Digital Switching System
 
 This 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. 