Re: LED at 60 HZ? (was RE: Radio Shack Strobes)

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Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 01:10 PM 2/24/2004 -0700, you wrote:
>Original poster: "Daniel Kline" <daniel_kline-at-med.unc.edu>
>An engineering friend says a high-brightness LED on a
>low-voltage transformer, like 6 Volts, connected through
>a suitable resistance, should flash at 60Hz in sync with
>the 60Hz mains. (50Hz for you all over there :)
>It seems too easy to me...he thinks that the LED may have
>a "persistence" issue. In other words, just because it
>turns off quickly doesn't mean it stops emitting quickly.
>Comments?
>Thanks,
>Dan K

Yes, it will work.. the real problem is that the LED flash is quite long 
(on the order of 8 msec). Say you have a white disk with a black stripe on 
it, tied to a 1800 RPM motor.  With that strobe, you'll see 4 quadrants, 
alternating white and grey.  This is because while the LED is on, the 
stripe moves 90 degrees.

What you really want is a real short bright flash, so that the image of the 
stripe you're looking at is nice and sharp, and not blurred.

I should think that one would be able to modify an inductive pickup auto 
timing light (It's a bit more expensive that the RS strobe).  You could 
build one of the AC line operated circuits with an ignition coil (2 uF 
motor run cap, light dimmer) to make a spark signal that the strobe can 
pick up.

Or, get whatever RS is selling now, and do the same reverse engineering 
that was done on the previous 2 or 3 models that have been fooled 
with.  I'm sure the basic circuit design isn't going to change much: line 
connected voltage doubler to charge the flash energy storage cap, resistor 
to charge a 0.1 uF or so trigger capacitor, neon bulb or diac to trigger 
transformer.


If you're a bit more ambitious, one could do some sort of short, but high 
power, pulse through the LED.  If you control the duty cycle, you could 
probably run currents that are 10-50x continuous current limit.  Maybe 
charging a capacitor off the AC, and using an SCR to discharge it through 
the LED, just like in a phase control dimmer.