Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
This Philips LumiLED app note gives some specs on automotive lighting. The one we bikies all tend to ignore is the surface area: greater than 37.5 square centimeters for rear combination stop-turn fixtures. Call it a scant 4 inches in diameter. You’ve never seen a bike light that large, have you?
LED combo tail stop light
Maybe the right thing to do is start with a street-legal truck light and build some electronics around it. This is a 4 inch diameter, 44 LED rear light with both taillight and brake light terminals. At 12 V, the taillight draws 10 mA and the brake light is 250 mA. Got it from Gemplers with a recent order, but they’re certainly not the optimum supplier if that’s all you’re buying.
Obviously, it’s unreasonable to run a 3 watt taillight on a bike, as the most recent crop of single-LED killer headlights are merely a watt or three. Battery life remains a problem.
At 10% duty cycle the brake LEDs would average 300 mW. That might be roughly comparable to the running lights on some cars these days.
With the taillight constantly energized and the brake flashing at 4 Hz, it’d be 120 + 0.5 * 300 = 270 mW.
That’s more reasonable. With a 50% efficient upconverter to 12 V, that’s half a watt. Start with 4 AA cells, triple the voltage, draw 100 mA, runtime is 1500 / 100 = 15 hours. Good enough.
And it ought to be attention-getting enough for anybody! The only trouble will be fitting the damn thing on the back of the bike; fortunately, ‘bents have plenty of room behind the seat, so maybe attaching it below the top seat rail will work.
Memo to Self: The rear reflector must be something like 3 inches in diameter, too. We ignore that spec, too.
A new fast NiMH pack charger that uses a thermistor to detect the abrupt temperature rise at full charge just arrived on my Electronics Workbench. The instructions say to tape (“Use rubberized fabric …”) the thermistor to a cell in the middle of the pack, a process which loses its charm fairly quickly.
The intent is to have the thermistor bead in intimate thermal contact with the cell, but air is a rather crappy thermal conductor. We can do better than that.
Sooo, off to the Basement Laboratory Adhesives Division we go…
NiMH cells have a steel shell, so holding the sensor in place with a magnet makes at least some sense. I used a pair of teeny rare-earth magnets (Electronic Goldmine G16913) bridged by a snippet of steel strap. One magnet points up, the other points down, the strap provides a magnetic path, and the whole assembly sticks to the cell like glue.
First epoxy setup
I trimmed the heatshrink tubing surrounding the thermistor back a bit, then applied enough epoxy to secure the magnets to the strap and smooth out the edges, leaving the thermistor sticking out in mid-air.
Although it looks risky, the epoxy doesn’t bond well to the (sacrificial, dead) cell. Doing it this way produces a nearly perfect AA-cell-shaped contour in the epoxy on the bottom of the magnets.
It’s JB-Kwik fast-curing epoxy, not quite so runny as its slower-setting and much stronger JB-Weld relative.
Epoxy covering thermistor
After the epoxy cured, I bent the thermistor down to contact the cell and dabbed epoxy over the bead. This puts the thermistor in good thermal contact with the cell. Epoxy isn’t a great thermal conductor, but it’s a lot better than air.
The alert reader will note that I wrapped a layer of masking tape around the cell for this operation. I wasn’t convinced I could pop the epoxy off the cell without cracking the thermistor leads, but that turned out not to be a problem.
Trimming the edges of the epoxy around the bead gave it a certain geeky charm.
And it works like a champ: get the assembly close to a cell and it snaps right in place. I align the thermistor more-or-less in the middle of the cell, although I suspect the temperature gradient from the middle to either end isn’t all that large.
Magnetically attached thermal sensor
Now, one could argue that this lump increases the thermal mass surrounding the thermistor, thus slowing the charger’s reaction time. That might be true, but the pack’s end-of-charge temperature rise seems considerably subdued now; the charger used to cook the living piss right out of the cells (with the thermistor taped down): I couldn’t hold them in my hand, so they were well over 150 °F.
Now they become just uncomfortably warm, which says they’re closer to 130 °F.
The charger’s single page instructions (two pages if you count the sheet illustrating the rubberized fabric taping thing) cautions “Stop charging when [the cell’s surface temperature] is over 70C or it feels very hot”.
Found this toxic spill while I was looking for a gadget on another shelf: it seems I left an alkaline D cell standing on my electronics parts & tools carousel for much too long.
Amazingly, although the cell’s leakage blistered the paint pretty badly, it didn’t affect the steel carousel!
I wiped most of the crud and dead paint off, then applied white vinegar (which is essentially dilute acetic acid) to neutralize the cell’s potassium hydroxide. The grabber tool sticking out from between the boxes had a pretty good dose of corrosion up the side, but soaking it in vinegar (wow, the bubbles!) removed that and a shot of penetrating oil expelled the rinse water.
It’s definitely not Duracell’s fault: the cell had a best-used-by date in 1997.
I need an LED taillight (and maybe headlight) with a metal case and far more LEDs than seems reasonable. This is a doodle to sort out some ideas… not all of which will work out properly.
