Cliff notes:

How do i use a PIC microcontroller to turn on/off AC outlets? (whether they
be part of an existing power strip or a custom panel) What alterations are
needed to be made to the outlet?

Long version:

Hi,

I'm a college senior computer science major, and am currently taking a
course on embedded interfaces. FYI: I'm a complete neophyte to embedded
systems and advanced electronics. (I got the basics down [i hope]) We'll be
using PIC chips in the course btw, so i'd like to use them as a foundation
before i learn other things.

My goal is to be able to create a system which is particular to one of my
hobbies (namely: reef aquariums) and i'm trying to figure out the
feasibility of certain things. I know it is possible to control AC outlets
(as i've seen other systems do the same) but i have no idea where to start
figuring out how. Basically i want to be able to use logic to determine
whether a certain outlet is allowing electricity through it. Dunno if this
is more of an electronics question than an embedded one.

I haven't decided whether to hack into an existing power strip, or if i'm
going to create a custom panel of outlets myself, but i imagine that in
either case the method of controlling the outlets would be pretty similar.

Anybody have any info on where i could start learning about something like
this?

thanks,
keith

"Keith Thornhill" <news@afex2win.com> schreef in bericht
news3aFf.347057$0l5.100601@dukeread06...
( ... )

Dear Sir,
Thank you for posting a well defined request!
The answer falls apart into several well-separated parts:

1) gather input information
2) define logics, i.e. build some controller to take
information and change outputs according to
input states & well defined rules
3) given the outputs, switch AC power

As I understand your request, you feel able to manage
points 1 & 2, and only request help for point 3.
If this holds true, the simple answer is:
"use solid state relay(s)"
Solid state relay is the industry name for
a triac and optocoupler and periphery
typically in a 4-pin TO220-like housing
Pins 1-2 can be driven from TTL-levels or like,
pins 3-4 are in series with the AC power circuit.

Hoping this is helpful & clear enough,
KA

Keith Thornhill <news@afex2win.com> wrote:

Then *please* keep your prying fingers well away from the AC power
network. Those things can *kill* you, or worse yet, any innocent
bystander, if you don't know what you're doing. And you you just said
you don't. Succeeding that course can't possibly be worth seriously
injuring, say, your girlfriend, can it?

It's mainly one of electrical installation safety, and thus *not* to
be taken lightly.


--
Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Thanks a bunch.

Do you have any suggestions for specific SSRs that will work well when using
a PIC to control them?

If anybody is reading this and has done something similar what i'm trying to
do (control AC outlets via microcontroller) then i'd love to pick your
brain.

-keith

"karel" <adelcoGENE@zeverSKYNET.BE> wrote in message
news:43e53547$0$2133$ba620e4c@news.skynet.be...

Keith Thornhill wrote:
Relays are a much safer option and easy to drive from a micro with a
transistor.

In article <1139104492.932181.265030@z14g2000cwz.googlegroups .com>,
cbarn24050@aol.com says...
Why? There is nothing inherently "safe" about a relay; an SSR can be
driven by the CMOS output of a PIC or other microcontroller directly,
and is optically isolated from the AC load. Most SSRs switch on the
zero crossing, while an electromechanical relay can and will arc.
The SSR is powered by the AC side, while an EM relay will require a
substantial amount of DC current when energized, and depending on the
load, may require snubbers to prevent transients from upsetting the
microcontroller.

--Gene

On 2006-02-05, cbarn24050@aol.com <cbarn24050@aol.com> wrote:

I'm curious: how are mechanical relays safer than an optically
isolated solid-state-relay solution?

A group I used to work for did a fair bit of mains-level AC
input/output stuff, and we used the SSRs quite a bit. The ones
we used came in potted modules with 4 pins, and IIRC, provided
something likew 10KV of isolation between the TTL side and the
110/220 VAC side.

--
Grant Edwards grante Yow! I would like to
at urinate in an OVULAR,
visi.com porcelain pool --

Grant Edwards wrote:

To answer that you need to look at the failure mechanisms of the component.
SSR's, when they fail mostly fail short-circuit. Relays are just as likely
to fail open as closed (depending on the means by which it failed). Also,
SSR's will exhibit a small leakage current into the load, whereas open
relay contacts do not.

If the OP was to utilise a Pulse Maintained Relay as a common element for
the power feed, then using SSR's for individual circuits would be OK in his
application. Pulse maintained relays are arranged such that any single
component failure de-energises the relay (hence isolating the load).

I would suggest that, as he is in the process of being educated, he takes
appropriate advice on AC power installation practice from suitable members
of his tutorial staff.

--
************************************************** ******************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
************************************************** ******************

Gene unlike an SSR, a relay breaks both lines, SSRs leak current even
when off, If you use relays with "force guided contacts" you can self
monitor to give a warning if the contacts get welded.

