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From: gpz@bridge2.ESD.3Com.COM (G. Paul Ziemba)
Newsgroups: sci.electronics
Subject: Re: 555 IC Timer help needed
Message-ID: <937@bridge2.ESD.3Com.COM>
Date: 25 Sep 89 20:28:28 GMT
References: <169@uwm.edu>
Distribution: na
Organization: 3Com Corp., Mt. View, CA
Lines: 61

duc@csd4.csd.uwm.edu (D. Tran) writes:

Tran>Greetings, I have a simple question that I'm sure someone can answer.
Tran>I'm building a simple circuit that contains a 555 IC timer chip.

Tran>                        Vcc
Tran>                         |__________
Tran>                         |          |
Tran>                         /        __|__
Tran>  R = resistor         (R1)    __|_____|__
Tran>  C = capacitor          \____|  4     8  |  
Tran>                         |    |7          | (Output)     __    __
Tran>                         /    |          3|---------> __|  |__|  |__
Tran>                       (R2)   |   (555)   |
Tran>                         \____|   TIMER   |
Tran>                         | +  |2          |
Tran>                         | |__|           |
Tran>                         |    |6    1    5|---> ???
Tran>                        -+-   |_____+_____|
Tran>                       (C1)       __|__
Tran>                        -+-        ---
Tran>                       __|__       GND 
Tran>                        ---
Tran>                        GND 

Tran>         1) How do I find the values for R1, R2, and C1?
Tran>			Is there a formula
Tran>            [ie. C1 (R1 + R2)...]

If you want a 50% duty cycle square wave, make r1 small with respect to r2.
Assuming r1 << r2, an approximate formula for the frequency is 1.1/(r2 * c2)
(I'm doing this off the top of my head; the data sheet has a more exact
formula).


Tran>         2) Can I replace R2 with a potentiometer so I can variate the
Tran>            output timing (ie. speed it up or slow it down).

Yes, but you should probably include a small series resistor so that the
total resistance between pins 7 and 2 doesn't ever go to zero.

Tran>         3) What does pin #5 get hooked up to?

Hook this to a .01uF capacitor going to ground. If you don't do this,
you may get some rather weird oscillations.


Also, you might be interested to know that you can turn the oscillator
on and off by controlling pin 4. Grounding it will shut off the 555 output.


Tran>       please email so we don't offend anyone.

Well, heck, I'll take responsibility for any offense my followup may
cause. I'd be interested in hearing what kinds of things people do
with pin 5 (I think that by placing a varying dc voltage on it one
can change the period of oscillation up or down 30 % or so).

 cheers,

 ~!paul