Counting The Spots

[Rich Grise]

On Tue, 19 Feb 2008 23:57:01 -0800, Ron Hubbard wrote:

On Feb 19, 4:58 am, "Phil Allison" <philalli...@tpg.com.au> wrote:
"Ron L. Hubbard"

Hmmm, i was hoping for something more... electronic...

** Depends what test gear YOU have available.

   Got a scope ?

   Got anything?

.......  Phil

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. ;-(

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

 

[Tim Wescott]

On Tue, 19 Feb 2008 04:43:34 -0800, Ron Hubbard wrote:

On Feb 19, 4:12 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com
wrote:
Phil Allison wrote:
"Ron Hubbard"

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?

** 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.

A nice practical application of basic physics there Phil.        :~)

Graham

Hmmm, i was hoping for something more... electronic...

Ron

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

 

[Ron Hubbard]

On Feb 19, 4:58 am, "Phil Allison" <philalli...@tpg.com.au> wrote:

"Ron L. Hubbard"

Hmmm, i was hoping for something more... electronic...

** Depends what test gear YOU have available.

   Got a scope ?

   Got anything?

.......  Phil

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

 

[John Larkin]

On Mon, 18 Feb 2008 20:53:30 -0800 (PST), Ron Hubbard <ryon@quik.com>
wrote:

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

Do you still have a record player? Put a dot on the turntable and
flash it with your strobe. Do some math.

John

 

[Tim Wescott]

Chris W wrote:

Tim Wescott wrote:
On Tue, 19 Feb 2008 12:52:54 -0600, Chris W wrote:

1. Yes, I am picking nits.

2. No it won't. The period gets longer as the angle gets greater and
the apparent torque on the pendulum gets smaller than a linear
projection of sin(angle). The difference is very slight, so only
clock wonks pay any attention to this.

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?

(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

 

[Chris W]

Tim Wescott wrote:

On Tue, 19 Feb 2008 12:52:54 -0600, Chris W wrote:

1. Yes, I am picking nits.

2. No it won't. The period gets longer as the angle gets greater and
the apparent torque on the pendulum gets smaller than a linear
projection of sin(angle). The difference is very slight, so only
clock wonks pay any attention to this.

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?

3. Even if you're a clock wonk all is not lost. In order to make a
pendulum's period independent of the magnitude of the swing you
can modify the pivot so that the center of mass of the pendulum
travels in a parabola, not in an arc of a circle. This can be
done by suspending the pendulum from a thin strip of metal that
is constrained to wrap around a pivot with just the right
curvature (a 'cycloid', IIRC).

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

 

[Tim Wescott]

On Tue, 19 Feb 2008 12:52:54 -0600, Chris W wrote:

I wrote:
You should really consider the pendulum idea. The time it takes a
pendulum to swing through one cycle is VERY consistent.

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.

1. Yes, I am picking nits.

2. No it won't. The period gets longer as the angle gets greater and
the apparent torque on the pendulum gets smaller than a linear
projection of sin(angle). The difference is very slight, so only
clock wonks pay any attention to this.

3. Even if you're a clock wonk all is not lost. In order to make a
pendulum's period independent of the magnitude of the swing you
can modify the pivot so that the center of mass of the pendulum
travels in a parabola, not in an arc of a circle. This can be
done by suspending the pendulum from a thin strip of metal that
is constrained to wrap around a pivot with just the right
curvature (a 'cycloid', IIRC).

--
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

 

[Chris W]

I wrote:

You should really consider the pendulum idea. The time it takes a
pendulum to swing through one cycle is VERY consistent.

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

 

[Chris W]

Ron Hubbard wrote:

Hmmm, i was hoping for something more... electronic...

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

 

[Ron Hubbard]

On Feb 19, 4:25 am, "Phil Allison" <philalli...@tpg.com.au> wrote:

"Ron Hubbard"

**  No  " L "  I sincerely hope.....

http://www.ajdesigner.com/phppendulum/simple_pendulum_equation_period...

......  Phil

No... But try adding an "O" and an "i" in the right places. ;-)

Ron

 

[Bob Masta]

On Mon, 18 Feb 2008 20:53:30 -0800 (PST), Ron Hubbard <ryon@quik.com>
wrote:

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

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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

 

[Phil Allison]

"Ron L. Hubbard"

Hmmm, i was hoping for something more... electronic...



** Depends what test gear YOU have available.

Got a scope ?

Got anything?



........ Phil

 

[Ron Hubbard]

On Feb 19, 4:12 am, Eeyore <rabbitsfriendsandrelati...@hotmail.com>
wrote:

Phil Allison wrote:
"Ron Hubbard"

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?

** 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.

A nice practical application of basic physics there Phil.        :~)

Graham

Hmmm, i was hoping for something more... electronic...

Ron

 

[Phil Allison]

"Ron Hubbard"

** No " L " I sincerely hope.....


http://www.ajdesigner.com/phppendulum/simple_pendulum_equation_period.php





....... Phil

 

[Eeyore]

Phil Allison wrote:

"Ron Hubbard"

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?

** 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.

A nice practical application of basic physics there Phil. :~)

Graham

 

[Phil Allison]

"Ron Hubbard"

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?

** 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

 

[Bob Monsen]

"Ron Hubbard" <ryon@quik.com> wrote in message
news:79d5cefb-2a7a-46dd-a9d6-5236f528ab3c@n75g2000hsh.googlegroups.com...

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

Video camera filming it and a watch. View it on slow speed. Count the
flashes...

Regards,
Bob Monsen

 

[Tim Wescott]

On Mon, 18 Feb 2008 20:53:30 -0800, Ron Hubbard wrote:

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

 

[Ron Hubbard]

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