How to Determine the Number of Revolutions?

Remember those old vinyl record turntables? They had a series of reflective bands at intervals around the edge. By adjusting the speed you could get them to synchronise with a given number of AC mains pulses so that the image appeared stationary.

You can see the sort of thing around the lower edge ↑ of this one.

You know the charity shop halfway down Wayne Mayne on the right? There's one in the window if you hurry.

there are a number of infared rpm detectors sold. all you need to do is attach a small reflective strip on the disc. Google RPM sensors.

Whe use a photo optic sensor switch from Sick Optics to read a colored mark on materials. The switching is simple NPN circuitry. This may work for you.

The switch model number is KT5-2N116 and can be found on their website.

Hope this helps.

Hello gdallas,

I use Sanwa make digital non-contact tachometer.

Since you are measuring rpm of silvery disc, i would suggest putting a black tape on a small portion and then stick the reflective strip which comes alongwith the tachometer.

Google the above mentioned bold keywords...may makes are available/

Good luck

All of above can solve your problem even you can use proximity switch or sensor for generating pulses and then count, but if RPM is too high then I am afraid, upon addition of any weight at only one place on disc there may be another problem of balance rotation.

Depending on what this "silvery" disc is made of you can use a black magic marker line and a strobe light. The marker can then be easily removed with alcohol and a cotton ball.

meke 60 hole on your disk use MOC7811 motorola optical sensor measur Frequency of pulsess just with any frequency counter that will be directly RPM

jarun magnet

as pulses are 60 you are measuring in 1 sec i.e. directly prop to RPM

You can use the reflective strips from the good old computer magnetic tapes used as tape marker.

This might be just what you need:

http://www.allelectronics.com/cgi-bin/item/TCH-4/search/LASER_DIGITAL_TACHOMETER_.html

You can get a tachometer that works with a strobe light sort of like the timing light for tuning up a car. A small marker with a felt tip will enable you mark the disc withoud damage to check the rpm. Clean off the disc with denatured alcohol.

If you do not find one anywhere else, go to a shop that sells stuff for model aircraft...they have such counters for engine RPM, non contact ones too....usually quite cheap.....