High-output, low-duty cycle LED strobe circuit

Hey, I didn't say everyone implements the standard* perfectly.

The PIC series has a long-standing hardware bug in slave mode. Libraries are often imperfect.

Master mode is usually not a problem, and starting with bit-banged master mode usually works. Hardware devices such as I2C SEEPROMs are done properly, IME. Not surprisingly, NXP (nee Philips) micros are quite solid in I2C support.

SPI, OTOH, is a dog's breakfast.

• formatting link

Best regards, Spehro Pefhany

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Point.

-- W . | ,. w , "Some people are alive only because \\|/ \\|/ it is illegal to kill them." Perna condita delenda est

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That's enough to dimly light up a really high efficiency LED.

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    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
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[About using a flyback coil to store energy for an LED pulse generator for stroboscope use... }

Yep, that'd be awkward, allright. With higher frequency input, you still want full charging of the inductor, so lets only allow 10ms (this would let the constructor reproduce 60 Hz and below, which is a useful range). To get a 1.5 mH inductor up to 0.3A with 5V (less a bit for a switch, and for sense resistor and coil resistance), takes

t =3D L (0.3A)/(4.5V) =3D 0.1 ms

meaning that 1.5 mH is about one hundred times too small. It also means that the turnoff transient at the regulated +5V supply is 300 mA.

Neither is good, so instead, use ten times as many turns on the primary winding (the 5V one) as on the secondary winding (the 1.5 mH one that drives the LED). Just as a car spark coil goes from 12V on the primary (with -300V spike at points opening), while the HV winding generates 30 kV, so the doubly-wound core will have the low-slew 5V side that ramps up, and the high-slew LED side that quickly dumps the energy into that LED. The 'blocking diode' in series with the LED will have to hold off about 50V during charging.

Ten times the turns means instead of 1.5 mH, the primary winding has N**2 * 1.5 mH, or 150 mH inductance, and takes 0.030A instead of 0.300A at full charge, so the transient when it turns off is only 30 mA. I'd still prefer to run this off a filtered unregulated supply if that's available.

Thanks. This ties in the with the "impedance matching" comment nicely. I think I see the picture, but I need to sit down a bit and drill it into my brain some more so that it stays put. But the basic intuition feels 'right', for now.

Appreciated, Jon

Actually, I have some here that look pretty decent at half an mA. At least, indoors.

Jon

Oh great. I had been thinking that PICs would make good I2C slave chips. What sort of problems are there?

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    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
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Exactly.

If you're like me, & grew up with red LEDs that were reasonably visible at 20mA, you'd have trouble picking them from modern LEDs at 500uA.

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    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
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I'm looking for ideas on how to make an LED flash so brightly at a low duty cycle that it's reasonably bright--maybe even close to what it would be if it were on DC.

I'm building a project where I need to flash white LEDs very brightly

30-50 times a second at about a 0.4% duty cycle.....

With a 0.4% duty cycle you will need to produce 250 times as much output over that time to equal the same as it would appear on d.c. (unrealistic!!)

However if low output devices like the ones you are using are ok on dc (giving 0.13Cd) try some high output devices (Lumileds, Cree, Seoul Semi) with optics if necessary (the optics are readily available for many of these devices.

Powering leds @ 1amp (or more) is quite straightforward and is commonly/cheaply done in some Infra-Red remote controls.

Use a decent high gain at current device such as the ZTX689B (amongst plenty of others)

put led as collector load to +ve, ~0.7ohm emitter to ground, small npn:- emitter to gnd, base to ZTX emitter, collector to ZTX base . (gives curent limiting @~1amp)

Resistor from ztx base to uP (the ztx will need only a few mA from your uP to switch 1 amp)

The duty cycle is so low that unless your supply volts are too large the ztx will stand the power dissipation. Any 6V 1amp supply ok- just ensure there is a low impedance (*not* a standard electrolytic) decent sized electrolytic across the supply.

hope this helps

[About using a flyback coil to store energy for an LED pulse generator for stroboscope use... with a primary side having ten times the turns of the secondary side]

Oops; got that backward, the switch on the primary (high-turns) side will have to hold off -50V during discharging.

Lovely. *mutter*

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    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
---^----^---------------------------------------------------------------

Yeah, that's where I was. I was still trying to figure out what you meant about the diode. Now it is back to making sense, again.

