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Custom AC power switch

My autistic daughter has burned up a few microwave ovens. She loves to cook, but places the popcorn inside and then presses 3 3 3 and START. Since that is

3:33 minutes and since the popcorn only wants 2 minutes, it often burns horribly or else bursts into flame.

Having a "popcorn" button on the microwave is completely useless. I've tried to teach her to use it, but she is about like a 3 year old and simply won't get the picture. So I considered the idea of plastic cards with pictures on them that she could "bring near" or "wave" by the front panel of the microwave unit, but I'd need to completely redesign the front panel electronics and ... I'd rather not attempt to bite that one off, just yet. It's an idea that would probably work, and I've tried to interest Amana/Maytag in the idea of fielding UL approved microwave units for "special needs," but the interest was nearly zero.

She's also very good at plugging things in, if the plugs are visible. Hot plate burners, toasters, microwave ovens, toaster ovens, etc. If it can be plugged and she thinks she might be able to make a melted cheese sandwich in the middle of the night, well.... I might wake up to a terrible smoke in the house, because she found a burner we'd set aside and plugged it in and got a pan and bread slices and cheese and forgot about the whole thing, after turning on the burner.

We have to be able to cook, ourselves, with some level of convenience. And locking everything up, keeping the main power breakers off for the main stove, etc., makes doing routine things not nearly so routine. Since the main problem in her habits is the microwave unit, I placed it into a cabinet with power where she cannot see or reach it. When I need to use it, I pull the entire unit out into my arms, reach back and plug it in, place it back and then use it. Then reverse the process, if I'm not otherwise distracted. If I do forget, I can often expect another burned bag of popcorn a few hours later.

I can arrange to place a hidden switch, I suppose. But I don't want to bring AC power out to a switch near someone's fingers and it wouldn't solve my forgetting to switch it back (having a 20 year old running around the house, with a playful mind of a 3-5 year old, leads to many distractions and I'm not getting any younger and my memory isn't quite as good as it once was.) So I started considering other options, such as low-voltage signaling, etc., for a remote power switch I could hide inline with the microwave unit behind the cabinet and which would automatically turn off the power supply, even if I forgot.

(I'm no analog power designer. A small time hobbyist and an embedded programmer type.)

I'm imagining this functionality:

  • A pneumatic switch driven by aquarium tubing, where I can snip off an appropriate amount from the point of the AC power controller back to some convenient location where I will place a bladder/bulb that I can squeeze when I want to enable to power to a device/circuit. This would start/initiate the circuit.
  • A self-contained power controller unit with an AC-in and an AC-out, and a nipple for that aquarium tubing (goes to the pneumatic switch inside, which feeds a microcontroller's input pin to notify it.)
  • The ability to control a single 15 amp circuit at 125V RMS AC, 60 HZ.
  • The system would do several things, once activated. First, a timer starts (programmable by means I can determine) and the power simply stays ON for this duration, no matter what. Second, if in that time the circuit draw exceeds about 0.1 amp, then the circuit stays enabled so long as that draw or more continues without abatement. If it does fall back to near zero, then the timer overrides if there is still more time. If not, the circuit goes OFF 20 seconds after the load is removed. I need to reactivate it, if I need to reuse it. * I'm not looking for dimming, here. This is an ON/OFF controller, not a zero-cross, cycle-by-cycle or phase-angle dimmer.
  • The time delay for the circuit can be programmed through the pneumatic switch, using a "long-held pulse" on the bulb that the micro would detect, followed by a series of short pulses to count up to the minutes I'd like to program into it. A long pause after that would commit the time to memory.

I can handle the mechanicals for the pneumatic switch, tubing, nipples, and bulb. That's an easy and cheap part. I can also handle the microcontroller circuit -- the digital parts of this. I believe I can also handle creating a modest power handling circuit to supply the necessary DC for the micro (less than a milliwatt total) and any zero-cross measurements of the AC cycles I may need.

However, what I do not feel comfortable dealing with or making the better design choices for turning the AC power ON and OFF, whether a relay, a TRIAC, etc., or designing well for monitoring mean AC load current. And dealing with safety issues I might not be aware of, even if the circuit is hidden away and not out where someone might touch it. Because it might be hidden, I have to be sure it is not going to create its own fire hazard.

I've been doing soldering and wiring for some 30 years now, so I'm now modestly competent at decent layout, adequate wire selection for the currents needed, and proper wiring harnessing.

I'm interested in existing devices to do this, first and foremost. I'd rather not fabricate my own solution if one already exists that is close to what I want. But if not, I'm interested in any viable options or ideas for the AC power control and for the AC load current monitoring functions, from the perspective of using a microcontroller.

Or suggestions about a "better way." (But on that score, no one else really knows or understands exactly how we live our lives here and I'm the only proper judge about what can work in our life style. My description above is necessarily reduced from the truer reality. Still, I'm open to ideas.)

