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5
Applications of Digital Integrated Circuits
You are expected to construct ONE of the systems:
EITHER
OR
(a) a Pulse Generator
(b) a Traffic Light Controller
In each case a basic circuit is provided and the prototype “breadboard” is to be used to
assemble the chosen system. Care should be taken to keep interconnections as short as
possible. The positioning of the oscilloscope earth lead in relation to the other circuit earths
can also be critical when observing high speed switching waveforms.
Use decoupling capacitors as recommended where necessary.
A small stock of discrete components (R, C, etc) is available in the laboratory. Consult a
demonstrator if you feel that you need components not already available on the bench.
Also available are printers to record the output and prepulse waveforms for the pulse
generator or the three traffic light controller outputs. A single printer output should show the
relative timing of the waveforms. Ask a demonstrator to check your display settings before
you attempt to record the waveforms.
5.(a)
Pulse generator
5.1
Construct the simple pulse generator with the component values and digital
integrated circuit types as shown in Fig. 3. It is intended that the Schmitt oscillator generates
a basic pulse repetition frequency, the monostable multivibrator form a short duration
prepluse and the final bistable circuit shape the output into a symmetrical square wave. The
output of the Schmitt oscillator is not a symmetrical square wave although its frequency is
determined by a single RC time constant. This is because TTL devices have significant
output resistance for high level outputs and draw significant input current for low level
inputs. The circuit can function over a very wide frequency range by changing the
capacitance value but the choice of resistance value is limited to a narrow range around 330
ohms.
5.2 Make any modifications to the circuit to ensure the observed waveforms are as “clean”
as possible then record the output and prepulse waveforms. Optionally you may like to
include the Schmitt output waveform as part of the timing sequence. Some thought will be
needed to arrive at the best method for triggering the display.
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