Building an LED microflash

[Zephod Beeblebrox]

I am having a little fun thinking around corners.

I have a huge array of LEDs and I want to pulse them on and off quickly (once).

A trip switch will trigger the LED flash briefly.

I need help with the circuit design. I'll try to show what I need done...


The LEDs are all in parallel and are 3v. The input voltage will be from a 3v DC supply (a coin cell).

A capacitor will charge and a charge indicator light will light up (could be a red LED). The LEDs remain off.

A switch closes the circuit over the capacitor causing the capacitor to discharge 3v into the LED array which glows briefly. I will have to work out the right capacitor value myself. When the capacitor has discharged, no morecurrent flows across the LED array regardless of whether the switch is nowopen or closed.

Can it be done simply and can anybody help with a diagram?

 

[Zephod Beeblebrox]

Please, I'm serious. This is for a personal project. I do need help - real help, not jokey, snooty time-wasting stuff.

 

[Phil Hobbs]

Just watch the diodes down the left-hand side.

Please, I'm serious. This is for a personal project. I do need help -
real help, not jokey, snooty time-wasting stuff.

(Context restored.)
Fair enough, but you're the one who picked the jokey nym, not me.
Serious folks on SED generally use their real names.

Putting all the LEDs in parallel will probably work OK for pulsed use,
though it's fairly far from optimal since there's nothing much to ensure
equal current sharing.

Knowing how many "a huge array of LEDs" means would help.

Knowing how long "briefly" is would also help.

Knowing what you're actually trying to do, e.g. make something cool
looking, doing time-lapse digital photography, or whatever, would help
as well.

And since you're asking for somebody to design something for you, you
might be better off in sci.electronics.basics. (I've cross-posted and
set followups there.)

It's okay if you don't know the answers in great detail, but a bit of
context would help us to help you.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[Zephod Beeblebrox]

Ok. Continuing in sci.electronics.basics

 

[[email protected]]

I am having a little fun thinking around corners.



I have a huge array of LEDs and I want to pulse them on and off quickly (once).



A trip switch will trigger the LED flash briefly.



I need help with the circuit design. I'll try to show what I need done...





The LEDs are all in parallel and are 3v. The input voltage will be from a3v DC supply (a coin cell).



A capacitor will charge and a charge indicator light will light up (couldbe a red LED). The LEDs remain off.



A switch closes the circuit over the capacitor causing the capacitor to discharge 3v into the LED array which glows briefly. I will have to work outthe right capacitor value myself. When the capacitor has discharged, no more current flows across the LED array regardless of whether the switch is now open or closed.



Can it be done simply and can anybody help with a diagram?

There should be no problem at all. We manufacture high-intensity white light LED lamps (with simple
built-in charger) with LEDs all in parallel. The
parallel connection provides the high intensity.
Basically, there should be a dropping resistor
for each LED, and the driving voltage should be
bit high, say 6 Volts, to take into account the
voltage drop in the dropping resistor. For added
protection, add a high Watt resistor that will
limit the total current into the parallel array.
That is, suppose 20 LEDs are in parallel, each
running at the nominal current of 20mA. So the
high Watt resistor would limit the total current
that flows into the array, and each individual
low Watt resistor will limit the current into
the LED connected to it. The calculations are
simple. Hope that helps.

 

[Martin Brown]

I am having a little fun thinking around corners.

I have a huge array of LEDs and I want to pulse them on and off quickly (once).

How large is huge? In here it tends to mean advert sized display boards.

A trip switch will trigger the LED flash briefly.

How short is briefly? 1ps, 1ns, 1us, 1ms 1s

I need help with the circuit design. I'll try to show what I need done...


The LEDs are all in parallel and are 3v. The input voltage will be from a 3v DC supply (a coin cell).

Coin cell isn't going to hack it as white LEDs need a bit more voltage
than that to reach their full brightness. Having the LEDs in parallel
isn't great either as the weakest one will get most of the current.

You need about 4v across one to get maximum light output output 20mA and
for a single pulse of current you can overdrive them somewhat provided
you don't exceed manufacturers maximum ratings. I'd suggest trying two
coin cells in series or better a pack of 4 AA cells at 6v.

See this basic tutorial which may help you
http://www.electronics-tutorials.ws/diode/diode_8.html

With pulse drive the lead inductance will save you from disaster. On DC
the weakest LED will suffer from too much current and the stronger ones
will be dim. Might not be too bad if they are all the same batch and you
weed out any obvious stragglers but not in general a good idea to do
this without a small series resistor in series with every one.

A capacitor will charge and a charge indicator light will light up (could be a red LED). The LEDs remain off.

I'd use a green one - looks brightest for least current.
But how long does it have to recover between flashes?

A switch closes the circuit over the capacitor causing the capacitor to discharge 3v into the LED array which glows briefly. I will have to work out the right capacitor value myself. When the capacitor has discharged, no more current flows across the LED array regardless of whether the switch is now open or closed.

Use a changeover switch so that the capacitor is switched from the
battery to the LED bank.

Can it be done simply and can anybody help with a diagram?

Battery --R-- -----Leds
| \ |
| _|_ |
| ___ C |
|__________|________|


Try R=1k and C=1000uF as a first guess.

The charge indicator LED wants to go across the capacitor with a largish
series resistor and maybe a diode in series so that it only lights up
when the capacitor is fully charged.

If you describe what it is for then you might get a much better answer.
Ring flash for macro photography designs in amateur electronics magazine
EPE have much more sophisticated drive, timing and trigger.

Basically using LEDs for flash requires running them aggressively close
to their maximum permitted drive voltage and power dissipation.

These days you can buy single batwing LEDs good for 7W continuous
dissipation on a decent heatsink.

 

[Fred Abse]

Are you claiming that Zephod Beeblebrox is, seriously, your real name?

Count the heads ;-)

 

[John S]

The calculations are indeed simple. 1 joule for 1 microsecond is 1 megawatt.

I think that's a bit misleading, John.

1 joule (a watt-second) _per_ microsecond is 1 MWatt.

1 joule _per_ second is one watt.

But, that's just my opinion.

Cheers,
John S

 

[Phil Hobbs]

Don't be like JF and start mincing words. You knew what I meant by "for."

The OP wants to make a microsecond photoflash from his LEDs, and the concept has
serious numerical problems.

IIRC an actively-quenched flashtube can get down to the few-microsecond
range. There's an afterglow, of course, but the peak is big enough that
that doesn't matter much. Doc Edgerton did fine with .303 bullets,
which are a teensy bit faster than air guns.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

 

[John S]

Don't be like JF and start mincing words. You knew what I meant by "for."

Yes, but maybe to OP did not. Perhaps I did not phrase my reply in a
tactful manner.

The OP wants to make a microsecond photoflash from his LEDs, and the concept has
serious numerical problems.

I can't disagree with that.

 

[whit3rd]

I have a huge array of LEDs and I want to pulse them on and off quickly (once).

The LEDs are all in parallel and are 3v. The input voltage will be from a 3v DC supply (a coin cell).

A capacitor will charge and a charge indicator light will light up (could be a red LED).

Your LEDs will take maybe 50 mA for a few microseconds? If there's a dozen of 'em,
the capacitor will have to supply 50 mA x 5 us of charge, and droop by less than
or equal to Vmax - Vswitch-Vload after those five microseconds.
So, it'll take a fast switch (probably NMOS power FET), and a capacitor that can handle
microsecond pulses.

A fast switch with higher voltage and lower current (LEDs in series) and an inductor
to store energy, is the more usual strobe-light design.