Jon

Reply to
Jonathan Kirwan
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I know you want to roll-your-own, but look for the "industrial" microwave ovens made by Amana et al over the past couple of decades. These are designed for use in cafeterias etc.

Unlike consumer microwave ovens, which seem to sell themselves over how many hundreds of buttons they can put on their front panels, these cafeteria-type units have a much simplified system, often just a set of buttons for some standard times. No enter, start, etc, necessary. Disable all but one of the buttons and you're done.

Tim.

Reply to
Tim Shoppa

Have you considered the X10 system of AC power controls? You can obtain IR and RF remotes, perhaps keep one in your pocket or turn appliances on from another part of the house.

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Paul Mathews

Reply to
Paul Mathews

horribly

I would suggest the following: a box in the supply line that passes power at low wattage. At higher current, it will cut teh power after a time you thing is safe (mabe 2 minutes for the popcorn).

For the case where you want to extend the time, put a button on the box that you have to push in the initial 2 minute period. This will leave the power on indefinitely, until usage drops (you switch off the device) and the operation is back at '2 minutes max'.

Now tehre is a problem with microwaves - at low wattage they cycle on and off. Same with hobs with te thermostat. This could mean they restart the cycle automatically. You may be able to avoid this using timers (do not allow another automatic 2 minute cycle for the next 30 minutes, and do not cut off 'high power' immediately in 'extend mode' when the usge drops.

It sounds pretty elegant: you start things normally, and if needed press the 'extend' button to keep the device on. Power the device off and things are safe again.

Your daughter can also use things normally, but they will cut off before they get dangerous.

Thomas

Reply to
Zak

How about buying her her own microwave. A cheap one with knob-controlled clock. Jigger the knob so that it can only be turned to do two minutes.

Also have a timeswitch and contactor that shuts off power to everything else the kitchen for those hours when it is not likely to be used by you. eg, midnight to 6am, or whatever.

--
Tony Williams.
Reply to
Tony Williams

horribly

to

the

I'm consider a blunt "adults are bigger" approach. The kitchen area over a power relay with the remote being placed on the ceiling. For an adult reachable with a chair. The light needs to work without the relay though.

Rene

--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net
Reply to
Rene Tschaggelar

I read in sci.electronics.design that Jonathan Kirwan wrote (in ) about 'Custom AC power switch', on Wed, 9 Feb 2005:

Would you be prepared to fit key-operated power switches in the appliances concerned?

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

You said you've got experience on microcontrollers, that certainly seems to me to be the easiest way of addressing the control / interface issues. With just two pushbuttons you can implement a fairly innocuous security code system, something along the lines of BLUE GREEN GREEN BLUE GREEN BLUE BLUE. Or for not too money you could get a full blown keypad, though that might actually serve your needs less well.

Check into TI's MSP430 microcontrollers. They've got a couple good app notes regarding using them to control things on the mains, and the development kits provide you with the JTAG emulator and software for about $100.

Reply to
Rob Gaddi

like

I suggest you use an MOCxxx opto-triac (to drive the output triac), which uses a low-voltage IR led in the input, well suited for the MSP430. Some are available with only 5mA trigger current, IIRC. It will provide galvanic isolation as well.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

I suppose I could. But the difficulty then would be that Athena would see us doing that and start trying (and probably breaking) various keys. We'd soon lose keys around the house, plus I'd probably need to get a locksmith involved so that all of the locks used the same key (it would be a terrible pain to have different keys for each lock.) Worse, it doesn't deal with the fact of my own forgetfulness or distractions. I can easily imagine leaving the key inserted. Or wasting time looking for one.

One of the things I'm also focused on are things that WASTE time in my life. Barriers to getting things done efficiently. Scanning around for keys, just to operate a device, is likely to add many short delays into my life. Not very attractive. Especially, since Athena has often lost whole collections of our keys because she becomes interested in things we are interested in.

Jon

Reply to
Jonathan Kirwan

Yes, but I also need to use the oven for myself, as well. Warm up soup, etc. Not to mention that microwave ovens isn't my only problem that this would help solve. I've many other electrical circumstances around the house that could use the same solution I mentioned. I can't go around looking for industrial solutions to each of these.

Jon

Reply to
Jonathan Kirwan

Been there, done that. She has access to the rest of the house and will use everything in it. Having her own was an old idea we tried and it kind of works, in the sense that she uses it at all and doesn't use the other one. But inevitably, she walks to another part of the house and decides to use that one, too.

Not convenient, though. My wife and I overlap our sleeping times so that one of us is usually awake. Most of the day is covered by one of us. We don't have normal hours that "normal folks" understand. And she doesn't sleep normal hours, at all. It's as likely that she will be awake for 35 hours as it is that she will be awake for only 15. She sleeps when she chooses to sleep.