The general notion is that one can put today’s crop of ultrasuperbright 5 mm LEDs to good use. While the Luxeon & Cree multi-watt LEDs are good for lighting up the roadway, they’re really too bright and power-hungry for rear-facing lights. Mostly, you want bright lights facing aft, but the beam pattern & optical niceness really aren’t too critical as long as you’re not wasting too many photons by lighting up the bushes.
I think, anyway. Must build one and see how it works. I know that a narrow beam is not a Good Thing, as cars do not approach from directly behind and it make aiming the light rather too finicky.
The problem with commercial bike taillights is that they use piddly little LEDs and not enough of them. If you’ve ever actually overtaken a bicyclist at night with a blinky LED taillight, you’ve seen the problem: they’re too damn small. Automobile taillights must have a very large surface area for well and good reason.
So the diagram in the pic explores the notion of arranging a bunch of red & amber LEDs in a fairly compact array. The shaded ones are red, the open ones are amber (with two more side-facing ambers to meet legal requirements), and there are eight of each. The OD is about 40 mm. Figure 5 mm LEDs with 2.5 mm of aluminum shell between them. If the center four LEDs were spaced right, an axial (socket-head cap?) screw could hold the entire affair together.
Turns out both the red & amber LEDs in the bags of 100 I just got from Hong Kong run at 2 V forward drop @ 30-ish mA, so that’s 16 V total for eight in series.
Four AA NiMH cells fit neatly behind the array, so the supply will be 4 – 5 V, more or less. The outer casing could be plastic pipe.
What to do for a battery charging port? Must be mostly weatherproof. Ugh.
Rather than a regulated supply and a current sink / resistor, use an inductor: build up the desired forward current by shorting the inductor to ground, then snap the juice into the LEDs. The voltage ratio is about 4:1, so the discharge will happen 4x faster than the charge for a duty cycle around 20%. At that ratio, you can kick maybe 50 mA into the poor things.
Governing equation: V = L (ΔI/ΔT)
If they’re running continuously, 2 V x 50 mA x 0.2 = 20 mW. The full array of red or amber is 160 mW, 320 mW for both. If you’re powering them at 10% duty cycle, then the average power dissipation is pretty low. Not much need for an external heatsink in any event.
A 1 kHz overall cycle means a 200 µs inductor charging period. With low batteries at 4 V and 50 mA peak current, the inductor is 16 mH. That’s a lot of inductor. I have a Coilcraft SMD design kit that goes up to 1 mH: 12 µs charge and 16 kHz overall. Well, I wouldn’t be able to hear that.
No need for current sensing if the microcontroller can monitor battery voltage and adjust the charge duration to suit; three or four durations would suffice. Needs an ADC input or an analog window comparator.
Automotive LED taillights seem to run at about 10% duty cycle just above my flicker fusion frequency; say between 50 – 100 Hz. If that’s true, red & amber could be “on” simultaneously, but actually occupy different time slots within a 100 Hz repeat and keep the overall duty cycle very low.
I’d like red on continuously (10% of every 10 ms) with amber blinking at 4 Hz with a 50% duty cycle. When they’re both on the total would be 60% duty.
The legal status of blinking taillights is ambiguous, as is their color; more there. Motorcycles may have headlight modulators. Bikes, not so much.
Battery life: assume crappy 1500 mAh cells to 1 V/cell. Red = 50 mA x 0.2 x 0.1 = 1 mA. Amber = 50 mA x 0.2 x 0.5 = 5 mA. Thus 1500 / 6 = 250 hours. Figure half of that due to crappy efficiency, it’s still a week or two of riding.
Rather than a power switch, use a vibration sensor: if the bike’s parked, shut off the light after maybe 5 minutes. It wouldn’t go off when you’re on the bike, even stopped at a light, because you’re always wobbling around a little.
Memo to Self: put the side LEDs on the case split line?
My buddy Mark One dropped off a pair of Maxwell PC10 10 farad Ultracapacitors. We both recall our respective professors saying that a farad is an impractical unit, there’d never be such a thing as a 1 F capacitor, and it would be the size of a barn anyway…
These are 25x30x3 mm.
The downside, of course, is that they’re rated at 2.5 V DC with an absolute maximum of 2.7 V.
On the other paw, they have a maximum current of 2.5 A and a whopping 19 A short-circuit current. Serious risk of fire & personal injury there…
Charged one up from an AA NiMH cell I had lying around on the desk, which took a while, then let it discharge all by itself while taking notes. The results look like this:
Now, maybe that’s not exactly the extreme top left end of an exponential drop, but it looks close enough:
V(t) = V0 * exp (-t/τ)
Pick any two points on the curve to find τ, the time constant:
V(t1) / V(t2) = exp (-t1/τ) / exp (-t2/τ)
Take the log of both sides and remember that the log of a ratio is the difference of the logs:
log V(t1) – log V(t2) = (-t1 + t2) / τ
Plug in the first and last data points to get:
0.02341 = 21.6 ks / τ
Reshuffle and τ = 923 ks. Close enough to a megasecond for my purposes.
