Playing with disposable flash...

[Externet]

Hi fellows.
My ignition timing light consists of a aluminium foil wrapped clothespin to grab wire #1, which turns on a neon bulb at every pulse. A photodiode glued to the neon triggers a white led to aim at the timing mark. Works perfect if not under strong daylight.
Nothing fancy to use it for a minute every 2 or 3 years, I don't care how pretty it is.

Planning to try now a disposable camera flash circuit; what modification should be done to decrease the brightness and increase the firing rate capability to ~under 10 per second ? Its power source to still be ~1.5V derived from car battery. A smaller capacity / same voltage capacitor ? A higher power capable inverter ?

Miguel

[Bigglez]

Greetings Miguel,

Planning to try now a disposable camera flash circuit; what modification should be done to decrease the brightness and increase the firing rate capability to ~under 10 per second ? Its power source to still be ~1.5V derived from car battery. A smaller capacity / same voltage capacitor ? A higher power capable inverter ?

Your first version sounds interesting, what limitations
are you trying to overcome?

The camera flash (as you know) has a high voltage
flash tube and support electronics. The energy from the
battery charges a capacitor and typical takes several
seconds from cold. Your strobe would require a
much faster recharge time (engine at 3600 RPM is
60Hz) whch is beyond the capability of the camera
flash charger.

Reducing the flash energy will go a long way to speeding
up the cycle time. The camera typically has a 50 to
150uFd capacitor and 300V (250 in smaller tubes).
energy = 1/2CV^2 = 2.25j to 6.75j.

If you replace that cap with, say, a 1uFd the flash
energy would be 45mj. At this level you'll have
to decide if the light is enough to see the timing marks.

The charging circuit is a limited current source so
the cycle time will decrease by 50 to 150 fold with
the smaller cap, for a 3 to 10 hertz range is still
not fast enough.

So, next you need to replace or modify the
charging circuit to five or six times stronger.

This will not be as simple as replacing the battery (!)
as the switching transistor is choosen for economy
(operates near thermal limit) and the transformer
is choosen for small size and cost (limited energy storage).

Perhaps your first experiment is to shrink the flash
capacitor (to 1uFd) and see if you can live with the
light level. If so, work on a stronger charger.

I found some NOS camera flashtubes at All Electronics
(years ago) and played with them for halloween.
I could get multi-flashes by using two tubes and
a monostable delay to sync two chargers. Your
engine strobe may require this type of crazy solution,
too.

Comments Welcome!

[Externet]

Your first version sounds interesting, what limitations
are you trying to overcome?

The neon stays on for longer than necessary triggering the led accordingly; that could improve with a capacitor in series to pass only a short spike. Mostly it is the light intensity as I do not have one of those superduper star white leds.

Perhaps paralleling a few disposable inverter sections (as I have a dozen of those camera flashes) will charge much faster. 5Hz should suffice.

Miguel

[Bigglez]

Greeting Miguel,

The neon stays on for longer than necessary triggering the led accordingly; that could improve with a capacitor in series to pass only a short spike. Mostly it is the light intensity as I do not have one of those superduper star white leds.

Why not use a monostable ('555 or similar) to generate
fixed length LED flashes for each trigger pulse?

You can use more then one LED, or over drive the
LED on the assumption that short pulses would not
harm it (them).

Did you see the article in NandV last for LED strobes?
Build A High Powered LED Strobe [Relevance: 1.20]
January 2008 - Fernando Garcia
Build A Ring Flash For Macro Photography [Relevance: 1.80]
January 2007 - Fernando Garcia

Perhaps paralleling a few disposable inverter sections (as I have a dozen of those camera flashes) will charge much faster. 5Hz should suffice.

I haven't tried that. A single flash unit has a voltage
feedback loop to stop the charger when ready, so
paralleling several chargers would require some form
of isolation, perhaps only a high voltage diode "OR"
from each charger to one storage capacitor is required?

Comments Welcome!

[thoman]

Considering that you have a nice, high current, high-voltage (12v vs 1.5v) powersource next to you have you considered reworking the cap section for the higher voltage.


Alternatively, buy a cheap automotive coil and wire your trigger to the inputs and drive the tube off the output. Not more charging issues. Now you just have to be worried about getting tossed across the room if you get a taste of 20,000 volts. BE VERY CAREFUL HERE. This method could easily get you entered in the 2008 Darwin awards for dumb things killing stupid people!

[haklesup]

I agree that modifications to a cheapo disposable flash would be extensive to increse duty cycle to a useful rate. These already are among the slowest charging devices when compared to even a budget flash unit.

