Pulsed drive of white LEDs?

You hit it with that last sentence. No time, *plus* procuring things quickly from where I live is next to impossible. I have to rely on what I already have, can get locally (which is of very limited range) or can scrounge from discarded stuff.

That should indeed work except for two things. I had intended to place the emitters between the two racers, battery powered without any cables leading to the sides, two beams each emitting outwards unidirectionally. The second factor is that an inward beam could spill over to the other side and hit the receiver for the other lane. That could be avoided by placing a baffle in the center, but I thought it would be best to make the center piece small, sturdy and low-profile.

Reflections could also be a problem. But I'm not rejecting the idea out of hand. It might just be the best solution under the circumstances.

I'd already considered laser pointers but, as I explained elsewhere, I anticipate problems with on-site alignment - four narrow beams hitting tiny sensors at a distance of more than 10 ft over uneven ground. And once aligned, both emitters and sensors would have to be fixed firmly enough against accidental knocks and vibration.

Put a single tuned circuit in the receiving amplifier and feed its output back to the light source. When the beam is unblocked, the whole system will oscillate, but you don't need anything more complicated than a diode to detect it.

There is only one tuned circuit , so temperature drift doesn't matter and alignment isn't necessary.

I built a system like this to work over a reflected path across a wide south-facing doorway at my local garage; the source and detector are in the same box but carefully screened from each other. It has been operating without problems for more than 10 years.

Thanks for all your interest and helpful replies. Even the suggestions I can't use directly were helpful in narrowing down possible solutions.

I've tentatively decided to go for a pulsed IR system. What particularly lit a light bulb in my head was when krw pointed out that the emitter and receiver do not have to operate with narrow beams. I had been stuck with the idea of collimating a thin laser beam on to a tiny sensor.

I think it will be feasible to guard against false detection due to reflections by limiting the angle of incidence with blackened tubes at both ends. I don't think there will be a problem aligning the path within a couple of degrees. A laser pointer beam impinging on a sensor 2mm wide from 15 ft would require an accuracy of something like 0.025º and that's simply not practicable in the given circumstances.

The remaining problem is interference between the two parallel paths, 7 inches apart, for each rider. Coded pulsing would obviate the problem, but I don't think I'll have time to try that out or get the parts. For the time being, I may just decide to use only one beam for each rider to mark the starting line (and detect false starts), and do without the less essential pre-staging marker for now.

Refinements can be added for later events - the full two-beam system, measurement of speed at regular intervals along the race track, elapsed time, reaction time, etc.

It is much easier to keep all the electronics together in one place and just fold the light path. The self-oscillating system I have described in another post will work perfectly well over a double 10ft path in bright sunlight (as long as it doesn't fall directly on the sensor).

Use a reflexor at the far side and keep the source and detector close together in the same box on the near side. Test a few easily-obtainable reflexors (such as number plate background material and the reflectors for bicycles and the sides of lorries) to check that they work at your chosen wavelength.

Rare earth phosphors I use have taus on the order of milliseconds.

The term 'phosphor' refers to both fluorescence and phosphorescence, now. Centuries ago, to any substance that seemed to emit light on its own, without combustion taking place. (So a radium watch dial, if sent backwards in time a few centuries, would probably be included by some earlier texts as a phosphor.)

Some phosphors (usually those that do NOT exhibit a nice exponential decay) can have decay tails that last well into minutes of time. One of the earliest recorded phosphors were of the this very persistent, phosphorescent kind. Some paintings in Japan or China used materials that came from volcanic activity acting on seashells and sulfur (with rare earth included, of course.) Although these have non-exponential decays in the microseconds, they also have long emission tails lasting well into seconds or even a minute or two.

Saying "phosphor persistence is pretty small" when talking about 200ns periods of time cuts off at least half of the modern meaning of the term, 'phosphor.'

Jon

Actually I don't think it would. I checked a couple of red laser pointers I have here. Both produce spot diameters of around 6 or 7 mm at 12 feet (the longest distance I checked). I also tried pointing one at my car number plate. The reflected light was just a blinding blob at that distance.

pimpom schrieb:

Hello,

I drived red, green and blue LEDs with pulses of 10 to 100 microseconds, no problem at all. But white LEDs may have a problem with the yellow flourescent dye, so I would prefer single color LEDs for short defined pulses.

