Keypad Control for Multiple Appliances

This circuit employs DTMF technique to switch on/off up to ten appliances. It can be modified to operate up to 100 appliances using the same keypad. -- S. Ramasamyr and G.Thiagaraj Kumar

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This circuit employs DTMF technique to switch on/off up to ten appliances. It can be modified to operate up to 100 appliances using the same keypad.

The controller uses telephone-type keypad with 12 press-to-on switches. These switches are arranged in four rows (R1 through R4) and three columns (C1 through C3) using seven lines that are terminated at corresponding inputs of DTMF encoder UM91214B (IC1). IC1 generates 12 distinct dual-tone signals corresponding to the switch pressed. This signal is routed to the receiver using a wired link. (It can also be used for remote control using IR or FM.)

The circuit diagram of the wire link unit is shown in Fig. 1. No modification is required in this unit whether you want to control 10 or 100 appliances. Only the subsequent stages differ for the two models. The wired link unit can be operated off a 9V PP battery or using a suitable AC mains adaptor.

Fig. 1: DTMF transmitter with keypad
Fig. 1: DTMF transmitter with keypad

The receiver unit, as shown in Fig. 2, decodes the received DTMF signal with the help of DTMF decoder IC KT3170/MT8870 (IC1) and provides the binary output according to the switch pressed in the handheld unit. It also provides StD signal that indicates the receipt of a valid DTMF code. When the system is initially reset, all the Q outputs of CD4013 dual D flip-flops (IC3 through IC7) are cleared and all the appliances are turned off.

Whenever a particular key is pressed in the control unit, the signal transmitted via the wire link is received by the DTMF decoder and it generates the corresponding binary code in its output lines A, B, C, and D. The delayed steering trigger output (StD) also goes high, which becomes low when the key is released.

Fig. 2: Receiver including appliance switching circuit
Fig. 2: Receiver including appliance switching circuit

The binary output of the DTMF decoder is connected as the input to the 4-line-to-16-line decoder CD4514 (IC2). One of the 16 output latches [IC3(A) through IC7(B)], corresponding to the input data, goes high as latch-enable (LEN) is kept permanently high (active), while the control input EN is connected to the StD signal, which goes high as long as a switch on the keypad (Fig. 1) remains pressed and all the output lines of IC2 remain low. However, when the user releases the keypad switch in the control unit, the transmission of the DTMF signal stops and the StD signal output goes low to output the already latched data. The output line of IC2 corresponding to the released key on the keypad goes high. Thus, a low-to-high transition occurs in one of the decoder output lines corresponding to the switch pressed and released in the hand unit (remote transmitter with keypad).

The positive-going pulse triggers the corresponding D flip-flop, which is wired in toggle mode to control the desired appliance. Hence, a particular appliance (say, No. 5) can be turned on by momentarily pressing the corresponding key (5). Subsequently, it can be turned off by momentarily pressing the same key again.

Keys marked ‘*’ and ‘#’ in the keypad act as master switches for turning on and off, respectively, all the appliances. When keys ‘*’ and ‘#’ in the keypad are pressed (one at a time), they generate signals that are decoded by IC1 as B and C (hex), respectively. The corresponding output lines (marked B and C) of IC2 are connected to Set and Reset terminals, respectively, of all the toggle flip-flops, to turn on/off all the appliances simultaneously.

The flip-flop outputs can be used to control relays through ULN2001 relay drivers or similar ICs. If relays are not suitable, these outputs can be used to drive optocouplers (such as MOC3040), which, in turn, control triacs to turn on/off the power appliances. This scheme provides a total isolation between AC mains and the controller and hence it is quite safe to operate.

The circuit for controlling up to ten appliances costs around Rs 400.


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