These boards are available for purchase from National
Semiconductor. This mod changes the Rf2 resistor on the
board to a variable resistance. Here you see the mod with a
RS 100Kohm potentiometer glued to the bottom right edge in series
with a 47Kohm resistor soldered to the bottom Rf2 pad. The tiny
surface mount Rf2 50Kohm resistor has to be removed first -
done with a fine tip soldering iron and a pair of forceps. The
glue used to mount the pot on the board is just a general purpose
glue (like Duco Cement). Very tiny work all the way around.
Here is the back of the board showing the ground wire running
from the pot and wrapped around the left edge of the board.
It is soldered to the bottom edge of the lower left (ground)
solder pad (above). Output with 2 alkaline AA cells is 5.27V
to 2.90V. Perfect for all sorts of experimentation, and with
the pot, you can dial in just about any voltage in-between.
A Luxeon Star LED seems to run nicely on the entire range of
Voltage with no problem (even though 5.27V is overdriving it
a bit). I may drop the voltage down a little by adding a 2.2Kohm
resistor in series (perhaps by cutting the green wire on the
back and placing it in line there). This will change the max/min
voltages to 5.09 and 2.87 respectfully.
A few hours and a serious
"DUH" later...
Well, it seems that I persist in proving myself to be a grade
A moron... I decided to add the extra resistance to the circuit
and also decided to clean up the board a bit by placing the
resistors on the back of the board instead of having the resistors
hanging off the edge of the board in the front, but in the process
of removing the 47K resistor from the front of the the board,
the surface mount pad TORE OFF! ARRRRGGGG! Luckily the trace
from the pad went directly to a capicator that I was able to
solder a shunt to. (Yellow arrow)
Here is a closeup of the capacitor I soldered the shunt to.
The shunt is simply a clipped off resistor lead. The red arrow
is pointing at the empty space where the Rf2 pad used to sit.
Here is the back of the board with the 2 resistors in series.
The lead on the left wraps around the board to the common ground
pad. Happily the board works fine. Min and max voltages are
5.1V and 2.87V.