How to find the capacitance? Charge the cap up fram a pair of NiMH cells, discharge it at a constant current using a battery tester, thusly:
10 uF Ultracap – 100 mA Load
That curve isn’t exactly linear, but it’s close enough that we can use the familiar capacitor equation:
ΔV/ΔT = I/C
Reshuffle to get capacitance over there on the left side:
C = I * ΔT / ΔV
The lower axis is minutes, not seconds, with truly poor grid values. Eyeballometrically, call it 4 min * 60 = 240 seconds.
Plug in the appropriate numbers and find that
C = 0.1 A * 240 s / 2.5 V = 9.6 F.
Close enough.
Knowing τ and C, find the self-discharge resistance R = τ/C = 96 kΩ. That seems pretty low, but at 2 V it amounts to 25 µA. The cap’s self-discharge current is rated at 40 µA, so that’s well within spec.
Now, admittedly, the cap doesn’t hold much energy:
NiMH 2 x AA = 1 Ah @ 2.4 V = 2.4 Wh = 8600 Ws = 8600 J
Ultracap 10 F @ 2.4 V = 1/2 * C * V^2 = 29 J
But, heck, it’s pretty slick anyway… it’ll make a dandy backup power source for a clock I’m thinking of making.
Memo to Self: Datasheet says to add balancing resistors that carry 10x the self-discharge current when stacking in series. That’d be 10 kΩ, more or less, which seems scary-low.
I recently bought two dozen Tenergy Ready-to-Use NiMH cells, rated at 2.3 Ah, with the intent of making up three 8-cell packs (identified as A, B, and C, for lack of anything smarter) for the amateur radio HTs we use on our bikes. However, one of the packs measured a consistently short runtime and I suspected one weak cell.
So I ran pairs of cells from the weak pack and found these results:
DSC-H5 Battery – Tenergy RTU NiMH AA Cells
Observations…
These are all measured just after charging, so they’re all the best you can expect from the cells. I haven’t done any self-discharge tests yet.
The overall capacity at 1 A load is roughly 65% of the 2.3 Ah rating.
The red trace falls far short of the others, so that’s the pair with the weak cell. I charged & tested those two cells individually, which are the lower two traces: cell A4 has 58% of nominal capacity. Admittedly, that’s 90% of the capacity of the rest, but, still …
I’ll use the other three pairs of cells through the Sony DSC-H5 camera, for reasons described there. Cell A4 is destined for the shelf…
Now, the question becomes: who should I buy the next batch of cells from?
My Sony DSC-H5 uses a pair of AA NiMh cells and, it seems, drains them rather rapidly. I’ve been cycling a motley assortment of paired cells through the thing and figured some measurements were in order.
Click on the graph to get a bigger image with readable labels:
DSC-H5 Battery – Old NiMh AA Cells
Some observations…
All of the cells, except for the Tenergy RTUs, have been cycled through the camera many times over the last few years. I charged the cells before testing, so these are hot-from-the-charger values without the usual self-discharge that afflicts all NiMh cells.
I picked a 1 A load for convenience. I think the camera presents a much heavier, although intermittent, load to the cells, as the actual runtime is far less than the 1.5 to 2.3 hours you see on the graph. In round numbers, the camera rejects the weaker cells in about 15 minutes, which means its load is much heavier.
The topmost blue-gray line is from the original pair of Sony Stamina cells that came with the camera, which still deliver decent runtime. Rated at 2.5 Ah and delivering very nearly that much into a 1-A load.
The green line is the same pair of cells loaded at 2.5 A, just to see what happens. They still work pretty well; the lower voltage is to be expected. A mere 0.14 Ω of lead resistance will account for that entire difference and I’m not sure how much the cells contribute.
The red and black lines are from the quartet of 2.2 Ah Energizer cells that came with an Energizer 15-minute (!) charger. They’re rated at “Min 2.05 Ah” and are still well within that spec. However, they deliver a relatively short runtime. I just noticed that the graph legend has the wrong capacity values for the red trace (cells C&D): oops.
The short purple line that dunks down in the middle of the graph is a new pair of the disappointing Tenergy Ready-to-Use cells, with a nominal capacity of 2.3 Ah and delivering barely 1.5 Ah.
The blue line is a pair of Tenergy 2.6 Ah cells with a similarly low actual capacity at a much lower voltage. They give a very brief runtime.
As nearly as I can tell, the only thing that matters for camera runtime is the battery voltage. Large currents cause a correspondingly large voltage drop, so even cells with good open-circuit voltage will fail early.
Internal cell resistance is probably the determining factor, as that increases with age. Even though the Energizers have plenty of capacity, they deliver it with a terminal voltage that’s too low for the camera.
The Tenergy RTU cells have a pitifully small capacity compared to their ratings, but they last much longer in the camera than I expected. Their output voltage stays above 2.3 V until fairly late in their discharge, so the camera remains happy.
I’ll continue using the Sony cells, along with a quartet of the Tenergy RTUs. The rest are destined for flashlights and such…
The Smell of Molten Projects in the Morning 