Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!rutgers!dptg!att!icus!limbic!gil From: gil@limbic.UUCP (Gil Kloepfer Jr.) Newsgroups: sci.electronics Subject: Re: 555 IC Timer help needed Summary: 555 timer info Message-ID: <568@limbic.UUCP> Date: 25 Sep 89 01:45:05 GMT References: <169@uwm.edu> Reply-To: gil@limbic.UUCP (Gil Kloepfer Jr.) Organization: ICUS Software Systems, Islip, NY Lines: 49 In article <169@uwm.edu> duc@csd4.csd.uwm.edu (Duc Minh Tran) writes: > > Vcc > |__________ > | | > / __|__ > R = resistor (R1) __|_____|__ > C = capacitor \____| 4 8 | > | |7 | (Output) __ __ > / | 3|---------> __| |__| |__ > (R2) | (555) | > \____| TIMER | > | + |2 | > | |__| | > | |6 1 5|---> ??? > -+- |_____+_____| > (C1) __|__ > -+- --- > __|__ GND > --- > GND > Pin 5 (control voltage) generally gets connected to a 0.1 or 0.01 uF capacitor. You may leave it unconnected with little or no ill side-effects. The equation for a 555 running in free-running mode (from the Radio Shack Semiconductor Reference Manual) is: Charge time (output high) t1=0.693(Ra+Rb)C Discharge time (output low) t2=0.693(Rb)C Total period T=t1+t2=0.693(Ra+2Rb)C Frequency of oscillaton f=1/T=1.44/(Ra+2Rb)C I believe that these equations, as they apply to the above diagram, are Ra=R1, Rb=R2, C=C1. Inasfar as replacing R2 to speed up and slow down the rate - sure you can. Whether you replace R1 or R2 with a pot would depend on whether you wanted the output active high or low for the "longer" amount of time. Also note that, although I don't see it in this data sheet, that in all other places I've read it is unwise to make R1 any smaller than 1K ohm. I may be wrong about this, but I thought I read it somewhere. It's probably a good rule of thumb. ----- | Gil Kloepfer, Jr. | ICUS Software Systems/Bowne Management Systems (depending on where I am) | ...ames!limbic!gil