Strobe lights are being made using LEDs. But are they comparable to say Xenon lamps, as used for car ignition timing lamps. These are very effective at "stopping dead" the perceived image of a spinning part. I'd like to make a compact, low power strobe, that won't give a blurred image. Any chance with LEDs?
an example for the needed flash time. A wheel rotates with 3000 revolutions per minute and it should move not more than 1 degree during the strobe flash. There are 50 revolutions per second, one revolution is 20 ms long. 1 degree is 20/360 ms or 55 µs. Some 10 microseconds will do.
If the flash is 1/1000 second long, the wheel will rotate 360/20 degrees within that time, 18 degrees, this is not a sharp image.
If you want to get a little fancy with the inductive pick-up (or you're too lazy or uninspired to make your own), you can get a professionally-made one through eBay;
**As PA has suggested, white LEDs possess too long a a persistence to be effective as a serious strobe light, but you can use RGB LEDs to provide a close approximation to white light. Speed is not a problem for RGB LEDs. I've used these in a recent project:
it was a complex project to measure sedimentation inside a rotating centrifuge using digital image processing. The strobe was only a subsystem for lighting the probe glases. I used about 20 RGB LEDs to select red, green, blue, yellow, magenta, cyan and white light. A IR reflective sensor was used for triggering. The duration of the LED pulse was selectable in steps of 10 µs up to 200 µs.
The circuit is not useful for an automotive strobe.
Thanks for the info. The strobe I want isn't for auto ignition timing, but to illuminate a "stopped" image of a spinning part, which does happen to be in an engine (with a glass window to see through). Colour isn't important as long as it's visible. It needs to be as compact as possible and heatsink type LEDs probably won't be easy for this, so I'll use the brightest probably 5mm diameter LED I can find. For the brief duty cycle, maybe I can get away with much higher current to get higher brightness.
Point taken about white LED persistence - good info. Does it happen in the LED, or the human eye? Those RGB LEDs are probably too big, but they look pretty nifty and I should buy one just to play with!
**As PA has already explained, the problem lies with the construction of all white LEDs. A white LED is actually a blue LED, with a fluorescent coating. The coating is the problem. It has a persistence.
**You could consider a green LED, as they are not only available in in quite high outputs, but the human eye is highly sensitive to green.
if you want to illuminate the spinning part with one 5 mm LED only, you need:
very low ambient light the LEDs should be close to the spinning part. only a small area should be illuminated a superbright LED should be used green light would be most efficient for human sight
The disposable camera option has one or two gotchas - the most common type is powered by a single AA cell and designed for single shot with a significant recharge time, it might be tricky getting enough power through the transformer for repetitive flash.
Winding your own bigger transformer on the basis of the original is complicated by an extra winding in series with the capacitor charging circuit that augments the base drive pulses - as the capacitor charges to maximum; the augment current tails off to conserve the battery. This feature offers no advantage in a repetitive flash application.
Some disposable cameras have a slide switch for the flash - but increasingly they have a push button and a few extra transistors for a latch controlling power to the flash circuit.
Its a matter of preference - but when I needed a 6V powered strobe for timing small motorcycles, I used a push-pull inverter.
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