EDNE February 2013 - (Page 45)
Photo meter assesses ambient light
Raju Baddi, Tata Institute of Fundemental Research, Pune, India
↘
Most PN-junction diodes can be
used as photodiodes. While not
optimized for this application, they do
work. When the diode is reverse
biased, it will produce a small photovoltaic output as the light level is
increased. LEDs are particularly suited
for this task because their housings are
transparent.
You can construct a simple circuit that will assess the condition of
ambient lighting and, because many
LEDs’ packages are tinted to enhance
their emitted color, may even yield a
reasonable evaluation of the detected
color. The results are not as effective
as those obtained using a high-quality
optical filter, which typically has narrow bandpass characteristics, but they
can be quite acceptable.
Though the design described here
does not produce the accuracy of
designs with laboratory-grade photodetectors and transimpedance ampli-
5V
+
(R)
(G)
(Y)
R4
1 TO 100M
BC559
R1
10k
Q2
Q1
BC549
1N4148
D1A
D1B
R2
2.7k
R3
2.7k
R5
+
−
M
1 TO 100 μA
D2A
D2B
Figure 1 If the simplest version is required, use an inexpensive volume-unit meter to
replace the 0- to 100-μA meter.
fiers, it can be quickly assembled
and will produce usable results at a
low cost.
Three LEDs are used; experimentation will indicate which device has
the best sensitivity to which color
(Figure 1). The ambient light falling on the LEDs causes some current
flow—typically in the range of 10 to
100 nA—through each LED, depending on the applied illumination level.
This current flows through the base of
a transistor, Q1, and is amplified. Q1’s
collector current then splits between
potentiometer R4, which acts as a firststage gain calibration, and the base
of Q2.
Q2 provides further amplification
and drives the left side of a bridge circuit (D1A and D1B). Note that R2/D1 and
R3/D2 form a balanced bridge. Q2’s collector current provides a slight imbalance
to the bridge. The meter, M, measures
this imbalance. R5 adjusts the sensitivity of the meter.
Set R4 and R5 such that the meter
has an appropriate deflection. R4 is useful for selecting the quiescent point; R5
is useful for adjusting the sensitivity.
Before building the circuit, check
whether the LEDs can be used as
photo sensors. To determine whether
a given LED is a good photodiode,
check the voltage across the LED using
a common digital multimeter set to
its most sensitive range—typically
200 mV. Typical output voltage should
be approximately 0.3 to 1 mV with
typical office illumination. EDN
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Table of Contents for the Digital Edition of EDNE February 2013
Cover
Digi-Key
Contents
RS Components
Microchip
Agilent Technologies
Masthead
EDN.comment
Digi-Key
Pulse
Maxim Integrated
Renesas
Signal Integrity
Mesago SMT 2013
Test & Measurement World
Rohde & Schwarz
Mesago PCIM 2013
Mechatronics in Design
Hot technologies: trends to watch in 2013
EPN
Hot 100 products of 2012
Add LED intelligence to improve light quality, efficiency, and cost
Design Ideas
EDNE
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Product roundup
Tales from the Cube
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