Dan's Data letters #182Publication date: January 2007.
Last modified 03-Dec-2011.
I have recently purchased (obtained it on release day, which was an interesting experience) a Nintendo Wii console. As you may very well be aware, the Wii is the return, some say apotheosis, of the wireless positionally aware controller. The first real commercial product I can think of in the gaming world since the ill-fated Power Glove, the Wiimote is a truly wonderful device. It has internal accelerometers to determine where it's going, and an infrared camera in it's snout to determine where it's pointing. Reference for this function is normally provided by the misnamed "sensor bar" attached to the Wii console.
For normal humans, this is an exceptionally grand idea. I am not normal. My television is a little glass lens in the front of my Viewsonic PJ656 projector, which happens to be bolted to my ceiling, twelve feet away or more from my components.
My brother has a similar setup, except that his components live near his projector, and he's got a fifteen foot spread to his screen. This causes quite the problem using the Wii, since the cable on the "sensor" bar is only ten feet or so.
Fortunately, the sensor bar is just an IR source, so we've set out to build a wireless sensor bar, basically building an IR source into the top of his projector screen. The circuit is simple as can be, battery, switch, resistor, IR LED, IR LED, red LED, battery ground. However, we've realized that we have a bit of a lack of knowledge, when we're trying to run up a resistor value that will keep everything happy and un-fried. The IR LED's are beefy 100mA units expecting a 2.0v forward voltage, and the red LED is a more normal 20mA unit expecting a 1.5v forward voltage. I believe that all we have to do is add the LED values together, giving us a 5.5v, 220ma LED array for purposes of calculating resistor values (100 ohm, by my best lights, into 12V, but dissipating an alarming 3300mW)
Am I checking out correctly, here, or am I going to be soldering together a device for creating Friodes?
Any help or advice you can provide on putting dissimilar LEDs in an array (series, parallel, hybrid - we're not picky) would be appreciated.
The installation is going to be running off of an 8 C cell pack, providing either 6 or 12 volts, whichever provides a more elegant solution. I figure with even 220mA worth of draw, if we can make a decently elegant circuit, C batteries should run for several months of constant use, and several years of intermittent use, and are small enough to be built into the frame of the projector screen without lumps or modifying sheetrock.
You can't string different LEDs together in a chain unless you want them all to run at the same current (by definition, everything in a series circuit has the same current running through it). Also, if you put LEDs in series then you add their voltages but the current remains the same - it just runs through one LED and then the next. If you put them in parallel the voltage stays the same, but the current adds.
It's no big deal to run your two LED flavours separately, though. It'd probably be a good idea to run your power-light red LED at a low current, too, so you can more clearly see when it's fading away and you need to change the battery - the difference between 20mA and 5mA can be surprisingly non-obvious unless it happens quickly.
There are, of course, many other battery-status indicator options - if you're feeling a bit clever, you could wire up an actual voltmeter, but a proper low-battery light circuit is hard to do without using a transistor. Such circuits are about as simple as transistor circuits get, though.
(If you're running from 12V, you could also wire in an automotive battery-level indicator doodad.)
Getting back to the LED power, yes, you can just run the two IR LEDs in series, giving you effectively a 4V-nominal, 100mA LED. The high current does indeed mean you'll be wasting quite a lot of juice if you want to run those LEDs from 12V - you'd want at least a one-watt 82-ohm resistor, and it'd be dissipating more than twice as much power as the LEDs.
If you decided to run from six volts, then you could use lantern batteries as your power source (or five NiCd or NiMH cells, for that matter), and you'd only need a half-watt 22-ohm resistor. Run time from a given battery milliamp-hour rating would be exactly the same; you'd just be using half as many cells.
Your power-light LED running at, say, 5mA, will need something like a 1000-ohm resistor from 6V or a 2200-ohm from 12V. Just give it its own little circuit in parallel with the IR LEDs. You can switch both together with a "dual throw" switch - that's one with two separate sets of terminals. Switches are normally specified by number of poles and number of throws; you'd want an SPDT or DPDT. More info here.
(I'm assuming, by the way, that your red LED's nominal voltage is actually about 1.8V, which is normal for red LEDs. The 1.5V figure is probably its minimum voltage, even slightly below which it barely lights at all.)
If the whole thing actually ends up drawing 105mA nominal, then you can only expect maybe 75 hours of continuous operation from a single string of C alkalines; twice that from two strings of C cells in parallel. D alkalines have about 2.5 times the capacity of Cs, and a 6V lantern battery (with four F cells inside it) has about 1.3 times the capacity of a string of four D alkalines.