Another easier route would be to modify a party strobe. The kind you get at the halloween store or party city for $10. Are you open to buying parts or is this an out of the junk box project? I guess it can't be much since you can buy a used timing light on eBay for $30 (incl shipping)

[Robert Reed]

And probably cheaper yet at Whitneys automotive store.

[MrAl]

Hi there,

How about just go to a higher power LED?
Although you are not supposed to run LEDs at a higher 'peak' current
than the manufacturer says, it could work to maybe 2x this amount
which would give you a pretty darn bright flash (50ms maybe?).
This would be safer and have long life too. Flash tubes are not made
for continuous use are they?

[Externet]

No, they are not for continuos use, but neither is the application.

[quote="Externet
Mostly it is the light intensity as I do not have one of those superduper star white leds. /quote]

Yes, this is about playing with whatever is in the junk box, not to buy already made units, that is too easy... The white led contraption I have works, just thinking of other means to do it in spare time.
Perhaps one of those 'star' leds and a lens would do it. Anyone has one to trade ?

Miguel

[sparkle]

Yet another xenon reference design:
http://www.zetex.com/3.0/appnotes/design/dn31.pdf

[MrAl]

Hi again,

Miguel, the price has come down a lot on the 1 watters if you want
to purchase one on the web, and you can get a lens with it.

What do you offer to trade for one of the 1 watter high power LEDs
made by Lumileds?

[Bigglez]

Greetings Miguel,

Mostly it is the light intensity as I do not have one of those superduper star white leds.
.....
Perhaps one of those 'star' leds and a lens would do it. Anyone has one to trade ?

Why a white LED? My first engine timing light was neon orange
(coiled glass neon tube behind a lens, connected to the No. 1
spark plug).

How about an orange or high efficiency red LED?

Today's LEDs are much better than early versions. I've
had to increase the ballast resistor in some designs
because the LEDs are painfully bright on 10mA.

Combine that with a fixed LED pulse (monostable) and
you may have a useful "low voltage" strobe.

Comments Welcome!

[MrAl]

Hi Peter,

It doesnt really matter to me that much, but i have only high power
white LEDs in stock.

I've used all sorts of colors for indicators, including red, green, yellow,
orange, and of course blue (one of my Li-ion chargers). Many times
it's best to use white however, because it has better color rendition
than any color LED could have. What if someone has a red mark
as their timing mark? I dont know if this is possible, but who knows.
White seems to show the true color of objects and 'marks', up to
a certain point of course. It makes sense to use white then, especially
if you already have high powered ones.

I have to say though, that some of those little high intensity red
LEDs are quite bright even at 20ma (the small LEDs). You could
overdrive them to maybe 100ma (short pulse) and probably still
get lots of life out of them. There is a slight problem that comes up
however with some of these LEDs, and that is that the radiation
pattern is not distributed as evenly across the field of view as
we would like. This means there is the possibility that the very center
of the pattern could be dark (typical with some 20ma red LEDs).
They are meant to be used as indicators, not light sources, so they
are not made to put out an even patttern. The white LEDs on the other
hand are made just for that. In fact, even the little 5mm whites have
a rather narrow beam and it's fairly even over the radiation angle.
Makes a nice light source, and can probably be overdriven with
low duty cycles to get a nice bright flash, just like the red, only with
the nice tight and even illumination beam.

I have a nice blue-green LED i got a while back, i used it to test
battery packs as a light load (doing some experiments). It's pattern
was NASTY!! I cant even discribe the pattern as strange as it was,
except maybe that it looked like two squares connected at one
corner each, forming two diamonds stuck together. What a nasty
pattern, not useable for much except an indicator. The color is
really nice though, a much nicer green than the standard green LED,
which sometimes looks a little yellowish-green rather than 'leaf' green.

Oh yeah, about the 'lens' for the LED...
If you get a standard lens used for close up viewing and mount it
in front of the LED you can get a nice small, round, high intensity
spot on an object that is placed in front of the lens (like for doing
automobile timing). The thicker the lens the higher the power usually,
and one of those eye loupes would work nice. The distance between
the LED and lens can be varied to get different effects: if the LED
already puts out a nice pattern than you can place it at a distance
equal to the focal lenght of the lens, but if it doesnt put out a nice
pattern you can get rid of some artifacts by moving it slightly farther
(or closer) away from the lens so it projects an image of the LED
that is slightly out of focus. This does help in many cases, and can
even get rid of those nasty connection wires that sometimes become
visible when using lens with LEDs. Move the LED slightly away from
the point of focus and the lines disappear, making a nice clean beam
pattern.
If not anything else, hot glue helps to mount the lens and LED relative
to one another perhaps in a plastic pvc tube or something.