I just bought a new strobe light for an experiment in biofeedback; supposedly it can flash up to 10 fps but I need to know when or if it's flashing at pretty much 7 fps. Does anyone know if there is any quick and dirty way to determine the flash rate?
Ron
Hang it on a rope long enough for a 1 second period, and do like your title says -- count the spots.
I think you want around a one meter rope, but it may need to be four -- do a web search on 'pendulum' & you should find the right number.
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Video camera filming it and a watch. View it on slow speed. Count the flashes...
Regards, Bob Monsen
"Ron Hubbard"
** Make a pendulum with a length of cotton tied to a small metal nut.Set it so there is 20 mm from the fulcrum to the centre of the nut.
The period will be 0.28 seconds or 0.14 seconds between peak excursions.
A 7 Hz flash should be able to stop motion at each peak excursion.
...... Phil
A nice practical application of basic physics there Phil. :~)
Graham
"Ron Hubbard"
** No " L " I sincerely hope.....
...... Phil
~)
Hmmm, i was hoping for something more... electronic...
Ron
"Ron L. Hubbard"
Hmmm, i was hoping for something more... electronic...
** Depends what test gear YOU have available.Got a scope ?
Got anything?
....... Phil
No... But try adding an "O" and an "i" in the right places. ;-)
Ron
Can you rig up a phototransistor to convert the light pulses to voltage pulses? Then you can use a frequency counter to measure the rate. Hopefully, your counter has a Period mode, so you can get an update on every flash, and not have to wait many seconds for decent resolution. You just have to know that 7 Hz = 142.857... msec.
No counter? You can use your sound card with the built-in frequency counter in Daqarta. It does the period-to-frequency conversion auotomatically, and you can enlarge the display to fill your screen if you want. (For personal/hobby use, Daqarta is US$29 for a lifetime license, but you can try it for 30 sessions / 30 days for free. After that it won't accept input signals, but the signal generator and analysis functions continue to work forever. If you can get all your measurements done in the trial period, you are all set!)
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
Cotton string is _filled_ with electrons. Without them, it'd fly apart with a pretty impressive bang.
You wanted to know if it was going "pretty much" at 7Hz. Phil gave you a solution that's better than mine, right down to the fact that he specified the length of the pendulum (although both of us neglected to stipulate that you have to change the length of the pendulum if you change planets).
You could do this with a power supply, and a resistor and a pin diode or phototransistor and an oscilloscope -- but isn't it easier to tie a nut on a string?
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Do you still have a record player? Put a dot on the turntable and flash it with your strobe. Do some math.
John
You should really consider the pendulum idea. The time it takes a pendulum to swing through one cycle is VERY consistent. That is why most mechanical time pieces use either a gravity or spring activated pendulum. It shouldn't be too hard to calibrate to some speed that will work for what you are doing. To make it easy just make the period 7 times what the period of the strobe needs to be, an even one second. To calibrate it, simply use a stop watch to time 20 or more cycles then shorten the string to be faster and lengthen it to be slower. Once you have its cycle set to that time, then adjust your strobe light till you see 7 stationary images of your pendulum. To minimize loss in the swing have the pivot be something very smooth and use light weight string tied loosely around the pivot and use a heavy smooth object at the bottom of the string (steel ball). That way it will swing for a longer time on it's own before you need to reset it.
-- Chris W KE5GIX "Protect your digital freedom and privacy, eliminate DRM, learn more at http://www.defectivebydesign.org/what_is_drm" Ham Radio Repeater Database. http://hrrdb.com
A point I forgot to make to illustrate this point, make a pendulum and start it's swing at say 60 degrees from vertical, time 10 cycles. Then do it again starting it's swing from 20 degrees. Even though it is moving a much shorter distance in the second test, you will see that the speed is nearly exactly the same. If you do it in a vacuum, with a zero friction pivot, it will be EXACTLY the same. Even with out the vacuum the friction from the air and a good pivot will be negligible at the speed it will be swinging.
-- Chris W KE5GIX "Protect your digital freedom and privacy, eliminate DRM, learn more at http://www.defectivebydesign.org/what_is_drm" Ham Radio Repeater Database. http://hrrdb.com
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
Hmm, I don't remember that little detail from my engineering physics class. Of course that just means, either we ignored it, or I forgot it. I do remember that in our lab experiment, I could not detect a difference in cycle time regardless of the angle using a hand stop watch and timing enough cycles for it to take between 30 and 60 seconds.
I'm not sure what you mean by the apparent torque on the pendulum gets smaller with greater angles? Do you mean the torque on the pendulum pivot? If so, why would that not get greater with a greater angle? I am considering 0 degrees to be the angle with the pendulum at rest.
Wait I just read that again and I think I know what you are trying to say... Are you saying that with larger angles the actual "apparent torque" will be farther from that calculated by projecting the vertical gravity vector onto the tangent vector of the pendulum? If I am understanding you correctly, can you tell me why that is?
Given the similarity of the equation for distance over time, at a given acceleration, and the equation of a parabola, that explanation does, as my calculus teacher liked to say, give you a warm fuzzy feeling.
-- Chris W KE5GIX "Protect your digital freedom and privacy, eliminate DRM, learn more at http://www.defectivebydesign.org/what_is_drm" Ham Radio Repeater Database. http://hrrdb.com
(snip)
To have a perfectly consistent period the torque on the pendulum would have to go as k * theta, with theta being the pendulum angle and k being some constant (with a free hanging pendulum k is determined by the pendulum length and acceleration due to gravity).
But for a pendulum that's suspended at a point, the torque on the pendulum goes as k * sin(theta). Sin(theta) is close to theta for small angles, but |sin(theta)| < |theta| for all theta != 0, so there you are.
This won't start to make a difference until you have cleaned up a lot of other sources of error, so for your first clock-building project I would suggest using a simple pivot.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
Yep, I do have an old Heathkit 'scope; very basic, nothing fancy. I also have-- somewhere-- a multimeter. That has a frequency counter, but it's unreliable below 20 Hz. I had always meant to make a good frequency counter, but I never got around to it. ;-(
Ron
If the scope is well-calibrated, then all you need is some kind of photodiode or something and a battery. Actually, if you had a calibrated marker oscillator, you wouldn't need the scope to be that well-calibrated. But if you had some bacon, you could have bacon and eggs, if you had some eggs. ;-)
But why 7 Hz? Are you trying to get seizures? Or maybe spoof those emergency vehicle things that turn your light green and all the rest of them red? ;-)
Good Luck! Rich
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