A stroboscope speed tester.

[epigramophone]

A technically minded collector friend who is not a member of this forum has built a stroboscope which uses a normal 78rpm strobe disc. This could be useful as LED's gradually replace filament light bulbs and fluorescent tubes.

My electrical knowledge does not extend beyond wiring a 13 amp plug, but my friend informs me that :

1. Setting up is tricky, because the resistors R3 to R14 have to be selected on test to get the frequencies right.

2. For speeds between the switched settings, the switch is operated backwards and forwards between the two nearest speeds and the governor is adjusted until the bars of the strobe appear to rotate at about equal speeds in both directions.

Here is the circuit diagram. I suggest you copy it before the image disappears.
I can pass any questions onto my friend.

[emgcr]

Thank you for this Roger and it sounds an interesting idea but I think more information is required please. Is this something he is thinking of making to sell ? If so, what might it cost ? How exactly does it operate ? You mention that setting up is tricky. Does this refer to the original design or to something every user would have to undergo every time it is used ? Photos would help.

[physicist]

I'm not the designer but I know enough to understand how this would work.

A 78 rpm strobe disc relies upon the flicker of an incandescent lamp powered by A/C mains.
In this case, the 100Hz at 78rpm in the image indicates that it is intended for a UK strobe
(50Hz mains). It would need different resistor values to work on a 78RPM strobe designed
to work with 60 cycle mains.

The circuit can generate a flashing light at a range of different frequencies depending upon
the resistance connected across IC3. Manufacturing the device is tricky because the correct
values for the resistors have to be selected by testing. An accurate frequency meter would
be the best way of doing this.

If you happen to have a multi speed strobe disc, a simpler device that only flashes at a fixed
100Hz speed (UK) would suffice to replace the flash that you used to get from a mains
incandescent lamp.

[AudioFeline]

This device isn't as relevant now as it would have been, because there are cheap laser tachometers which can be bought from China that are accurate enough for turntables.

There is also a circuit downloadable from VinylEngine for a LED light, configurable for US or UK frequencies, that doesn't require the setup that this circuit appears to require.

Edit: a link to that VE page: https://www.vinylengine.com/diy-strobe.shtml

[emgcr]

Although I have changed all bulbs in the house to LED, I have kept an angle-poise lamp with small, well-focussed, incandescent bulb shining on the turntable of my every-day gramophone, thus a traditional multi-speed stroboscope will still work well and seems easier/quicker to use than the alternatives---laser tachometer, mobile phone etc.

[epigramophone]

I think my friend would make these to sell if there was sufficient interest, but AudioFeline's point about the availability of cheap laser tachometers is probably the logical and simpler answer.
Until recently I preferred the low tech approach of placing a piece of paper between the record and the turntable, counting the revolutions against my watch, but now I have this mechanical marvel. I don't know how accurate it is, but it is fun to watch :

[PeteLeoni]

FYI, the Android ap "Strobily" is a fantastic app for a standard strobe disc. No longer do you need a neon or florescent light. Strobily uses the led on your phone. Very crisp clear and accurate.

[Hoodoo]

I either use a free iphone strobe app or simply count the number of revolutions in 30 seconds.

[mrrgstuff]

A long time ago I did build my own electronic tester: https://youtu.be/x5ZZIaDR1aY
However, I although I still use it, the availability of commercial RPM testers seem a better bet: https://youtu.be/EDhr652jlDg
More recently I've also started using a phone app which works well for me:
https://youtu.be/FRHwDRF082g
https://youtu.be/hLoFjPrpM1A

[CarlosV]

FYI, the Android ap "Strobily" is a fantastic app for a standard strobe disc. No longer do you need a neon or florescent light. Strobily uses the led on your phone. Very crisp clear and accurate.

I tried it and works fine! Thanks for the tip.