Page 131 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Useful Electronic Circuits and Construction Techniques to Get You Going

124 Chapter Six

components can be exchanged by sucking up excess solder around the leads and
pulling out the component. If extensive modification needs to be done in sensi-
tive areas, such as around the inputs and feedback resistors in transimpedance
amplifiers, flux cleaning may need to be done again. If you change the layout,
change the paper drawing to match. If you need to review the design in the
future it should correspond with what you see.
The main advantage of this well-practiced approach is its speed. Small cir-
cuits can be drawn, laid out, and soldered in an hour or two. If you keep col-
lections of designs for preexisting transimpedance amplifiers, synchronous
detectors, modulators, power supplies, filters, etc., they can be easily reused
with very little new layout work. With simple circuits you may have a suffi-
ciently good mental picture of the mirror-image underside view to avoid the
transparent paper step. The only remaining task is to file away your design so
that it can be reused in the future and perhaps to label the board’s external
connections. A word processor set to print small labels in 4- or 6-point type for
gluing onto the board is about right. A digital photograph of the board can be
very useful; scan the completed board, top view and underside, on an office
flatbed color scanner. The depth of field is usually adequate.
If you need a couple of identical simple boards, the blob board method is still
the best approach. Once you need more, and the performance has already been
verified, this approach represents too much work and a conventional PCB is
probably a better approach. Once the graph paper layout has been designed and
tested, it is usually straightforward to transfer to a PCB layout. Almost all of
the layout thinking and optimization has been done already. The main differ-
ence is that the blob board doesn’t allow tracks to be placed between IC pins,
so the PCB can be a little denser.

6.4 LED Drive and Modulation
It may seem a little trivial to discuss how to drive an LED, but it is not uncom-
mon for users to treat them like a lightbulb, connect them to a battery, be
impressed at how bright they are, but disappointed in their short lifetime! The
problem is that they are diodes, and Shockley said that the current flowing
through them varies roughly exponentially with the voltage across them. Hence
although you can drive them from a low-impedance voltage source such as a
DC power supply, even small increases in supply voltage will lead to large and
possibly destructive increases in current. It is therefore much more pleasant to
use a higher-impedance source that defines the current and allows the diode
voltage take on whatever value it needs.
To use the LED you need to know the maximum forward current that it can
bear (typically 20mA for a common visible LED, perhaps 200mA for a high-
power infrared device) and the maximum reverse voltage it can handle without
breaking down and passing damaging currents (typically only a few volts). You
get these figures from the data sheet. Then we can choose the voltage source
V b and series resistor. There is a lot of leeway. A low voltage and small resistor

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.