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A stroboscope is a tool which uses a bright flashing light to allow a user to view objects which are rapidly moving in a periodic manner. When the frequency of the flashing light is matched to the speed of the object, the object can appear to be stationary.
Stroboscopes are commonly used to examine moving machinery in an industrial setting. The stroboscope allows the equipment to be inspected or monitored while it is operating.
If the strobe frequency is high enough, the observer won’t perceive the flashing of the light, similar to how one does not notice the fact that a movie projection actually consists of individual frames.
This article details construction of a control circuit that can drive an array of LEDs for use as a stroboscope. The LED array can be made by straightforward modification of inexpensive LED flashlights, or a custom array can be built.
One note of caution. Some people who have epilepsy are susceptible to seizures caused by rapidly flashing lights. Do not use the device shown here, or any commercial strobe light around anyone who is known to have such a susceptibility!
Here is a YouTube video demonstration of just a few ways that this stroboscope unit can be used to view moving objects.
Stroboscopes are commonly used to examine moving machinery in an industrial setting. The stroboscope allows the equipment to be inspected or monitored while it is operating.
If the strobe frequency is high enough, the observer won’t perceive the flashing of the light, similar to how one does not notice the fact that a movie projection actually consists of individual frames.
This article details construction of a control circuit that can drive an array of LEDs for use as a stroboscope. The LED array can be made by straightforward modification of inexpensive LED flashlights, or a custom array can be built.
One note of caution. Some people who have epilepsy are susceptible to seizures caused by rapidly flashing lights. Do not use the device shown here, or any commercial strobe light around anyone who is known to have such a susceptibility!
Here is a YouTube video demonstration of just a few ways that this stroboscope unit can be used to view moving objects.
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The stroboscope works by producing very brief yet very bright pulses of light. If the frequency of the light pulses is correct, the rotating object will be illuminated at the same position during each flash of light, giving the appearance that is stationary. This phenomenon is called the stroboscopic effect.
The stroboscopic effect is really a phenomenon of aliasing, which is the result of under sampling. If you are familiar with digital signal processing, you may be aware that a signal that is sampled can produce “aliases” depending on the sample rate used. A signal that is sampled at less than twice its frequency can produce a result called an alias, which has a lower frequency than the original signal.
If a signal is sampled at a rate which is exactly the same frequency as that of the input signal, then the sample will take place at the same point in its cycle, and so the same value will always be read. Because the sampling results in the same value each time, the resulting sampled signal representation appears as DC value instead of the alternating waveform of the actual signal. The original signal has therefore been downshifted in frequency to a DC signal. The same concept applies to the stroboscope. The actual rotational speed will be downshifted to zero in the eyes of the observer, giving the false perception that the object is standing still.
The object will also appear stationary when the strobe frequency is some integer fraction of the rotational frequency, such as one half, one third, or one forth, etc. This is because these cases will also result in the object being illuminated when it is in the same position each time.
If the strobe frequency is slightly lower or higher than the rotational rate (or an integer fraction of the rotational rate) of the object, it will appear to rotate slowly forward or backward. In these cases the rotational speed has been downshifted to a speed slightly greater than zero in the eye of the observer.
The appearance of a rotating object illuminated by a strobe can show more complex behavior if the rotating object has multiple identical sectors, like the spokes of a wheel or the blades of a fan or propeller. In these cases, the object can appear stationary when the period of the strobe frequency is an integer multiple of the rotational period divided by the number of sectors. If the individual sectors are similar enough in appearance that they are identical to an observer, then the object will appear stationary for any strobe frequency where any of the identical sectors is in a given position. As the strobe frequency is swept, the observer may notice several points at which the rotation appears to first slow , then stop, and then begin rotating in the opposite direction.
The stroboscopic effect is really a phenomenon of aliasing, which is the result of under sampling. If you are familiar with digital signal processing, you may be aware that a signal that is sampled can produce “aliases” depending on the sample rate used. A signal that is sampled at less than twice its frequency can produce a result called an alias, which has a lower frequency than the original signal.
