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From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: 555 Monostable?
Date: 8 Nov 2002 15:07:43 -0800
NNTP-Posting-Date: 8 Nov 2002 23:07:44 GMT
"Kevin Patterson" wrote in message news:...
> The 555 circuit that I have made is a simple monostable timer, that triggers
> a relay on the output(pin3). My problem is that the timing interval starts
> on the first press of the trigger (pin2), any other trigger presses within
> the time interval don't restart the timer.
> My scienrio is this, I want to enable a relay for a set period (4 seconds),
> if the trigger button is pressed within this 4 seconds I would like the
> timer to start again (i.e. 4 seconds after the last press)
> The circuit can be found at this link http://www.eleinmec.com/article.asp?4
> Does anyone know how I could modify my monostable 555 circuit (NE555N IC),
> to allow retriggering from the trigger input.
If you simply put a diode between pins 2 (cathode) and 6 (anode) of
that circuit, it should do what you want. You might also put a small
resistance in series with the diode to limit the current. Diode
leakage current should be small compared with the current in R, so if
R is 1 megohm, be sure the diode leakage current is under a microamp
or so. The series resistance to limit the current would be chosen
from Rlimit = Vs*2/(3*diode_peak_current_rating). For example, if you
can use a 1N4004 diode and it has a peak current rating of 30A and
Vs=15V, you should use 1/3 ohm resistance minimum. 10 ohms would be
fine in this case. (You should probably pick the lower of the diode
peak current rating and a safe current rating for the switch that
won't decrease its life through pitting of the contacts.) If R is
100kohms and C is 47uF in your circuit, then the 10 ohm resistor will
discharge C in a millisecond or so, so you don't have to worry about
holding the switch down for a long time...a very short tap would do.
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