An investment proposal

First, credit where credit is due. I read Mike the Mad Biologist. His post pointed me here, which mentioned an atrociously written Washington Post article (bad arithmetic, erased without comment, then corrected, with no reflection of the vastly different results in the text). This got me to think about buying efficient light bulbs as an investment, and so I did some interest calculations, as if the bulb were paying a mortgage.

I’ve long been interested in LED lighting, and have installed some myself in places where it was a clear win (underneath cabinets and on bicycles). I’ve been wary of “LED light bulbs” for some time, because to be fair, diplomatic, and objective, up till now, most of them have been overpriced crap. But last weekend or so, I was at the Home Despot, saw some LED light bulbs, read the labels, decided that it was worth my while to try one of them. I got a 14 watt bulb claiming to be the equivalent of a 75W bulb, installed it, and so far, so good — it’s bright, good color temperature, and instant-on, as expected. Lifetime is TBD; claims to have a 5 year warranty (but did I save the receipt? Whoops, need to remember to do that when I buy more), also claims to have a 25000 hour life, which is reasonable for LEDs. Also claims to be dimmable, but the reviews consistently say “no, not really, not like you’d expect”.

So, if I view this bulb as an investment, what is the rate of return? Let’s benchmark it against an incandescent bulb, since that is what the Post did. I initially decided to assume that I was saving 60 watts per bulb (15 instead of 75), running it 6 hours a day, and paying $.12/kwh for electricity (actually, I pay $.15, I just checked last month’s bill). Electricity savings come to about $1.30 per month.

Assume, also, that an incandescent bulb costs a dollar, and has a lifetime of 1500, so include the cost of the bulb in each month’s savings. At 6 hours per day, the bulb savings are 12 cents per month. Total savings are $1.42/month.

Assume the bulb will last 10 years (21600 hours at 6/day). I paid $30 for the bulb, plus sales tax, rounded up, is $32.

So, supposing I made an investment of $32, and it paid me back $1.42 per month for 120 months, at which point, no more payments, just as if I were loaning money to someone else for an itty-bitty mortgage. Spreadsheets have a “RATE” function that will determine the interest rate given a present value, future value, payment amount, and number of payments; in this case, -32, 0, 1.42, and 120. And out pops 4.4 percent. Not very exciting, though it helps that it is free of taxes, since it is money saved, not money earned. And if you only ran a bulb 3 hours a day, and only saved 45 watts, and only paid $.10 per killowatt hour, only 1.2%. That’s not much of an investment, is it?

But those interest rates are PER MONTH, not per year. So really, 1.2% — that’s 14.4% return, per year, tax free. If you return to the original assumptions, the $32 investment in an LED light bulb pays out at 52.8% per year.

What’s the risk in this investment? I see three possible risks that could cause it to fail to pay out.

First, you can only save money that you have; if you go bankrupt, then it’s not interesting that you aren’t paying money to the electric company, because you’re already not paying money to the electric company. However, all investments are vulnerable to bankruptcy risk.

Second, electricity in the next few years could become incredibly cheap (pigs could fly, too).

Third, the bulb could fail. Obviously, it pays to save your receipt; that gives you insurance of some sort for up to 5 years. If the bulb fails in five years instead of lasting ten, then the payout is not as impressive. If electricity is too cheap, or if you don’t run the bulb enough hours per day, it won’t pay out (3 hours per day, $.10/kwh, 45 watt savings, 5 years, will not pay off). But, if a bulb is on even 4 hours a day at $.10/kWH, the interest rate is 7.4%, or 3 hours at $.12/kWH, is 5%. Where I live with $.15/kWH electricity, 3 hours a day, 45 watts less, failing at 5 years, pays off at 14.8%. At 6 hours a day, 45.8%. I really like the idea of an investment that pays me 14.8% annual interest, tax-free, when it “fails”.

