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Long time delay

M

M.Joshi

Jan 1, 1970
0
Arfa said:
"Ralph Mowery" [email protected] wrote in message
A 555 is good for delays of a few seconds to a few minutes, but t
get
a
45
minute delay will require a huge timing capacitor and the tim
delay--
won't--
be very accurate at all. You really need to use a counter to scal
down--
a--
higher frequency for such long delays.

-

This is not strictly speaking, a problem - if you use the CMOS versio
of
the 555, from Harris, for instance. This is capable of producin
delays-
from-
" microseconds to hours ", quoting from the Maplin data, fo
instance.
The
only reason that the bipolar 555 had trouble with large values o
caps,-
was-
the typical leakage of these exceeding the trigger / threshold pins'
input
currents, when large value timing resistors were used to go with the
large
value caps. This limited you to a meg or so and a few tens o
microfarads-
if-
you wanted a stable predictable timer. This is not the case with th
CMOS
version, where the input currents are in pico amps, so you can us
large
value resistors and caps to get much longer delays. In terms of sheer
simplicity, I'm with Dave on this one, although your clock an
divider
solution is an elegant way of achieving the same result, and one that
I've
used in the past ;-)-

Another problem with long time delays is how to calibrate the delay.
If
you
do use the 555 or similar version then it could take a week to se
the
time.
Remember it is 30 minuits between checks. The beter way is to us
some
kind
of devider chain so you can set the timer up. He may want to use th
555
and a 100:1 devider. I have not checked to see what is out on th
market
now but there should be lots of chips that will devide by almos
anything.
Probably running an oscillator very fast and deviding it down will b
more
accurate and maybe easier to set.
If the device is operated off AC then deviding the 60 hz will be
good
place to start.

-
Provided you use a good quality low leakage cap, and high stabilit
high
value resistors, the delay time should be predictable with reasonable
accuracy, using the standard 555 timing formula. Shouldn't take mor
than 3
or 4 runs to get the time delay down to what you need within a fe
seconds.
It seemed to me that the OP needed a 'simple' solution, which the 555
circuit is, requiring only one resistor and one capacitor besides th
device
itself ( the CMOS version doesn't need either the supply decouplin
against
crowbarring, or the control pin decoupling ).

This is in contrast to the clock and divider solution, which I accep
is
totally predictable, but requires a divide ratio of 162000 ie 18 bit
to
arrive at 45 minutes,even using a low speed clock such as the 60H
line
supply ( which will also need clamping and shaping to form a suitable
clock ). Also, some ANDing will need doing on the output of the divide
to
get an output at the required time delay. Not as simple as the time
chip
solution, properly implemented.

Arfa


Thanks for your reply guys.

I think the 555 I.C solution may be the simplest. I have some NE555'
- I believe they are the CMOS type?

Is the formula T = 1.14*R*C

Does the output sink high/low during the timing period?

Can the output signal be used to drive another I.C?

Thanks
 
D

Dave Plowman (News)

Jan 1, 1970
0
I think the 555 I.C solution may be the simplest. I have some NE555's
- I believe they are the CMOS type?

IIRC, NE555 aren't CMOS. Look for something like the ICM7555. And apart
from allowing much longer timing intervals, the quiescent current is tiny.
 
A

Arfa Daily

Jan 1, 1970
0
Dave Plowman (News) said:
IIRC, NE555 aren't CMOS. Look for something like the ICM7555. And apart
from allowing much longer timing intervals, the quiescent current is tiny.

--
*I'm not being rude. You're just insignificant

Dave Plowman [email protected] London SW
To e-mail, change noise into sound.

Agreed. NE555 is the normal bipolar version of the IC. As Dave says ICM7555
is a CMOS

Arfa
 
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