[sci.electronics] A pathetically simple question...

reynhout@wpi.wpi.edu (Andrew Reynhout) (09/29/89)

   Help!  This is an amusingly simple question, but I've been working on it
for some time now, and can't come up with an answer...

   I need a circuit that will, on application of power, cause a delay of 5-20
seconds before allowing current to flow through (a different part of the circuit)
   Basically, it should take, say, 12v, wait 10 seconds, then trip a relay.
It's for an alarm circuit.  (I didn't see much point in paying $159 for $30 of
components and a few hours of work.)
   I tried a deceptively complex setup with a FET and a timing cap and all sorts
of other nasty things, but I can't get ANYTHING out of it.

   Any help would be greatly appreciated...
   Andrew

arnief@tekgvs.LABS.TEK.COM (Arnie Frisch) (09/30/89)

To build a reproducible time delay of several seconds to 
a minute - or more - a good approach is to do it digitally using
CMOS.  Build an inverter based oscillator out of two resistors
a capacitor and two inverter sections (any good CMOS application
manual will show how).  Follow this with a multi-stage counter.
The counter will have to be reset at power on - This will take a 
diode, a capacitor and a resistor.  Limiting yourself to 1 uf
capacitors and 1 meg resistors and using a seven stage counter IC,
you should be able to get delays up to 300 seconds.

I don't do schematics for free!

Arnold Frisch
Tektronix Laboratories

jans@tekgvs.LABS.TEK.COM (Jan Steinman) (09/30/89)

<I need a circuit that will, on application of power, cause a delay of 5-20 
seconds before allowing current to flow through.  Basically, it should take, 
say, 12v, wait 10 seconds, then trip a relay...>

Check out the CMOS 4528 dual mono.  These suckers will delay up to *days* with 
careful parts selection.  (your basic 10 Gohm resistor: clean a piece of glass 
with MEK, draw a thin line of india ink on it... the leads are a problem :-)  
They're re-triggerable, and lack a lot of the nasty behavior of the infamous 
555.  Plus, you get two of them in a DIP.  For a 10 second delay, try 10 Mohm 
and 1 uf.  Capacitor leakage current will reduce the effective time constant -- 
use non-electrolytic caps with resistors over half a meg or so.

							   Jan Steinman - N7JDB
						  Electronic Systems Laboratory
					Box 500, MS 50-370, Beaverton, OR 97077
						(w)503/627-5881 (h)503/657-7703

larry@kitty.UUCP (Larry Lippman) (09/30/89)

In article <4377@wpi.wpi.edu>, reynhout@wpi.wpi.edu (Andrew Reynhout) writes:
>    I need a circuit that will, on application of power, cause a delay of 5-20
> seconds before allowing current to flow (a different part of the circuit)
>    Basically, it should take, say, 12v, wait 10 seconds, then trip a relay.
> It's for an alarm circuit.  (I didn't see much point in paying $159 for $30 of
> components and a few hours of work.)
>    I tried a deceptively complex setup with a FET and timing cap and all sorts
> of other nasty things, but I can't get ANYTHING out of it.

	I am assumning that this is NOT for an OEM application where you must
have this circuit function on a PC board along with other circuit elements.

	All you need is a time delay relay, with a delay-on-operate mode.
The simplest time delay relay is of the thermal delay type, as manufactured
by Amperite or GV Controls.  You can often find such thermal delay relays
(which are usually made for octal sockets, although 7/9-pin miniature also
exists) in surplus stores for a buck or so.

	A far better approach is to use a solid-state timer, which can be
purchased new for as little as $ 25.00, and surplus for $ 5.00 or less.
Such a solid-state timer - which is available in many package types -
will do EXACTLY what you want and will provide an isolated set of relay
contacts.

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
<> UUCP  {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry
<> TEL 716/688-1231 | 716/773-1700  {hplabs|utzoo|uunet}!/      \uniquex!larry
<> FAX 716/741-9635 | 716/773-2488      "Have you hugged your cat today?" 

morris@jade.jpl.nasa.gov (Mike Morris) (09/30/89)

In article <4377@wpi.wpi.edu> reynhout@wpi.wpi.edu (Andrew Reynhout) writes:
>
>   Help!  This is an amusingly simple question, but I've been working on it
>for some time now, and can't come up with an answer...
>
>   I need a circuit that will, on application of power, cause a delay of 5-20
>seconds before allowing current to flow through (a different part of the circuit)
>   Basically, it should take, say, 12v, wait 10 seconds, then trip a relay.
>It's for an alarm circuit.  (I didn't see much point in paying $159 for $30 of
>components and a few hours of work.)
>   I tried a deceptively complex setup with a FET and a timing cap and all sorts
>of other nasty things, but I can't get ANYTHING out of it.

Look in a catalog for an Amperite Delay Relay.  They come in the same package
as the thermeonic high-voltage-depletion-mode-JFETs (a.k.a. Vacuum Tubes),
and are just a heater and a pair of bimetalic contacts.  They come in 
6v, 12v, 24v, 110v and 220v, and in times ffrom 1 second to 3 minutes, 
normally open ort normally closed.
the part number format is obvious:
     <voltage><contact style><time in seconds>
         6NO180= 6v NO 3 minute
        12NC1   12v NC 1 second
        110C30  110v NV 30 second
Note that the heater can take DC or AC, it doesn't care.  I've seen both
octal format and 9-pin-miniature versions.  At the surplus store they are
in the $1 region.  I saw them new in a Newark catalog a couple of years ago.

Mike Morris                      UUCP: Morris@Jade.JPL.NASA.gov
                                 ICBM: 34.12 N, 118.02 W
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gbell@pnet12.cts.com (Greg Bell) (10/02/89)

jans@tekgvs.LABS.TEK.COM (Jan Steinman) writes:
>They're re-triggerable, and lack a lot of the nasty behavior of the infamous 
>555.  Plus, you get two of them in a DIP.  For a 10 second delay, try 10 Mohm 

Speaking of the nasty behavior of the 555, is there any way to "desensitize"
the trigger input so that it doesn't trigger when you turn on a flourescent
light? (or a TV, or ...)  The problem is that the 555 is fast... ie. it
reacts to extremely brief negative going spikes on the trigger line, right?

I've tried slowing the thing down with bypass caps, but it never helps much...


    Greg Bell_________________________________________________________
      Hardware hacker          |
      Electronics hobbyist     | UUCP:  uunet!serene!pnet12!gbell
      EE major at UC San Diego |