Subject:[tt-forum] LM-6) Light meter construction and use:

Date:Sun, 09 Sep 2001 19:58:53 -0700
To:tt-Forum <tt-forum@yahoogroups.com>

If after the PS you are curious about how fast the darkness is clearing
up. Then take your light meter out side near noontime once a week or
month and plot a graph of light intensity versus Time.  This way you can
predict when you will have enough light for efficient outdoors growing. 

A useful light meter can be made really cheaply for under $20 from a
harbor freight tools using a low cost digital multi-meter (part number
30756) $6.50 to about $13 depending on sale price at the time. See:
http://www.harborfreight.com/.  And enter the part number under "find
item number:" then click "GO". A silicon solar cell can be purchased
from radio shack for about $5 and hooked directly using any 2 wires with
2 Radio Shack Banana Plugs (cat no. 274-721c) for $1.59.  

The solar cell comes with a nice little plastic box.  Solder the leads
on. One trick with this -- solder it without flux on the solder. Melt a
big drop of solder on the wire on a peace of paper to take off the
rosin, let cool, transfer to the back of the solar cell. Holding the
wire heat enough to get it to stick.  This worked best for me. Other
wise once the flux gets on the back of the solar cell it becomes hard or
nearly imposable to get solder to stick. One can add a small amount of
foam rubber to hold the solar cell to the front face.  Close the box and
use five-minute epoxy over the crack to seal it up and keep it from
opening.  

Readings are taking using the DC milliamp and microampere current
scales.  The unit can be calibrated before the PS if needed.  Compare
with a know light meter, the sun for high end, and light bulbs or a
candle for the low end. For measuring Latitude no celebration is
needed.  I plotted the following as a typical calibration curve. 
http://home1.gte.net/ob/LuxMeter-A1.gif.  Note the different curves
that resulted because of the different light spectrum for each type of
light. Bright sun has more infrared, florescent and cloudy skies have
much less.  To use this curve one would take a reading in milliamps and
follow over to the curve that is appropriate and then over to the LUX
axis to see what the light intensity is.  I used a LX-101 LUX meter as a
standard. This can be purchased at Electronix Express 1-800-972-2225 for
$65.95 and has a part No. 01LX101. 
 
Typical maximum measurements of 150,000 LUX are found on a bright sunny
day. With cloudy conditions it can get down to approximately 10,000
LUX.  For a typical clear day the sun goes down with the most rapid
light change (steepest slop) between 100 LUX and 5 LUX. See
http://home1.gte.net/ob/SUNLUX1.gif. At the intensity of where
photocells typically switch we get about 10 LUX change/min.  Thus it
becomes possible to measure the length of the day fairly precisely using
these devices.