Jon

Much of that seems to be the shift in wavelength towards our eyes' optimal sensitivies, while still being 'red', though I don't mean to take away the idea that there have been some improvements elsewhere.

Jon

Modern high brightness red LEDs are orders of magnitude brighter than the ancient red LEDs that were available in the early days.

--
    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
---^----^---------------------------------------------------------------

Read the whole thread. 5 things stand out:=20

1) Put the LED in the collector side (you need the voltage advantage). 2) Bypass the LED circuit right damn close and maybe isolate it a bit with a ferrite bead. =20 3) Even still you may have to jack up the collector/LED supply.=20 4) The LED output increases far less than linearly with overcurrent.=20 5) White LEDs are phosphor coated near UV devices, the phosphor decay rates will have major impact in fast pulsed modes.

There are two basic categories of decay rates for wavelength shifting effects -- fluorescence and phosphorescence. These are driven by two different quantum mechanisms, the latter requiring an improbable state transition in order to gain a longer decay time.

Some phosphorescent decays are long at room temperatures, with taus on the order of a millisecond or two for some cheap, commonly used materials. Fluorescence, on the other hand, tends to be very fast -- with nanosecond taus -- because the usual de-energizing state transitions are quite fast. They are on the same order as for what one observes when light passes through glass in an optical lenses, for example. (At rates of roughly 10^8 absorption/ emission transitions per second, broadly speaking.)

I've not tested white LEDs at all, but I do recall that many use a P46 YAG (a combustion-synthesized form of Y_3 Al_5 O_12:Ce+3(2) phosphor), which has a tau on the order of some 70ns, memory serving. (I seem to recall two emission peaks, one around 530nm and another around 550nm, and I don't have my books handy to look this up to see which is which here.) In any case, this is far, far faster than the OP's driving period.

For an interesting page on lifetime measurement, see:

The term 'phosphor' is often applied to anything exhibiting a Stokes shift with any useful efficiency -- conflating both fluorescent and phosphorescent mechanisms into the term. So it isn't always possible to tell what mechanism is operating, merely because a material is called a phosphor.

There are also ZnSe substrate white LEDs that completely avoid the use of a phosphor of any kind.

Jon

I'm replying to myself here because I have a solution that works for the moment.

It seems that it was enough to just have no current-limiting resistor in the collector. We'll see how that goes with 12 LEDs; I may yet have to get a low output impedance cap to avoid flicker.

Here's a short video demonstrating what I have so far on my breadboard with this project:

Thanks to all who responded.

--Mark

Thanks, Mark. That was really great in terms of letting me see exactly what you are doing. That disk almost looks like those ink jet printable dvd disks I've seen.

My wife didn't understand the video, at first. I had to add more explanations to get her to accept that it wasn't just shining a flashlight on a wheel with letters on it. Part of her distraction about that was due to the time you spent talking about the IR LED, which made her think that you thought it was the main attraction when it was just part of the mechanism you wanted to highlight but wasn't the feature attraction. I explained it more, then she got it. I particularly just added the idea of a car's timing light. Then it was much clearer to her.

Her immediate jump was then towards using this NOT for displaying letters but instead for doing animation -- she draws cartoons and is a writer. As a toy device, she thinks that this would be far more interesting than just some character output device, which really doesn't get her attention nearly as much.

You are probably already familiar with drawing stick figure cartoons on the margins of a book and then flipping the pages to see the action. A huge difference that this kind of device contributes to something so low-tech is that the series of images are laid out on the perimeter do NOT have to be seen in the same sequence. In other words, a set of ... say, 50 images ... can be played in a wide variety of ways, allowing not just one "story" or two, but a great many different ways of being played out. Not all the same length of time, either. In the end, a lot could be done. Even a game, perhaps, that plays out differently with different people making different choices in order to impact the way it proceeds.

It __may__ even be possible, with additional thinking, to consider allowing end-users to do their own images -- even hand-drawn on those dvds I mentioned -- with feedback coming from timing marks they also draw on the perimeter edge with a pen. Some means of adjusting those marks (making them wider and using the center, perhaps, allowing them to go either way in adjusting the timing?) might allow them to "fix up" what they are doing until it looks pretty good. Okay, practice will help, too. But it might be fun. How to specify transition orders might be a problem. Just thinking right now, that's another issue perhaps.

Anyway, some neo-gaming possibilities are suggested.

Jon