From: Phil Hobbs
Subject: Re: 555 Timer question
Date: Tue, 08 Oct 2002 11:26:52 -0400
Organization: IBM T. J. Watson Research Center
References: <0001HW.B9BDB985000A4AA7165FEAC0@news.covad.net> <3DA2E532.B3DEAC58@alcatel.be>
NNTP-Posting-Date: 8 Oct 2002 15:26:55 GMT
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Danny Hendrickx wrote:
> Hi Dave,
> a 555 in standard astable circuit can not reach 50 % duty cycle. I once
> saw a circuit where the bottom (?) resistor was bridged by a diode to
> make sure the cap had equal charge/discharge time, making a 50% duty
> cycle possible. This way the cap uses the top resistor to charge, and
> the bottom resistor to discharge. Frequency regulation will still
> require you to change both resistors at the same time though.
It's much easier than this. Instead of using the resistors to Vdd to
charge the cap and the discharge pin to reset it, which is where the
asymmetry comes from, just use the output pin.
Connect the cap to pins 2 and 6 as usual, but leave pin 7 open, and run
a single R to the output (pin 3). If the top and bottom thresholds are
symmetrical about Vdd/2, that will give you a 50% duty cycle. With a
bipolar 555 (does anyone still use those?) the supply rejection will
suffer, but with a CMOS 555 it's just as stable as the old method.