White LED strobe

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N9AOK
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Re: White LED strobe

Post by N9AOK » Sat Nov 15, 2003 1:43 pm

Here's my circuit if anyones interested..nothing sophisticated. Using a 555 as others here suggested.<p>http://www.homefires.us/bluemarble/PAGE ... /UVSTROBE/

N9AOK
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Re: White LED strobe

Post by N9AOK » Sat Nov 15, 2003 2:11 pm

I think I got pretty good stop motion with an ON time of 8 to 15msec ..beyond that and you would see movment of an object while the LED was ON. <p>If you really need less ON time, you would probably really need to dump the current through them....(current that I didn't have available easily with a 9V battery). I never drove an LED beyond about 150ma but as I said before it didn't seem to increase much brightness after the first 100ma. My current measurement accuracies may be in question as my junky old scope has some problems and the method of inverting and adding channel 2 to channel 1 had issues when I upped y range for the higher currents.

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Chris Smith
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Re: White LED strobe

Post by Chris Smith » Sun Nov 16, 2003 9:48 am

Yes its simple and cheap. One way mentioned in N&V a few years back places a 123ap transistor [100mw size] back wards in the circuit, ups the voltage until the diode junction breaks down and recovers, and its natural frequency is in the 100 to 200 nano second range at around 30 or 40k? <p> It takes over 100 volts [130v?] to break down that junction, but the time is short and no damage occurs to either diode portions.<p> Other ways are to set matched pairs of transistors in parallel, one has a slight delay from the other, one goes off, the second follows a slight bit behind, and the on portion is a short pulse followed by a dump or short to the whole circuit which makes an off. <p>Others like the one I did have a 555 into to Two FET configuration and although it was crude, one amp into the LED was as bright as a four cell mag light. <p>The secrete to getting the flow high enough, is voltage because of ohms law or "R". <p>The higher the pressure at the gate, the more current will flow, chopped off by the gate to a reasonably short time, equals high short current pulses. <p>I still have some LD60 laser diodes that require up to 100 amps max, 100-200 ns, and will shine down a fiber optic cable some 25 miles, and still have a signal at the other end.

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jollyrgr
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Re: White LED strobe

Post by jollyrgr » Mon Nov 17, 2003 3:34 pm

Sorry not to get back in such a long time but here is what is going on....<p>I did find the kit from Ramsey as stated in my second posting but thanks Tommy Volts in any case. <p>Dumping a strobe into an LED won't work, I do not believe. The capacitor in a strobe is charged, the gas exicted by a high voltage pulse, then you get a huge "short circuit" of the capacitor by the ionoized gas. You basically drain most of the capacitor during a flash.<p>N9AOK is on the path that I was expecting to take. My problem is I've used timers to flash LEDs but never got a "pulse" effect like a strobe. A 555 timer and a driver circuit is what I have used in the past. But this just made the LED blink. It must be the high current with modified duty cycle that is the trick. Too short and the LED shimmers, too long and it blinks. <p>I did finally break down and put a bid on eBay from one of the "Hong Kong" sellers for High Brightness White LEDs. It has been two weeks now and FINALLY today my package is at the Post Office (waiting to be signed for). (Amazingly they can sell these and ship them here for under $0.25 each in lots of 100 and US companies want over a dollar each.)<p>I would have probably had problems getting my circuit to work as I was expecting to use a 1mS on time. But I will try the 8 to 15mS as suggested. <p>The reverse polarity transistor seems like an interesting idea but I did not want to use high voltage. I was hoping for something that would work on nine or twelve volts without doing a step up voltage.
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N9AOK
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Re: White LED strobe

Post by N9AOK » Wed Nov 19, 2003 9:58 am

I'm interested in your results. Please repost here or email me if you would<p>[email protected]<p>Thanks,
Todd Snyder

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Chris Smith
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Re: White LED strobe

Post by Chris Smith » Wed Nov 19, 2003 4:00 pm

There are other diode junctions at lower break down voltages that you can chose. But the osc circuit is also small,.... a 555 and a tiny coil.

