Relay

http://en.wikipedia.org/wiki/Relay

--
Randy M. Dumse
www.newmicros.com
Caution: Objects in mirror are more confused than they appear.

    Although it's a bit dated, I'd recommend "The Art of Electronics"
by Horowitz and Hill.

    Relays have a coil, which requires power, and contacts, which
switch the output.  Coils come in many voltages and current
ratings.  Most relays require more power to the coil than a signal
from a computer, what's called a "TTL level" signal.  So you usually
need a "relay driver" IC to get enough power just to run the coil.

    Many contact options are available, but the usual ones are
"normally open" (N.O.), "normally closed" (N.C.), "single pole
double throw" ("S.P.D.T"), which is like a 3-way lightswitch,
and "double pole double throw" ("D.P.D.T."), which is like
two 3-way lightswitches ganged together.  If you're going to
turn something on with a computer, use "normally open".

    To switch AC power, you need a relay that's rated for it.
A typical rating for something to control a light would be
5 to 20 amps, 120 to 240 volts AC.

    Wiring on the AC side of the relay should be 18 gauge
wire, bigger if the load is more than 5 amps.  Get a relay
with tabs for insulated crimp-on connectors
on the AC side.  That's what everybody uses today.
You'll need a supply of insulated crimp-on connectors
and a crimping tool; most big hardware stores have this.
Provide strain relief on power cords, so if somebody
yanks a cord, it doesn't pull wiring apart.  The relay
should be in the "hot" side of the AC line (which connects
to the narrow pin on the power plug), not the "neutral"
side (the wide pin on the power plug.)  It is customary
to use white wire for neutral, red or black for the "hot" wire,
and green (always) for protective ground.  If something has
a metal case, it needs a 3-prong plug, and the case is
connected to the green wire, which grounds the case when
the plug is plugged in.

    When working on this stuff, use a GFCI-protected outlet
as your power source.  This reduces the shock hazard, since
the GFCI outlet will cut the power if you touch a hot wire.

    Here's an example of a typical relay for switching an
AC load given a 5 volt DC control signal.

     http://www.nteinc.com/relay_web/pdf/R45.pdf

Note the first relay in the table. It will switch 30 amps,
and needs 900 milliwatts, or 0.9 watt, applied to the coil.
0.9 watts / 5 volts = 0.18 amp, or 180 mA.  So you need
a control signal with at least 180mA at 5 volts.

     For practice, wire this up and use a 5 volt source
(actually a 9V battery won't hurt the relay) to
activate the relay.  That's a good test.

     If you look around on the web, you'll see lots of
circuits for relay drivers suitable for connection to
some computer output signal (the RTS line on
a serial port being a classic example, because you can
control it from software).  In practice everybody uses
a relay driver IC rather than discrite components.

                John Nagle

Hi,

Maybe have a look at using solid state relays, they don't need much power to
switch. Something like:-
http://cgi.ebay.de/Opto22-Hockey-Puck-Solid-State-Relay_W0QQitemZ230008723129QQihZ013QQcategoryZ100184QQrdZ1QQssPageNameZWD1VQQcmdZViewItem


Regards
Ian Dobson

Home of the Atmel based UDP mobile web cam http://www.planet-ian.com
All mails scanned with av-filter.pl (F-Prot / perl)

Relays can be a lot of fun. And you can get started without even having one.
Just look up "ladder logic" and start by learning just how they have been
used to get full automation into the factory floor and sequencing the lights
at your busy intersections, and all the controls in your furnace, washing
machine, dryer, coffee maker, microwave etc. They are the true forerunners
of computer automation.

Ladder logic is a superb primer for all electronics that drive motors,
lights, valves, etc.

Wayne

have you seen the Cubloc controller

  http://www.comfiletech.com/index.asp?PageAction=Custom&ID=2

aside from basic it allows Ladder programming.

regards,
colin

--

www.minisumo.org.uk

(Remove the "No Spam" to reply by email!)

Thanks Colin. I went to their site to see what it's all about and really
don't know what it is they are selling nor how they apply it to the real
world. The words are there, but apply to anything from a single PLC type
device to a Moon Rover. For example, the use of the word BASIC, to me that
implies sequential programming and when they say ladder then I see
conditional programming. But try as I might, I could not verify that
opinion. \

But what the heck... this technology changes so fast that it's hard to keep
up. When I started designing machines my basic control were cams... like the
Swiss watchmaking machines. Now I'm into DNC.... maybe.

Here's a good place to start with the really basic approach to what I was
talking about earlier.

http://www.opamp-electronics.com/tutorials/digital_theory_ch_006.htm

That was useful.  Even though I've worked with digital logic for decades, I
had never seen the term "ladder logic" before.  I've even seen schematics
in that format (like washing machines and pinball machines) but had never
realized there was a term for it.  It's always fun to learn something
new....

--
Curt Welch                                            http://CurtWelch.Com/
curt@kcwc.com                                        http://NewsReader.Com/

http://CurtWelch.Com/

http://NewsReader.Com/

You're my kind of person! So few of us around willing to admit that we are
in a continuous learning mode if we are to survive in our profession....
even if retired.
Wayne

    Heck, ladder-logic programming has been a mainstay of industrial
automation control systems for decades, and is still going strong in
tech-conservative industries.  Even for newer systems that support more
complex languages, Ladder Logic is still grandfathered in -- the
IEC-1131 standard still supports it.
    It's looking a bit long in the tooth now, though -- a lot of brand-name
LL setups support all sorts of exotic functions "buried" in LL function
blocks, b/c the demands placed on industrial automation keep getting
more and more complex, and LL has some real limitations, especially when
you start getting into sequential logic and mathematical operations.
Unfortunately, some industries have gotten into a trap where LL is the
only programming language any of their personnel know, and they
can't/won't invest in retraining.  So their hardware vendors have to
keep building larger and larger tower-of-Babel constructions of LL that
are liable to collapse under their own weight...
    Whoops, sorry, drifted off onto a typical rant, there.  :)
    Personally, I like IEC-1131 compliant systems -- you can mix&match
multiple programming "languages" (LL, C-like ST, register-based
bit-banging, and others) in the same project file, using whichever
language you know best and/or works best for a particular function
block.  Hasn't exactly caught on like wildfire in the industries I work
in, yet (the customers I deal with a *very* conservative), but it's
pretty much inevitable.

Your question really hit me. Sorry to flood you with so much info... Just
that I love relays!

I ordered a few timer relays some time ago for a project which quickly died
and I'm sitting here with two that I am willing to give you on the condition
that you respond from time to time with your experimental results so I can
duplicate them with the ones I'm keeping.

They are Macromatic with specs you can get from their online site
http://72.14.253.104/search?q=cache:3VANVNDztP4J:www.macromatic.com/literature/catalog/pdf/ss-6.pdf+macromatic+SS+60228&hl=en&gl=us&ct=clnk&cd=4

they are standard 8 pin and clearly illustrated timing sequences with a
variable knob and really, really neat!

If you have some alligator clips and a voltmeter and anything with a 24 volt
supply you should be able to have a ball by one triggering the other, to
trigger the first on x minutes of signal or not...

If you are interested, send me a private email (WaynelundNOSPAM@att.net  and
I'll send you my mailing address to which you can send a self addressed and
paid envelope from the post office and I'll slip them into the pack and they
will be on the way.

Wayne