Advanced appliance timer

  
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This project introduces important type of transducer, an optical coupler. An optical coupler together with a square wave generator and counters can be used to construct a timer for appliances. That is, an appliance can be turned on at a desired time for a specific time interval, Figure 1 shows a block diagram of such a timer. As shown in Figure 1,
Advanced appliance timer - schematic

the output of the square wave generator is used as a clock (trigger signal) for the divide-by-N counters. A divide-by-N counter is a digital IC that produces a single output pulse for every N input pulses, where N is an integer. The integer N is commonly called the modulus of the counter. There are basically two types of divide-by- ­N counters: fixed and programmable. In Figure 1, the output of the divide-by-N Counters is then applied to a one-shot (monostable) multivibrator, the output of which in turn drives the optical coupler. The output pulse width of the controls the time an optical coupler is on. The output of the monostable multivibrator is also applied to the reset control circuit, which halts the square ­wave generator and also resets the circuit for restart operation. The resistor-capacitor combination used for the monostable determines its output pulse width. When the output pulse goes high (logic 0 to 1), the optical coupler is triggered and the appliance turns on. On the other hand, when the output pulse width of the monostable goes low (logic l to 0), the optical Coupler is disabled and the appliance turns off. At the same time, the negative transition of the pulse also triggers the reset control circuit, which in turn shuts the square wave generator. Thus the circuit is ready for the next. Now let us see how the 3011 is used in the appliance timer. Figure 2 shows the schematic diagram of the timer, which is...



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