If a signal is sampled at a rate which is exactly the same frequency as that of the input signal, then the sample will take place at the same point in its cycle, and so the same value will always be read. Because the sampling results in the same value each time, the resulting sampled signal representation appears as DC value instead of the alternating waveform of the actual signal. The original signal has therefore been downshifted in frequency to a DC signal. The same concept applies to the stroboscope. The actual rotational speed will be downshifted to zero in the eyes of the observer, giving the false perception that the object is standing still.
The object will also appear stationary when the strobe frequency is some integer fraction of the rotational frequency, such as one half, one third, or one forth, etc. This is because these cases will also result in the object being illuminated when it is in the same position each time.
If the strobe frequency is slightly lower or higher than the rotational rate (or an integer fraction of the rotational rate) of the object, it will appear to rotate slowly forward or backward. In these cases the rotational speed has been downshifted to a speed slightly greater than zero in the eye of the observer.
The appearance of a rotating object illuminated by a strobe can show more complex behavior if the rotating object has multiple identical sectors, like the spokes of a wheel or the blades of a fan or propeller. In these cases, the object can appear stationary when the period of the strobe frequency is an integer multiple of the rotational period divided by the number of sectors. If the individual sectors are similar enough in appearance that they are identical to an observer, then the object will appear stationary for any strobe frequency where any of the identical sectors is in a given position. As the strobe frequency is swept, the observer may notice several points at which the rotation appears to first slow , then stop, and then begin rotating in the opposite direction.
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I did something similar about 5-6 years ago. I started with a prototype using a 556 dual timer to check the concept and then built a final version using a PIC driving an LCD display and a single 5W LED.
I built my strobe for a microscope illuminator so that I could observe the motion of the cilia of microorganisms. I controlled the flash period using a rotary encoder and it was settable between 1/5 sec and 1/256 sec in one continuous range which would display on the LCD. The flash duration was settable from about 5 to 250 uS. I drove 15A current spikes through the LED (5W LEDs can handle it as long as you keep the duty cycle low).
It was really surprising what could be seen using the strobe. Cilia that normally move too fast to be seen became clear as day and one could see patterns in their motion like the wave-like pattern you can see in centipede's legs when they are moving fast. Really amazing when you consider that a single-celled organism has no nervous system to coordinate the motion.
Have you looked at using rotary encoders instead of a pot with an analog input? It is so much easier to use an encoder - a simple lookup table is all that is needed to determine rotation direction and counter increment/decrement. It's lightning fast and It will greatly simplify your code. Most rotary encoders, even the cheap ones (see here: http://search.digikey.com/us/en/cat/sensors-transducers/encoders/1966131?k=rotary%20encoder), will work more reliably and last longer than a pot.
This page has a great explanation of how to read an encoder in assembly language: http://mcmanis.com/chuck/robotics/projects/lab-x3/quadratrak.html
I do see the benefits of an encoder, and I have some sitting around that I've intended to use on projects but just haven't ever gotten around to it. I may do a second version of this project that uses an encoder instead.
I would also have liked to add an LCD for frequency and on time readout like you have, but I built this stobe project using a custom general purpose PCB that was only laid out for an 8 pin PIC processor.
All I added was a 15V 5A power supply, a 5V regulator to power the board from the 15V supply, power FET switch to switch current pulses into the LED, and the encoder. The 5A supply charged a capacitor that rapidly discharged through the LED to get 15A pulses.
One nice thing about encoders is there is no mechanical stop- you can just keep spinning and spinning the thing eliminating the need for range switching. You can make the increment/decrement function speed-sensitive so that you get fine control for slow rotation and quick jumps for fast rotation.
A well-done project. For people that do not want to use a microprocessor-based solution, the 555 timer could be implemented to perform the main function. In spite of the lower accuracy (could be improved by careful calibration) the circuit would be simpler and easy to put together.