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LED Strobe & Contactless Tachometer

This versatile LED Strobe & Tachometer can be used to observe and measure the RPM of rotating machinery. It offers three different measurement methods and the readout is via a 2-line LCD module.

By John Clarke

Feaures

  • RPM and frequency readout on LCD panel
  • Generator or triggered strobe
  • Can be triggered via slotted disk or reflective light
  • Adjustable flash period
  • Fine frequency adjustment in generator mode
  • Wide frequency range
  • 1 RPM resolution
  • Divider options when triggering
  • Triggering indicator
  • Readout averaging
  • It’s easy to measure the speed of rotating machinery with this versatile project. It uses three different "contactless" sensing methods, making it ideal for checking the RPM of objects such as rotating shafts, fans and model aircraft propellers.

    In addition, the strobe feature allows rotating machinery to be effectively "frozen" for close visual inspection. The strobe is based on a high-brightness white LED and can also be used to provide basic stroboscopic speed measurement. Alternatively, speed measurements can be made using either an infrared reflective optical pickup or a slotted disk/photo-interruptor pickup.

    Strobing

    Many people consider strobes as just a party effect, for use in discos and other venues. A typical disco strobe flashes at about four times a second and the strobing effect makes people appear to move in a jerky manner. That’s because, at night, you only see each person’s position when the strobe flashes. The intermediate positions between flashes are not seen.

    Strobing rotating machinery gives much the same effect, depending on the strobe frequency and the RPM of the rotating part. If the strobe is set to flash at a rate of once per rev, then the rotation will appear to stop. The reason for this is simple – the machine will be in the same position each time the strobe flashes.

    In fact, the effect is so convincing that it can be dangerous. You must be alert to the fact that the machine must not be touched, since it is still actually moving and could cause serious injury.

    Other strobe effects also become apparent as the strobe frequency drifts out of step with the rotational frequency. For example, if the strobe flashes slightly faster than the rotational speed of the machine, then the machine will appear to rotate slowly backwards. Conversely, if the strobe flashes at a slightly slower rate than the rotational speed of the machine, the machine will appear to rotate slowly forward.

    One area where this is often apparent is in western movies, where the wheels of a stage coach initially appear to slowly rotate backwards and then stop while the stage coach is still moving. That happens because movies are shot at a rate of 24 frames/s and this has the same effect on the wheels spokes as a strobe.

    Initially, the wheel spokes are travelling too slowly to keep up with the strobing effect of the frame rate. Then, as the speed increases, the wheels appear to stop before finally appearing to rotate forwards.

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