukenick)
Newsgroups: sci.electronics
Subject: Re: 555 IC Timer help needed
Message-ID: <3356@ccnysci.UUCP>
Date: 6 Oct 89 15:04:36 GMT
References: <169@uwm.edu> <568@limbic.UUCP>
Reply-To: sukenick@ccnysci.UUCP (SYG)
Organization: City College Of New York
Lines: 16

>Pin 5 (control voltage) generally gets connected to a 0.1 or 0.01 uF

As previously stated, pin 5 can be used to alter the output frequency.
One circuit that I recall was to have the output of another 555 timer 
connected to pin #5  (one variation was to have the output go through
an RC ie: pin #3 (ic2) connects to Pin #5 (ic1), with a cap at pin
#5 (ic1) to ground or power.  This will give you a siren or whooping
sound depending  upon the values of resistors used.

>The equation for a 555 running in free-running mode (from the Radio
>Shack Semiconductor Reference Manual) is:

Be very careful with equations: I've seen different equations for 555's,
duty cycles, frequencies, etc. depending upon the manufacturer. 
Also, some will work even after pin 7 is shorted to pin 8, others
will blow out immediately if the resistance is lower than 1k or 200 ohms.