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Digital
Switching System
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circuit can control any one out of 16 devices with the
help of two push-to-on switches. An up/down counter acts
as a master-controller for the system. A visual indication
in the form of LEDs is also available. IC1 (74LS193) is
a presettable up/down counter. IC2 and IC3 (74LS154) (1
of 16 decoder/demultiplexer) perform different functions,
i.e. IC2 is used to indicate the channel number while
IC3 switches on the selected channel. Before using the
circuit, press switch S1 to reset the circuit. Now the
circuit is ready to receive the input clock. By pressing
switch S2 once, the counter counts up by one count. Thus,
each pressing of switch S2 enables the counter to count
up by one count. Likewise, by pressing switch S3 the counter
counts downwards. The counter provides BCD output. This
BCD output is used as address input for IC2 and IC3 to
switch one (desired channel) out of sixteen channels by
turning on the appropriate triac and the corresponding
LED to indicate the selected channel. The outputs of IC3
are passed through inverter gates (IC4 through IC6) because
IC3 provides negative going pulses while for driving the
triacs we need positive-going pulses. The high output
of inverter gates turn on the npn transistors to drive
the triacs. Diodes connected in series with triac gates
serve to provide unidirectional current for the gate-drive.
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