It would, of course, not be hard to run the contraption from a plugpack instead of batteries, which would solve the run-time problem. Otherwise, though, you're going to have to change or charge batteries fairly often. Those IR LEDs suck a lot of juice.
(Check the specs for them, though. Their nominal, as opposed to maximum, current may actually only be 50mA or something.)
I saw your blog post about very bright fluorescent bulbs, and I was wondering, is it okay to use a 200 watt CFL bulb in my "100 watt max" fitting?
I ask, because I assume the limit is due to the massive heat that incandescent bulbs generate, but obviously this isn't an issue if CFLs produce a lot less heat.
It's probably OK.
(It's definitely OK if the "200-watt" CFL is actually just outputting the same amount of light as a 200-watt incandescent bulb. CFLs are often specified with a big-text "incandescent equivalent" number and a small-text actual wattage; a "200 watt equivalent" CFL will probably actually draw less than 40 watts.)
As you say, most bulb wattage limits, especially in things like table lamps, are there because high-wattage incandescents will melt the fitting.
Big CFLs run pretty darn warm, but they waste far less energy as heat than an incandescent. A 200W CFL should indeed toast its fitting less than a 100W incandescent.
Note that the difference may not be as large as you might think, though. CFLs output far more light per watt than incandescent bulbs, but watt-for-watt their heat output is not nearly as different. A 100W CFL will be three to four times as bright as a 100W incandescent, but it won't be vastly cooler.
The heat from CFLs is spread over a larger area, though, and no part of any CFL gets to the plastic-melting temperatures of an ordinary light bulb. Which is just as well, seeing as CFLs have plastic bases.
The wiring to the socket, unless it's very old, should also be fine with heavier loads. Not far heavier loads - using an adapter lead to run your toaster from a light socket is not a great idea - but wiring and switches (not necessarily dimmers...) should be OK with twice the power, even in 110-120V countries like the USA where a given wattage requires twice the current it would in a 220-240V country like Australia.
(If the wiring is very old, you should replace it anyway, of course. Even if it's new, it may be hideously unsafe; there are lots of perfectly normal-looking houses that have nightmarish electrical setups. The worst case scenario, if that fitting is wired with old coat-hangers or something, is that a 200W lamp will start a fire in your ceiling. But lots of single-bulb outlets get rewired to support a multi-bulb fitting with 200 or more watts of incandescents plugged into it, and nothing bad happens.)
The total load on the lighting circuit for your house could be an issue if you ran high-wattage lamps in all of the fittings, but this of course shouldn't be a problem either if you've got a 200W CFL in one fitting and normal CFLs in all of the others.
So, overall, I'm not going to give you a total guarantee about this, because I don't know the electrical oddities of the place you live in general and your house in particular.
But if I were you, I'd just screw the thing in there and see what happened.
(UPDATE: Look on the 250-watt CFL, ye mighty, and despair!)
Also, I've got this piece of metal that's always warm...
Four years ago I found a little Tupperware container with some mercury in the bottom. Being that its so fascinating to look at, I kept it. I keep it in my car as a traffic toy when there's too many red lights. People have started to get nervous about it being in the car. Could the fumes leak out of the container? It's never been opened.
Yes, your container certainly could be leaking. Plastic (in general) is at least somewhat permeable to mercury vapour. Mercury also vaporises quite enthusiastically when it's warm, which it often will be in the average car. I bet the air mercury levels in your car are quite high if it's been sitting there with the windows closed in the sun in summer.
Don't freak out, though. "Quite high" does not mean "your brain is even now turning black and rotten". It just means "Occupational Health and Safety people would not be pleased".
Cars are not generally very air-tight (old VW Beetles are an exception), so unless you do a lot of driving in a hot car with the vents set to "recirculate", I doubt you've actually received a particularly alarming dose of mercury from this source.
I've said it before, I'll say it again: Metallic mercury is bad for you, but it is not Death Incarnate. Organic mercury compounds are extremely toxic, but metallic mercury is not. One milligram of mercury can kill the average human if it gets into the right tissues, but mercury metal, fortunately, isn't good at doing that. Back when people drank mercury to treat syphilis (apparently it has an interesting "fatty" flavour), an enthusiastic consumer could have many tens of thousands of times the fatal dose in his body at any given time, and still go on to live for several more years, before dying of the same (largely unaffected) disease, not mercury poisoning.
If it is your aim to live to be a thousand, then the mercury dose you've received may well be a problem. I'm sorry to say, however, that something else is very likely to kill you before the mercury does.