Paul E. Bennett wrote:

I personally make that last paragraph *stronger* .
I'd feel much safer for him and those around him if he used a
UL/CSA/(IIRC)VDE listed unit which had attached line cord, outlet(s) and
isolated logic level inputs.

"Hans-Bernhard Broeker" <broeker@physik.rwth-aachen.de> wrote in message
news:44kshtF2m9pgU1@news.dfncis.de...
That's simply paranoid. Of course it can kill you. But how many engineering
students does it actually kill?

In article <1139151512.199568.134840@o13g2000cwo.googlegroups .com>,
cbarn24050@aol.com says...
No, a _DPDT_ relay breaks both lines, not "a relay".
I have never seen a consumer product that monitored its relays for
failure.

Another post implies that a relay is safer because it is just as likely
to fail open as closed. Depending on the application, this just means
there are two failure modes to handle, rather than one.

The OP is looking to control aquarium equipment, not provide human life
support, so let's not over-engineer this.

OP needs to understand how to control high voltage with a reasonable
amount of safety. In the past weeks, there was a discussion here about
using mains power to replace a $0.50 1.5V alkaline battery, with a
design that was clearly a hazard. There is a difference between a
design that is "safe" and a design that is "failsafe".

Certainly, an SSR can be used as the basis of a "safe" design.

--Gene

Hans-Bernhard Broeker wrote:
The safety would be the first thing they teach on such course.

Sorry Gene i was just answering your question. SSR's are not permitted
in applications where failure could lead to a dangerous condition, for
example in the gas valve control of your dometic boiler, unless they
have an independent backup disconnection device. While it's true that
self monitoring relays are not used in the home they are mandatory in
industry in machine stop systems, probably only a matter of time before
they make it into the home. Not only that but a relay is usually much
cheaper than an SSR.

Nappy <noemail@all.com> wrote:

One would already be a lot more than enough.

And besides, this guy is far from being an "engineering student".
He's a computer science major who freely admitted that he knows
_zilch_ about electronics.

--
Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Gene S. Berkowitz wrote:

But does OP have background to understand routine hazards?

To be accurate, I said I'm not familiar with advanced electronics.

i'm fine with basic circuitry. i'm simply trying to see what things i will
need to accomplish my goal so i can start investigating how to utilize them.

i appreciate the concern for my well-being, but this thread seems to have
gotten a little off-course. i'm still looking for some concrete methods for
exactly how to control an SSR with a microcontroller to switch AC power.

-keith

"Hans-Bernhard Broeker" <broeker@physik.rwth-aachen.de> wrote in message
news:44n3ovF30n54U3@news.dfncis.de...

On Sat, 4 Feb 2006 16:40:34 -0600, "Keith Thornhill"
<news@afex2win.com> wrote:

Do you mean that you want the system to be able to sense when a load
is connected to an outlet and drawing power?

Yep the same, but it would be easier to build your own panel. Kinda
hard to stuff everything inside an existing outlet strip depending on
the outlet strip in question.

pretty cheap through surplus outlets. It doesn't matter what
type/brand of SSR you use as long as it is rated to handle the
expected load. I don't know, but I would expect that nowdays most are
capable of handling inductive loads such as flourescent lighting that
utilizes a ballast transformer or an air pump.

An EMR isn't that much more difficult to drive. One transistor, one
diode, and one resistor per relay and a common power supply for all
the relay coils. If RFI might be a problem (doubtful) you could always
add an RC snubber across the contacts to help reduce it. Contact
arcing and RFI are more of a problem when switching inductive loads.

If you want to learn even more you could roll your SSRs using triacs
and opto drivers. I wouldn't reccomend it, but it would certainly be a
learning experience.

Don't let em scare you off tho. Just be careful and use good common
sense. A little reading up on AC wiring and safety may be in order
before you start building anything.

This PDF will give you some helpful info about SSRs.
http://www.clare.com/home/PDFs.nsf/www/an-145.pdf/$File/an-145.pdf


Mike

No. I simply want to be able to manipulte an outlet to arbitrarily control
whether or not current is allowed to pass through it. the on/off state of
the outlet in question can be remembered by my chip.

thanks for the info.

On Sun, 5 Feb 2006 15:49:48 -0600, "Keith Thornhill"
<news@afex2win.com> wrote:

The low volttage control input of an SSR simply looks like a current
limited LED to the micro. Ground the - terminal connect the + terminal
to an output pin. When the pin goes high the relay is on.
Connect one of the SSR output terminals to the hot side of the outlet
to be controlled and the other terminal to the hot wire from the AC
source. IIRC the hot lead is the white wire in home wiring in the USA.
The rest of the outley wiring is normal, you're just breaking the hot
lead and inserting the SSR output terminals inline with it.
That's about all there is to it. If the load is sufficiently high
current you may need to mount the SSRs on a heatsink to keep them
cool.

Mike