Fixed times are simply going to get in the way. No question. And I'm really trying to get rid of impediments, things that delay me for 20 seconds or 50 seconds are to be eliminated, not created.

Which is why I want a very convenient way that I can hide to turn things on that will always be there for me when I need it, without having to think about it or go find something or take a long walk to the power control panel downstairs.

Jon

Reply to
Jonathan Kirwan

She's 20 years old and very, very strong and agile. She has already moved entire groups of furniture in order to reach things we've placed in hard to get locations. She will move ladders around, get chairs, etc.

However, there are some things that can be made just enough "too complex" for her to activate. For example, if I were to set up a bulb I'd squeeze in a certain pattern to activate the power. She'd never learn it, though she would know it is connected to the idea and would probably try.

Also, when she knows there is something that works for me and she cannot get to it, she will pester me for hours on end to let her get access (if she cannot do so, herself.) I want to reduce this, not increase it. If she has access to a bulb, she will squeeze it. But I can arrange things so that it simply doesn't work for her. Under those circumstances, she will give up and only try occasionally. And she won't bother me about it, as she has access.

Chances are, as well, that if I make that aquarium tubing go far enough away from the device involved, she won't make the connection. She might, but I've got a much better chance that she won't. She'll just think I use magic to turn on the power. Much the better.

She's learned over all theses years most of the normal methods used by real human designers for human end-use. So whatever it is, it should NOT look like one of those normal arrangements. This is one reason why keys/keylocks are a bad idea in this context. She knows about them.

Jon

Reply to
Jonathan Kirwan

You can get the zero-voltage crossing kind. Just put 5mA + some margin through it for "on" and 0mA for off. The optocoupler worries about the zero crossings.

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Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

I don't want to set up something like that. I'm looking to keep things simple, effective, to cover my own memory loss should I be distracted and have to deal with something else. Automatic shutoff is important, but not while the device is being used.

And she absolutely LOVES remotes. Carries them around, loses them, etc. I'd really be just asking for more delays in my life, that way.

Jon

Reply to
Jonathan Kirwan

I'm using them, currently. I'll have to check into the apnotes, though, and see what you are talking about. However, as my knowledge of TRIACs is very limited, I suspect that I'll need to do some studying to be sure how to properly control them via a micro-pin (I don't want to destroy the fragile output pin driver.)

Jon

Reply to
Jonathan Kirwan

Thanks for the suggestion. I'll look them up. The 5mA is about the limit I'd want to consider for the MSP430 outputs.

I assume I'll need to re-trigger at each zero-cross? Should I be concerned enough here to make sure I don't trigger until the AC voltage reaches some threshold for staying on? Or just re-trigger it in a short burst of pulses long enough to be sure that's true?

Thanks, Jon

Reply to
Jonathan Kirwan

I read in sci.electronics.design that Jonathan Kirwan wrote (in ) about 'Custom AC power switch', on Wed, 9 Feb 2005:

That's why my proposal was tentative. Are you in touch with any support groups? People have had your problems before; maybe they also have some solutions?

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate

I am. In fact, I've been active in volunteering about 300-500 hours a year for perhaps 15 years now. So I know a lot of people in this area. I recently spoke with a home contractor who is part of a corporation that specializes in special construction for children like mine. In fact, he spent a few months working here and teaching me while he worked on various aspects. When he and I talked about this particular need (which he understands well), he had no real suggestions and instead countered that if I came up with something that could be UL approved, he'd probably use it in his own work, regularly.

It wasn't my intention to bite of UL approval. I just want to solve my own problems, to be honest. But if I work out the 'bugs' on something like this, through experience here at home and perhaps a few other friends I know who could also use it, perhaps it would be worth going the extra mile then.

Certainly, the marketplace is smaller than "every home." But it is a much easier market to field a product into, as advertising response rates would be much higher and the places to advertise or get the word out are well known.

Of course, like everything else, the success would depend highly on knowing the market well and providing the right conveniences at the right prices.

Jon

Reply to
Jonathan Kirwan

Without getting into technicalities I am guessing that, with say, the microwave oven you want to modify the oven to work up to 2 minutes maximum when your daughter uses it. Then when you or your wife come along the microwave oven will work normally with no fuss.

Some sort of radio control gadget comes to mind. You and your wife wear a pendant round your necks like a piece of jewellery that emits a radio signal. A gadget inside the microwave oven or hidden behind the power outlet point receives the signal and disables the 2 minute timer. The oven can then work normally.

You use the oven for how ever long you set it and it keeps working even though you walk away out of radio range. When the oven times out from whatever your cooking it reverts back to the 2 minute maximum time again.

Is that the sort of thing you are looking for?

Regards, John Crighton Hornsby

Reply to
John Crighton

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