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From: rep@genrad.UUCP (Pete Peterson)
Newsgroups: sci.electronics
Subject: Re: 555 timer duty cycle
Message-ID: <1452@genrad.UUCP>
Date: Wed, 25-Nov-87 09:45:21 EST
Article-I.D.: genrad.1452
Posted: Wed Nov 25 09:45:21 1987
Date-Received: Sat, 28-Nov-87 18:35:19 EST
References: <9376@shemp.UCLA.EDU>
Reply-To: rep@genrad.UUCP (Pete Peterson)
Organization: GenRad, Inc., Concord, Mass.
Lines: 26

In article <9376@shemp.UCLA.EDU> khayo@MATH.UCLA.EDU (Erazm J. Behr) writes:
>Is there any way to wire the 555 to get an "on" time of
>less than 50% of the cycle? (I know, "just add an inverter" -
>but I'd like to know if it can be done without adding extra
>active parts). Sorry for this, I'm sure the answer is in the
>library somewhere ...... 8-O
>-----------------------------------------------------------
Information from Signetics applications note:

Normal astable operation has a resistor Ra from Vcc to pin 7;
a resistor Rb from pin7 to pin6/pin2; a capacitor from pin6/pin2
to ground.

To get shorter duty cycles you connect a diode from pin7 (anode)
to pin 6 (cathode) and it is recommended that you put another
diode in series with Rb (anode of diode to pin6, cathode to Rb.).

The signetics apps note claims you can get duty cycles from 5% to 95%
with this configuration and recommends a minimum value of 3k for Rb.

This configuration gives charge path: Ra->diode->capacitor and
discharge path: capacitor->diode->Rb->discharge transistor (in 555),
so you can determine high and low times more or less independently.

	pete peterson
	{decvax,linus,wjh12,mit-eddie,masscomp}!genrad!rep