Appliance timer

Appliance timer

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.

Working of the Circuit

As shown in Figure 1, 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.

Circuit Connection and description

Now let us see how the 3011 is used in the appliance timer. Figure 2 shows the schematic diagram of the timer, which is designed to switch an appliance on 23 minutes after the initial startup time. The appliance remains on for approximately 12 minutes. The 556, a dual 555 timer, is used to form the astable and monostable rnultivibrators. The time between the pulses as well as the output pulse width of the timers is limited by the size and cost of the timing components. In the circuit of Figure 1, the time period of the pulse output waveform of the astable muitivibrator is T=0.69(RA + 2RB)C = 11.5 minutes, even though a maximum time period of approximately 20 minutes is possible. The output pulse waveform of the astable multivibrator works as a clock for the SN74177 4-bit binary counters/latches.
The SN74177, a DIP, consists of four dc-­coupled master-slave flip-flops, which accept frequencies of 0 to 35 MHZ at the clock 1 input and 0 to 7.5 MHZ at the clock 2 input (see Figure 2). It is fully programmable and triggers on the negative-going edge of the clock pulse. A, B, C, and D are the data inputs and QA, QB, QC, and QD are the outputs. When the count/load (pin 1) is low, the outputs directly follow the inputs. However, when the count/load is high (logic l) and the clock inputs are inactive, the outputs remain unchanged. The clear terminal (pin 13), when taken low (logic 0), sets all outputs low regardless of the states of the clocks. In Figure 2, the SN74177 is used as a 4-bit ripple-through counter in which output QA must be externally connected to the clock 2 input. The output of the astable multivibrator is applied to the clock 1 input. In addition, the count/load is connected to +5 V and clear is taken low each time the timer is used. In fact, this arrangement enables the 74177 to perform simultaneous divisions by 2, 4, 8, and 16 at the QA, QB, QC, and QD outputs, respectively. That is, the outputs at QA, QB, QC, and QD will have timer periods of 23, 46, 92, and 184 minutes. However, only the QA output is used in Figure 2. To achieve longer time periods, output QD of the 74177 can be used to clock the next SN74l77, and so on. In addition, by using all four outputs of the 74177, three more appliances can be controlled, although at different times.

The QA output of the 74177 with a time period of 23.0 minutes is applied to the monostable multivibrator, which has a pulse width of approximately 12 minutes. Note that the wave shaping network composed of R1, C1, and D1 is used
between trigger pin 8 and the VCC pin 14. This arrangement assures proper operation of the monostable multivibrator, since the negative pulse width of the input trigger will be shorter than that of the output pulse width of the monostable multivibrator. Thus, in Figure 2, 23 minutes after the timer is initially set the appliance will come on and will remain on for approximately 12 minutes. At the end of 12 minutes, the output pulse width of the monostable goes low (logic 1 to 0), which disables the 3011 and the appliance turns off. Al the same time, the negative transition in the output pulse of the monostable multivibrator also triggers the reset control circuit and shuts the astable mullivibratur off.
The reset control circuit is composed of a single J -K negative-edge-triggered flip-flop 74H102 with preset and clear. The Q output of the flip-flop is connected to reset pin 4 of the astable multivibrator and is normally high (logic 1). However, when the output pulse of the monostable goes low, it switches the Q output low (logic 0), which in turn shorts out the reset terminal. This halts the astable operation and prevents further clocking of the 74177 counters. The end result is that the appliance goes through one cycle (off-on-off) and then stops. Initially, to start the timer, apply the power to the timer by placing switch SW1 in the on position; then depress the START switch SW2. With SW2 depressed, the J-K flip-flop is preset (Q 1), and the reset terminal of the astable multivibrator is pulled high (logic l). The switch SW2 also clears the QA, QB, QC, and QD outputs of the 74177 counters. Now the timer is on and the appliance will come on in approximately 23 minutes and will remain on for approximately 12 minutes. At the end of 12 minutes the appliance turns off and the Q output of the flip-flop switches low. This action shuts off the astable multivibrator and hence the timer. To restart the timer, simply depress the START switch SW2. When the timer is not in use, switch SW1 should be in the off position. In the circuit of Figure 2, since the maximum on state rms current supplied by the MOC3011 is 100 mA, the power rating of an appliance whose operation is to be controlled must be ≤11 W = (110 V) (100 mA). However, for appliances with higher power ratings (>11 W), a triac with appropriate electrical specifications (V and I) may be used between the MOC3011 and an appliance. For more information on using triacs with the MOC3011, refer to the MOC3011 data sheets. Finally, to verify the operation of the appliance timer, a light bulb may be used as the appliance.


Resistors (all ¼-watt, ± 5% Carbon)

R1, R5 = 10 KΩ

R2, R7 = 5.1 MΩ

R3 = 390 Ω

R4 = 4.7 KΩ

R6 = 5.1 KΩ


C1, C3, C4 = 0.01 µF

C2 = 130 µF

C5 = 10 µF

C6 = 1 µF

C7 = 100 µF


IC1 = NE556 timer

IC2 = SN74177 4-bit counters/latches

IC3 = SN4H102 single J-K flip-flop or 7476 dual J-K

IC4 = MOC3011 optically isolated triac driver

D1 = 1N914 signal diode


SW1 = SPDT switch

SW2 = Push-button switch (normally open)

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