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