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Re: White LED strobe

Post by toejam » Sun Nov 23, 2003 6:23 pm

Hi Chris; i always wondered about using coils with 555's can they dump a few amps for a real short pulse? Do you substitute the coil for the timing cap? What value coil?Great stuff!Thanks.

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Chris Smith
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Re: White LED strobe

Post by Chris Smith » Mon Nov 24, 2003 8:19 pm

You can use a transformer with a 555, with or with out a booster transistor to carry more amperage. <p>My first 555 coil step [1980s] up with out additional transistor still produces over 400 volts, at milli amps. <p>After you store any amount of energy in a cap, preferable around 100 volts @ 100uF or so, then the cap can dump that power almost instantaneously into a fast switched circuit which lights up lasers or LEDS with heavy amps being delivered because of the voltage and ohms law. <p>The switch also must shut off the power, or the caps simply discharges all of its energy, and thus burns out the diode junction. <p>100 amps, at 100 volts, at 100 nano seconds on time, 1000 cycles per second, is actuall much less than 3 volts, 20 milli amps at constant current.

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Re: White LED strobe

Post by toejam » Sat Nov 29, 2003 7:49 pm

The problem i am having is how do you get that fast shut off? Is it possable to time two mosfets so one was entering the switching on transition the other is entering the switching off transition?
thanks,tj

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Chris Smith
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Re: White LED strobe

Post by Chris Smith » Sun Nov 30, 2003 6:46 pm

Yes this can be done with a short length of wire, but a diode must be inserted into the circuit to stop "ringing". <p>Remember each nano seconds is 17 inches long, and no saturated coils are permitted. <p>But also remember the RAMP or start up time of any transistor, this often is far in excess of the total time that you are trying to achieve? <p>This is the delay in a non square wave form of the transistor in transition between just starting and full saturation or the fully open circuit. <p>Check the specs, and use this "ramp time" to be your delay between one starting transistor, a signal out to the other transistor via a flip flop, and this signal starts the second transistor to shut down [or start up to ground] and the other shuts off the total current after this delay, shutting off the total circiuit, or shunting it to ground.

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Re: White LED strobe

Post by toejam » Mon Dec 01, 2003 6:58 am

Thanks Chris,
tj

Moe74
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Re: White LED strobe

Post by Moe74 » Wed Dec 17, 2003 10:53 pm

What might work is a 3909 IC. It can fire a LED from 1.5 vlots Vcc. I think it works in charge pump configuration, firing the LED from a cap.

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Re: White LED strobe

Post by Tparker » Thu Dec 18, 2003 10:45 am

I made a strobe flasher to alternately drive 2 LED arrays out of 3 555 timer chips. 2 of the chips are configured to run at approx. 8HZ and the third chip running at 1HZ (.5sec. on /.5 sec. off) controlls on/off of the first 2 chips. The third chip has the "diode mod" to run at a 50% duty cycle. Gives me approx. 120 "quad" flashes/min. The arrays are composed of 56 Lumiled's 70ma "Superflux" spider red LED's. I am driving them at approx. 100ma with no problems. Being 30 degree LED's. the arrays are somewhat directional but from a distance they look like xenon strobes.

toejam
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Re: White LED strobe

Post by toejam » Tue Dec 23, 2003 12:07 pm

the easiest way to measure light output would be a photo exposure meter.the problem you may have to deal with is the response of the meter with respect to time and wavelength. I dont know the response of either a selenium or silicone photo generator, that would be nice to know. There are other photo sensors such as photodiodes and transistors as well as the old cad sulphide photo resistors. Any of these devices can be used to measure light intensity, but you must know what part of the light spectrum they respond to as well as thier response time.I wouldnt mind having that info.

toejam
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Re: White LED strobe

Post by toejam » Tue Dec 23, 2003 12:08 pm

the easiest way to measure light output would be a photo exposure meter.the problem you may have to deal with is the response of the meter with respect to time and wavelength. I dont know the response of either a selenium or silicone photo generator, that would be nice to know. There are other photo sensors such as photodiodes and transistors as well as the old cad sulphide photo resistors. Any of these devices can be used to measure light intensity, but you must know what part of the light spectrum they respond to as well as thier response time.I wouldnt mind having that info.

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