Did you notice that the Washington Post thought it was more important to tell you about the terrible government-subsidized light bulbs, when they could have been giving you this useful information instead? Says something about their priorities, doesn’t it? Hard to believe that anyone would think time spent reading that would be well-spent.

USB charging on a bike

I’ve spent way too much time and money tinkering with bicycle electronics, but I am so incredibly pissed at off at the high price and mediocre quality that you get if you buy retail instead of DIY. I had a clever idea yesterday that would allow easy, simple combination of a USB charger with a cheap, brain-dead rectifier/doubler.

5V power buck power supply for bicycle use

It has to digest a wide range of input voltages, and it has to be efficient enough to deliver 2.5 watts of power.
This version has several flaws (spec’d wrong buck diode, holes were not large enough; needed to bias D+ and D- to make iDevices happy about charging), but with some tinkering, it works (tested with iPod Classic and Nano). Further research needed to figure out why it won’t charge my Motorola phone. Further research: mini-USB is a 5-pin connector, and pin #X (they call it that) is shorted to pin 2 and connector to pin 4 (GND) with a 200k resistor. For now, I will just buy a cable that claims to work for this purpose.

This time for sure, bicycle light design

Not to be confused with this post.

Trying two new things, and one of them certainly worked in a prototype.

Thing #1, is to make the printed circuit board fit in a 1-inch tube. That means I can stuff it either in my steerer tube, or in my seat tube, out of sight and mostly protected from the weather.

Thing #2, which is a little bit trickier, but made a little bit easier by the amazing op-amps we can buy nowadays, is to build a current monitor that works from a few-millivolt signal above ground. This means that no matter what is attached in the way of lights, it will pull “the right amount” of power from a hub dynamo.

LED Color rendering

A quick and half-assed comparison, inspired by an argument on SlashDot.

I took (more than) five pictures, of my hand, holding a bunch of different colored wires, in indirect daylight, under neutral, cool, and warm LEDs, and under the light of all three LEDs combined. Camera was Canon SX100IS, ISO 100, Cloudy white balance, let the camera pick aperture and shutter.

Executive summary: true daylight is best of all, then mixed LEDs, then neutral, then you’re probably not happy with your light.

Undercabinet lights, basement kitchen

This is pretty much a copy of what I did in the upstairs kitchen, but with more attention to detail because there’s no molding to hide behind. It’s a little heavy in its use of aluminum.

Further adventures in packaging

Things are looking up, slightly. Some time ago I picked up a “Really Useful Box” at Staples. The rectangular board fits nicely inside. A larger version of the same box, would include enough room for the battery switch and battery. The box itself is not entirely watertight; it needs a bead of silicone around the rim. Note: Really Useful Boxes has a bizarro shipping policy for tiny boxes; $10 will ship up to 5 boxes, which arrive in an comparatively huge double-layered cardboard, suitable for shipping something heavy and delicate, not a handful of tiny plastic boxes.

Here’s the box, with the shunt resistor and “indicator” LED siliconed onto the outside.

Opened:

Bottom view:

Stacked on the larger box:

A simple, functional home-made headlight mount

Headlight mounts always end up looking a little clunky, and are a little hard to adjust. This one works better than most I’ve tried, and looks better too (at least, relative to the others). You’ll have to take it on faith that the shape is pretty good, but the shape is pretty good, meaning, it throws a lot of light down the road, not too much too high, and makes a nice puddle around the front of the bike. I solve the light-in-eyes problem with low-beams; this one’s not too bad, but the amber low beams, mounted low, aimed low, are vastly better.

Standlight packaging, round two

Figured out the wax thing (probably). I suspect that the BuckPuck has an overheat circuit, and was still hot from the melted wax when I tested it.

The official bottle-cage container was not at all durable, and really a waste of money. Instead, I am using a tighter-fighting, cheaper, with reliable screw-top, Trader Joe’s Peanut Butter jar. Also, I decided to ditch the 8xAA battery pack, and will either use a 9 volt battery, or am 8xAAA pack, which can fit into a little bubble wrap to protect the electronics, while still leaving it loose.

