1
\$\begingroup\$

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?

\$\endgroup\$
3
  • 1
    \$\begingroup\$ Why won't a stroboscope work on the turntable directly? \$\endgroup\$
    – DKNguyen
    May 10, 2020 at 3:43
  • 1
    \$\begingroup\$ have you considered using a stroboscope app for your smartphone? \$\endgroup\$
    – jsotola
    May 10, 2020 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, 2020 at 10:12

2 Answers 2

Reset to default
6
\$\begingroup\$

Try 120Hz. The light varies with amplitude of the voltage and ignores polarity.

\$\endgroup\$
6
  • 1
    \$\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, 2020 at 3:46
  • 1
    \$\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, 2020 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, 2020 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, 2020 at 10:45
  • 1
    \$\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, 2020 at 5:12
0
\$\begingroup\$

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).

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.