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This is a spin-off from Roscoe's turntable article. Remember, he's using a Garrard as the basis for his next turntable (see "Next turntable project....").

I thought in the past to build a handheld strobe to set platter speed using our standard strobe disks. Note that I have a pretty big hand!

I just went down a long, incorrect path by assuming that I needed to reproduce an accurate 33-1/3 Hz signal and an accurate 45 Hz signal. After finding a way to do this (using a 1.8432-MHz TTL oscillator from Jameco - PN 27879 - and a few counters and logic gates), I finally realized that common strobe disks are designed for double either 50 or 60 Hz, depending on the country's AC-power standard. A real Homer moment - all I have to do is make an LED flash at an accurate 120 Hz and I'm done!

Given that the oscillator is accurate to +/- 100 ppm, dividing the frequency down to 120 Hz should provide a signal accurate to +/- 7 parts per billion. That accurate enough for most people, maybe even my OCD friends! Maybe I should plan to include a wall wart in case I kill the batteries, and maybe a battery test? Use a white LED and, with enough power, it could be used as task lighting, though I don't know the battery life for that use - good to have a wall wart.

Or I could get this for $15 and settle for 50 ppm: http://www.tindie.com/products/bot_thou ... le-strobe/

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- Guy

Or, you could just get one of these: http://www.radioshack.com/120v-neon-min ... 20707.html

Roscoe

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I can explain it to you, but I can’t understand it for you.

That depends on two things:
(1) The Shack is going belly up (though I can get on elsewhere)
(2) AC Power frequency varies by a good +/- 1/2 Hz depending on load, which in turn is dependent on time of day. As more load is added or removed, the frequency changes because generators cannot immediately compensate.

None of this may matter as long as your turntable maintains constant speed regardless of voltage or frequency change (within reason.) I'm not a musician with trained ears, so as long as the music isn't wowing around and the speed is close, I probably won't notice.

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- Guy

RS isn't belly up today

Probably well over 99% of the turntables with built-in strobes are using strobes that run off line frequency....

Roscoe

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I can explain it to you, but I can’t understand it for you.

But we're the DIYers - we need to push the envelope!

I agree that, for me, actual speed is less important than constant speed. I just thought I'd throw this out there as an intellectual exercise because I forgot what I was doing and had to remember my basics.

I mean, why do we build turntables with shot-filled platters, frictionless bearings, and wild motor controllers? It must be to approach the limits of high-fidelity performance - or because I'm just cheap and like to build things.

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- Guy

I certainly couldn't afford to be where I am with commercial products...

Roscoe

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I can explain it to you, but I can’t understand it for you.

You could always use the same chip I'm using for the 50Hz generator for the Garrard. ( http://www.mouser.com/Search/ProductDet ... 5-UCC3750N ) With a different crystal, it'll do 60Hz, rectify but don't filter it, and you'll have a 120Hz pulse to light an LED, neon, or whatever....

Roscoe

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I can explain it to you, but I can’t understand it for you.

I'm interested in your idea, and I reviewed the chip when you first posted it. But it feels like cheating if I just copied your circuit, and I've had my idea (or intention) for years so I feel like I should follow through with it, if only as a point of comparison. That was Reason #2 for posting it - offer another, though similar, approach to the same idea. Reason #1 was to reveal my screw-up with how strobe disks work. I frequently learn more from others' mistakes than I do from their successes.

The world (and industry) is changing to the point that comments will soon appear saying we can do this with a microcontroller! Which I'm sure I can, and maybe I should look into that, too.

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- Guy

You could, probably, do it with a Raspberry Pi, which can also double as a digital music server.

Roscoe

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I can explain it to you, but I can’t understand it for you.

Depending on the type of motor, you may want the strobe to track the line frequency. If it is a 60-Hz synchronous motor, the strobe should track the line as the strobe rate will vary with the motor speed. Of course the record speed will also track the line frequency, as this frequency varies back and forth a small amount in the power line regulation process. The average is always 60 Hz, but you will not want to manually try to hold your TT speed accurately by continuously tweaking the speed with a fixed-frequency strobe because it will be varying back and forth as the platter speed varies. Of course, this is an incentive for having crystal-controlled dc or ac motors that do not depend on the line frequency.

David