New effort, being tested:

Closeup:

In a (particular) bottle cage:

Lid off, showing holes for wires (not yet sealed) and easily accessed standlight-controlling chip, with switch and over-voltage LED. The battery switch PCB (itself coated in wax) flips up out of the way to allow access to the battery.

I’m not 100% sure on the wax potting; it adds weight, which is not for everyone. I’ve also laid out the circuit on a larger rectangular board with room for zip-tie-downs for the large capacitors, which is a lighter-weight way to immobilize them. However, it doesn’t fit as well into a cage-compatible container.

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Interestingly, the battery controller is very sensitive to static discharge; touching any part with a finger after turning it off, tends to turn it back on. I’m not sure this matters, but it would be nice to make it be a little less excited about turning on.

Standlight tweaking and pack-ugh-ing

I’m still tweaking the software. Latest mistake on my part was getting the numbers backwards for the on/off counter, so right now it is a little too happy to turn on. This doesn’t necessarily run down the battery; if the battery comes on when the bike is moving “too fast”, the effect is to dim the lights slightly as the system voltage drains into the battery, and the batteries are (slightly) charged.

I added a switch to allow me to force the batteries on; in theory I could use the bike as a battery charger, though it would take hours, but the main goal is to permit the lights to stay on arbitrarily long if you need light (fixing something by the side of the road, for example). With the bike stopped, toggling the switch off-on-off turns the lights off immediately. Usually.

“Debouncing” the circuit has been a little tricky. When the microcontroller decides to “turn off” the lights, it is really turning off its own power, as well as the power to a beefy trio of capacitors, an op-amp, and a little switching power supply. It doesn’t just go to sleep, and sometimes thinks it sees signals as things settle unevenly towards zero (somewhat like HAL singing Daisy). So, timers and wheel turns are taken on faith as they occur, but for purposes of detecting switch movement, and for resetting the software to its on-the-road state, there are counters. A net of 313 (1/10th second) samples must agree before the switch is judged to have a value. For purposes of deciding that the bike is no longer sitting still, 8 zero crossings (one foot of motion, can be back-and-forth) must occur. This is partly necessary because there can still be voltage to the microprocessor even when the lights are out (surprise!) as the big caps slowly drain.

Packaging has been a pain. Originally I had intended to pot the controller in wax, but when I did that, for some reason it didn’t work. It half-worked, but I couldn’t get the lights to go on. Quite depressed, I got it out of the wax, checked the wax to see if it was conductive (shouldn’t be, but you never know), and discovered that now everything was working perfectly. Maybe I goofed somehow on the light test.

So, instead, and especially for this one where I am still taking it apart and putting it together again (mostly to swap out the microcontroller), things are held in place with elastic cord. I’m not entirely happy with the bulk, though part of the problem is that the bike I am testing on has no water bottle cage. An 8-pack of AA cells is probably too big, especially given that the batteries are only used intermittently.

The rear light is a win. That is a P-clamp, plus some aluminum angle stock, plus a bare LED on puck for a taillight, all but the lens painted with nail polish to keep the weather out.

I think the front light would be acceptable if I had a better place for the cylinder-o-circuits. The light bracket is made from two bell mounts, two longs bolts or screws, some aluminum stock, a mirror (to cut off some glare from other cyclists and pedestrians) and some mounting nuts. The cutoff is not good enough; I think that low beams (amber, either down low or aimed low, mounted on aluminum angle stock) will be necessary.

Side view:

Head on:

I imagine it would be tidier if the blue, orange, and green wires came out the bottom of the circular PCB, through aligned holes in the bottom, additional (blue, red, black, white) ran up to the battery switch, and then an AAA battery pack was cradled in something squishy below the switch.

Wow. (Been experimenting with tweaks.) I think a few noise-reducing capacitors might be in order. Sometimes a few taps on the bike are enough to turn the lights on.