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I needed to measure a turntable's number of revolutions per minute, but didn't have any incandescent lamps around. So I rigged an LED to an ESP8266 SOC and programmed it with delays to output a 60hz square wave to the LED. The scope confirms 60Hz +/- 0.2. Unfortunately, it doesn't work as I can't get a reading on the stroboscope disc.

I know I am missing something .. and it might have something to do with a DC square wave vs. an AC sine wave. Can you show me the direction?

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    \$\begingroup\$ Why won't a stroboscope work on the turntable directly? \$\endgroup\$ – DKNguyen May 10 at 3:43
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    \$\begingroup\$ have you considered using a stroboscope app for your smartphone? \$\endgroup\$ – jsotola May 10 at 4:59
  • \$\begingroup\$ it's not clear to me how you are reading the signal. I assume the ESP+Led is used to emulate the signal. What are you using to actually feed the signal to the scope? \$\endgroup\$ – RJR May 20 at 10:12
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Try 120Hz. The light varies with amplitude of the voltage and ignores polarity.

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    \$\begingroup\$ To the OP: Did he program it to toggle at 60Hz (turn on for 1/60th of a second then turn off for 1/60th of a second) which would result in 30Hz blinking)? Or to actually blink at 60Hz (which would be turning on for 1/120th and then turning off for 1/120th of a second)? \$\endgroup\$ – DKNguyen May 10 at 3:46
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    \$\begingroup\$ @DKNguyen valid question, but the point of this answer is that an incandescent bulb driven by 60Hz AC is bright 120 times a second and dim 120 times a second. \$\endgroup\$ – hobbs May 10 at 7:23
  • \$\begingroup\$ Right, and the neon strobe light in my old Technics turntable is just the same- 120 pulses per second- it's directly across the 60Hz mains (with a series resistor, of course). \$\endgroup\$ – Spehro Pefhany May 10 at 7:51
  • \$\begingroup\$ @hobbs: No, an incandescent light has no significant brightness variation at all. A fluorescent lamp would. \$\endgroup\$ – Dave Tweed May 10 at 10:45
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    \$\begingroup\$ Thanks everybody. And sorry for the late response. StackExchange's Emails went into junk. Anyway, I changed the frequency to 120hz and it works fine now. More info and background: rollofone.com/?p=2598 Thanks again. Michaela \$\endgroup\$ – Michaela.Merz Jun 2 at 5:12
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Where did you buy your strobe disc? If you bought it in 50Hz land, e.g. in Germany, (unless you imported it from 60Hz land) you'll need to strobe it at 50Hz (which as Spehro says, implies flashing the LED at twice that frequency ... 100 Hz).

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