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Simple Strobe using LED

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I just made one of these and it works fabulously well. Easiest for those of use with V/T gear because then a low voltage AC source locked at line frequency is easily available.

Those of you who are still in the silicon valley can just increase the zener value and series resistor to suit the power transformer in use.

The basic idea is to use half wave rectification off the AC supply and run this through series string of a zener diode, a current limiting resistor and an LED. I use the 6.3V AC windings for the heaters in the V/T power supply for my TT and using a 5.1V zener and 120R resitor gives a theoretical pulse width of 5ms once per cycle for a duty cycle of 25%. Visual inspection indicates a duty cycle around 30-40% so there must be some slippage somewhere. (No, I don't have calibrated eyeballs. Just wave the strobe about in a dark room and estimate the lengths of the light streaks and the spaces between them to give the duty cycle).

A couple of notes of explanation: First, this is designed to use line frequency as the timing standard. Most places this is about as accurate a frequency as you'll find - when I measure the line frequency where I live it usually varies by less than 0.05% and I think that's more the accuracy of my cheap meter.

Secondly, you need to take account of whether the strobe disc you use is designed to run at line frequency or at double line frequency. Most strobe discs use double frequency because an AC light source generally lights on each voltage crest therefore twice per waveform. For instance you will find a lot of discs use 180 markings for 50Hz / 331/3 RPM (180 x 33/1/3 / 60 = 100, not 50). I made my own disc by printing a pie chart in Exel, if you have a double frequency disc you can just use a full wave rectifier instead of a half wave.



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Topic - Simple Strobe using LED - Mark Kelly 04:05:30 04/13/04 ( 5)