(And just because plastic is permeable doesn't mean tons of mercury has actually been escaping. If the container was really leaky, you might well have noticed your mercury blob getting smaller over time.)
All the same, though, it is not clever to keep that stuff in a plastic container, or in your car.
The reason why you don't want it in your car in any container that it could possibly get out of (apart from the fact that a spill in your car would be worse for your health, though less stinky, than three litres of full-cream milk on the back seat) is that mercury will rapidly eat any aluminium that it touches, and a lot of other metals will dissolve in mercury more or less quickly. If you splash mercury around inside the cabin of an ordinary car it may not hit anything important that's made of a metal that the mercury will destroy - iron and steel, in particular, are immune - but if you've got an aluminium transmission tunnel or something, you could be seeing the road through the floor of your car surprisingly soon.
So, by all means, keep your mercury. It is, indeed, fun stuff, and treating it with sufficient respect is not difficult. But transfer it to a glass bottle with a tight-fitting impermeable lid (I recommend a standard laboratory bottle with a ground-glass stopper - you can get old perfume bottles with that kind of stopper in junk stores for cheap, and a little wax or grease on the stopper makes them very airtight indeed), and keep it at home, somewhere where it's unlikely to get smashed. Size the bottle to the mercury, if you can - a little bottle is less smashable than a big one.
Oh - your mercury is also likely to be pretty dirty by now. To clean the oxide crud off it, you can pour it through a small hole in a piece of paper folded into a funnel, which should be sitting in a plastic or glass funnel for support. Get the setup right first, because mercury is very dense, so it can easily get away from you when you're pouring it around. Humans do not normally deal with liquids that have 13.5 times the density of water; you just can't react fast enough to catch the stuff when you discover you've screwed up.
Filter paper will work better than plain paper, but a coffee filter is fine, and any old paper will work in a pinch. This isn't a normal filtration operation; the mercury won't actually soak through any kind of paper. The roughness of real filter paper is just better for collecting the "slag" from the mercury's surface. So blotting paper would probably be pretty decent.
Needless to say, DO THIS OUTDOORS.
Find further instructions here.
I bought my husband what I thought was a useful item, the "My Cool Gadget Survival Steel". Of course he discarded the instructions and for the life of both of us we can not figure out how it works, or even if it does work. Can you direct me to a site that might have brief directions?
I'm guessing that the gizmo you bought is this one.
I can't find a picture of it that's big enough for me to be sure how it works, but I'm guessing that it, like a number of other products in this market segment, gives you a hard steel scraper and a peg of ferrocerium, which is "mischmetal" with added iron. Basically, it's a giant lighter "flint". Scrape the steel smartly over the ferrocerium and you'll make sparks. There's not much of a trick to that - it's getting the sparks to get your tinder glowing, then coaxing a flame out of it, that's more difficult.
Some of these gizmoes also have a wooden part, which you're meant to shave slivers off with a knife to make emergency tinder. It's still not very good tinder, but I suppose it's better than nothing. Some devices have a magnesium block, slivers of which will burn very hot if you can get them lit with sparks.
(You can make your own excellent tinder by taking some cotton cloth and toasting it into "char".)
Note that all this is very much for emergencies. The standard highly-reliable firelighter that many outdoorsy people actually use 99% of the time is a plain old Bic lighter, which you can just keep in a watertight container, and which will light twigs directly without all of that messing about with tinder.
Even when it's out of gas, you can use the spark from a normal flick lighter to ignite tinder. The shower of sparks from one of the big ferrocerium doodads works better, and the really big ones like the "BlastMatch" can get a flame out of paper in one shot if you're lucky. But there's really not that much difference, in practical terms.
"Windproof" lighters of one kind or another (including ye olde Zippo) can be superior to the Bic. But in the real world, when the weather's bad enough that a Bic won't light your tinder or kindling, then you probably won't be able to get the fire lit with any lighter. Butane lighters of all sorts won't work when they're very cold (because the gas refuses to boil - if it's actually straight butane then its boiling point is only minus one degree Celsius), but all you have to do to cure that problem is keep the lighter in an inside pocket.
For sane and sensible reviews of lots of survival gear, I recommend Equipped To Survive. It's got lots of stuff about practical, cheap survival kits. There's a page about firelighters of all kinds here.
For Nifty Caveman Tricks, some of which are genuinely useful and most of which are just cool things to try while camping (and which might, just possibly, save your life one day, but which almost certainly won't), check out Wildwood Survival.