Page 133 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

126 Chapter Six

LED reverse current or voltage by connecting a silicon diode either in series or
in reverse parallel, respectively. The load resistor should always be large enough
to avoid exceeding the current output capability of the generator.
This works fine with square-wave generators and on/off LED output. If you
want instead to sine-wave modulate the output, you will need to bias the LED
with a DC current first. One way is to use a DC supply and resistor to give about
half the maximum allowed LED forward current, and AC couple the bipolar
sine-wave generator to the LED, as shown in Fig. 6.2c. The AC generator can
then add its current up to the LED maximum and reduce it to zero. As before,
make sure the generator can drive the series resistor load without damage. The
capacitive coupling and series resistor lead to a high-pass characteristic, so C
has to be large enough not to attenuate the frequency you want to use. Choose
RC >> 1/f mod and use a nonpolarized capacitor. Using a little care in setting up
the other components, the capacitor can be dispensed with altogether. To set
up the modulation amplitude, observe the LED output on your receiver. Too
much modulation current or too little bias will lead to a clipped sine-wave
output.
If you use a lot of LEDs, in routine testing it is useful to have an active current
generator set up for about 10mA. This can be a single-transistor circuit as in
Fig. 6.2d. The red LED acts as an indicator and also provides a relatively con-
stant voltage of about 1.6V to the transistor base. The base-emitter junction
loses another 0.6V, to give 1V across the emitter resistor. One volt and 100W is
10mA, which also flows through the collector and LED. Whatever the LED and
even if the LED terminals are shorted together, the current remains near to
10mA, so the circuit is quite robust. Last, there are even simpler integrated
circuit current generators available such as the LM334 (Fig. 6.2e). This is a high-
precision current regulator that can deliver up to about 10mA. Both Figs. 6.2d
and e only need a small battery with 6V or 9V for long-life operation.
When you want to drive the LED from low power circuitry, including opamps
and CMOS logic, their current capability may be a bit marginal. In that case
use a bipolar or FET transistor used as a switch, as in Figs. 6.2f and g. Any
small npn-transistor will be fine, such as a BC548, 2N3904 or ZTX450. You can
even avoid wiring the two resistors by choosing the new “digital transistors”.
Infineon does a good range, such as the BCR108, BCR112 which can sink
100mA. Rohm’s range (e.g., DTC114EKA) is very similar. These are surface-
mount devices. Arrays of small switching transistors in a single package are
also convenient. Intersil’s CA3081E contains seven common-emitter npn-
transistors in a DIP16 package. For the FET any small n-channel enhancement-
mode MOSFET with gate threshold voltage less than about 3V is convenient.
The VN2222LL is widely available, and for higher currents Zetex does a good
range such as the ZVN2106A series. These have a channel on-resistance less
than 2W and can handle 450mA continuously. As they are operated as switches,
both drivers need a series resistor to limit LED current.
It is not always necessary to drive the LED at or below its nominal current.
The main limitations to drive come from temperature rise of the LED chip,

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