Bye

Clever!! ...Jim Thompson

Modulate the LED and look for the modulated signal. Great aid to {semi-)manual alignment. Plus it improves S/N a bunch. All monochrome LEDs are good for MHz, whites may be good for many kHz, depends on the phosphor.

Two

Or any other fine particulate source, like superfine flour in a eductor type sprayer, or just kick up some dust.

Design case point. What do the maintainers think? How will they act? There may be more to the customer than the representatives you meet.

Ah yes, the three corner reflector trick. Reflects the incoming beam parallel to and adjacent to itself. Then the detectors are adjacent to the emitters and no wire to run and most alignment issues reduced or eliminated. Thanks Adrian, i had missed this.

A simple source of "three corner reflectors" is a bicycle shop ;-) ...Jim Thompson

I have what I consider to be "major experience".

My experience is in 2 parts, both disfavoring pulsing white LEDs in attempt to increase ratio of visual perception of brightness to amount of current or power delivered to the LEDs.

One is personal experimentation into this bit of LEDs appearing brighter when pulsed. I found that human vision is a "good integrator" when pulse rate is fast enough to achieve lack of human-perceptible flicker.

On that line, I also found that the legend of benefit from pulsing arose from the benefit being applicable to a widely-used kind of LED having a nonlinearity favoring efficiency and more-important-still ratio of lumens to average-mA varying directly with instantaneous current throughout at least most of the range of instantaneous current under consideration.

I discuss this a bit more in a web page of mine:

(slightly outdated due to excessive consideration to low-power LEDs at the times when I wrote that long ago, enough long-ago for high-power LEDs

Principles when translated to such higher currents of modern high power LEDs still apply.

- Don Klipstein ( snipped-for-privacy@misty.com)

This physical efect has a name that I suspect is good enough to web-search on:

"corner cube reflector"

It appears to me that plenty of bicycle reflectors including most other than "aftermarket", some bicycle taillights, and some automotive taillights use this principle.

In most vehicle-mounted retroreflectors (one more keyword), there is at least one area of clustered small-sizede corner-cube inits with "cell size" around 3 mm IIRC, and depending on (successfully) that a refractive surface towards the incoming light source (boundary between air and plastic) refracts the incoming and outgoing light rays equally so as to maintain retroreflective property of the array of "corner cubes".

Many of these vehicle retroreflectors rely on arrays of small "corner cubes", whose surfaces are angled sufficiently close to parallel to incoming light rays desired to be retroflected, so as to achieve great retroreflection from air-plastic boundary via "total internal reflection". That principle of physics has its presence being a function of angle of a ray being incident to a boundary from higher refractive index material (such as plastic) to a lower refractive-index adjacent material (such as air).

There are also retroreflective objects marketed to cyclists (and elsewhere) involving very small glass spheres made of glass of whatwever variant has refractive index that favors a significant and notable bit of retroreflection. One thing that comes to my mind is "Scotchlite" (tm) by

- Don Klipstein ( snipped-for-privacy@misty.com)

Retrorefletors are available cheap at the local hardware store. The colored and white ones, about 3 inches in diameter get nailed to anything that people don't want hit by cars at night in rural areas.

I had friends, back in WV, who lived at the curve in the road.

Continuing problems with cars hitting trees in the front yard, and finally one actually hit the house, penetrating part way into the living room.

So they added a row of boulders at the edge of their lot.

Drunk comes along and kills himself when he hits boulders.

His family sued my friends.

Fortunately judge threw the case out.

=======

Another WV story...

US33 near Judy's Gap (Pendleton County), WV, makes an abrupt left turn (eastbound), right at a farmers barn.

Last time I drove that at night, it scared the hell out of me... tooting down the highway, there's a glow ahead that gets brighter and brighter as you get closer.

Finally you can read in fluorescent paint, "BARN" ;-) ...Jim Thompson