and D2 are necessary to prevent IC6D to trigger IC6B.
The Output Stages
IC7 and IC8 form the output stage of the ACU. The output
stage is made up of two LM311 Integrated circuits. Although
they appear on the schematic as operational amplifiers, they
are, in fact, differential comparators. The output of a comparator is an open collector or open emitter stage. This allows
us to combine two or more outputs into a single output.
The LM311 has another interesting feature. Normally,
pins 5 and 6 are part of the balancing circuit. This can be
used to offset small differences in supply lines or input
circuits. In our case, by tying pins 5 and 6 together, we can
switch both input and output of the LM311 on or off. To
effect this switching, we connect a 2N3904 transistor from
pins 5 and 6 to ground. For example, pin 13 of IC5B is high
during the first five pulses of our signal. Transistor T1 is
fully turned on and IC8 is therefore turned off. At the same
time, pin 12 of IC5B is low, transistor T2 is not conducting
and IC7 is operating as a normal amplifier. Build IC7 and all
its components first and check that the output only shows
the first five pulses of the original signal. Build IC8 and its
components; its output will be the two pulses from the
regeneration stage. You can test this stage first by
connecting a 1K ohm resistor from pin 7 to V+. If you are
satisfied, remove this temporary resistor and connect pin7
of IC7 to pin 7 of IC8 to combine the signals.
When building the output stage, note the subtle difference between the two LM311s. The input to IC7 is a string
of negative pulses, but the input to IC8 is a string of two
positive pulses. IC8 therefore must invert these pulses. NV
ACKNOWLEDGEMENT
It has been said that no man is an island. As a corollary, no
project is without external sources and stimulants. To that end,
my thanks go to Krishna Blake of the West Island Aeronautics
Club for suggesting the project and lending assistance whenever necessary. To the manufacturers of electronic components
whose websites are a wealth of useful information and to all the
individuals who find the time to fill their website with whatever
I seem to require at any particular moment.
6ISIT /UR 7EBSITE !T
HTTP WWW ELEXP COM
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36
January 2006
