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■ FIGURE 1. The schematic of our practical
LED-based photometer. Because its output is
voltage, a microcontroller can use it to make
measurements of light intensity.
There is a simple circuit that can
do this: the transimpedance amplifier.
After a little research (thank
goodness for the Internet) and
emailing Mr. Mims, I found the
inexpensive transimpedance amplifier
circuit for the LED photometer.
At the heart of the
transimpedance amplifier is the
TLC272 operational amplifier, or
op-amp, which is inexpensive and
available from companies like
Jameco, Mouser, and Digi-Key.
The transimpedance amplifier
makes use of inverting feedback to
convert an input current into an
output voltage proportional to the
value of the feedback resistor. Since
the input is a current and the output
is a voltage, the transimpedance
amplifier is said to have a gain
measured in ohms.
So, for example, a
transimpedance amplifier with a gain
of one kilo-ohm converts a 1.0 mA
input current into an output voltage
of 1.0 volts.
The current output of the typical
LED is very small; therefore, the
transimpedance amplifier requires a
large value feedback resistor to
create a practical output voltage.
I have found that a 100 kilo-ohm
resistor is needed for IR LEDs and a
one mega-ohm resistor is needed for
visible color LEDs.
Create the single-sided printed
circuit board (PCB) for the circuit
from the pattern that is available at
the article link. If you are unable to
produce PCBs at home, then
assemble the photometer on perf
board using the schematic provided
here. If you can create the PCB, then
you can follow Figure 2 for the
proper placement of the
Start the construction by first
selecting an LED for the photometer.
Be sure to make note of its peak
frequency which is normally given
■ FIGURE 2. This shows the placement of components
for this article's PCB mask.
May 2013 63