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Old 19th Mar 2019, 2:02 pm   #48
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: Turntable Stroboscope

As to 'persistence' Tony Duel made a good point in post #6 regarding his (quite involved) circuit. Namely:

"The LEDs are on for 1/16th of the time. A short flash (rather than 50:50 on/off ratio) gives a clearer 'still' pattern of the strobe disk".

With an asymmetrical square wave, the LED is on for a shorter time than with a symmetrical square wave, so the flash period of the LED is shorter. Hence, if the LED has some ‘persistence’ rather than go straight off, it fades till the next ‘on’ period, so at 300 HZ might not seem to go off at all but may appear both to the naked eye and the strobe disc as a continuous light. To cite Tony's circuit, the LED would be on for only 6.25% of each cycle, and off for 92.75% so the flash would be more clearly defined, rather than a symmetrical square wave which would be high for 50% of each cycle, and low for 50%.

We know that LEDs quite unlike incandescent light bulbs in which a filament glows and take time to heat up and cool down. What isn't known, is to what extent say ultra-bright low current LEDs have 'persistence', and the only way of knowing that is to try one on a normal 50:50 square wave 300Hz signal.

As to your scope, you've said that you now have a stable 300Hz signal from your oscillator circuit, so if you feed that into the 'Y' input of your scope it should have do difficulty in displaying that to enable you to see the waveform, and the voltage. Don't know if the output of the oscillator would be high enough voltage to drive a low current ultra bright LED directly. Again, the only way of knowing is to try one.

What we do know for certain, is your own undoubted 'persistence' as amply demonstrated both with this project, and with the 'basket case' Bush AC91, which you persevered with until it behaved itself, long after many would have become disheartened and thrown in the towel!

Good luck with the project.

Hope that helps a bit.
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