Alight‐emitting diode (LED) and operational amplifier can be used as an affordable method to provide a digital output indicating detection of an intense light source such as a laser beam or high‐output LED.1 When coupled with a microcontroller, the combination can be used as a multiple photogate and timer for under $50. A similar circuit is used as a data logging system for a mirror galvanometer, providing a record of changes in the Earth's magnetic field as small as a few nanotesla (nT).
REFERENCES
1.
When the only output required is a digital “high” or “low,” this is a simpler and cheaper method than has been detailed elsewhere [c.f.
David L.
DuPuy
, “A sensitive photodiode photometer for laboratory experiments
,” Am. J. Phys.
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(Oct. 1978
),John W.
Jewett
Jr., “Get the LED out
,” Phys. Teach.
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Gordon
McIntosh
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,” Phys. Teach.
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(Nov. 2006
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From $30 to $65 at www.sparkfun.com/commerce/categories.php?c=103, depending on the number of I/O pins required.
3.
Available at www.phidgets.com/products php?category=0&product_id=1018. Because of the design of this board, a quad bilateral switch such as the 74HC4066 will also be needed for every four LEDs.
4.
We have used the FGM‐3 available at www.fatquarterssoftware.com for about $55.
5.
Approximately $10 at www.amazon.com.
6.
Shawn Carlson, “The amateur scientist,” Sci. Am. (Jan. 1999 and March 2000 columns).
7.
James
O'Connell
, “Magnetic torsion pendulum
,” Phys. Teach.
38
, 377
–378
(Sept. 2000
).8.
A. A.
Bartlett
& Robert
Stoller
, “Lecture hall galvanometers
,” Am. J. Phys.
41
, 1116
–1117
(Sept. 1973
).9.
As determined by the model at www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp.
10.
The LCD used is shown at www.sparkfun.com/commerce/product_info.php?products_id=462. A cheaper solution would be to use an array of seven‐segment displays, but this requires a more complicated design.
11.
geophysics.ou.edu/solid_earth/notes/potential/igf.htm.
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© 2011 American Association of Physics Teachers.
2011
American Association of Physics Teachers
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