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
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
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
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
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.