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 amplifiers, 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 first-stage 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.