DTMF Based Remote Control System

Different gadgets have been introduced to meet our requirements ranging from basic needs to classy needs of human. With this development in the recent years, day by day people have grown more sophisticated. Putting human comfort as a main concern, hereby we have presented a circuit to remotely control a number of electrical or electronic gadgets connected to it. The unique feature of the circuit DTMF based remote control system in comparison to other remote control system like IR remote control is that this circuit employs FM transmission and reception, for this reason it can be used for comparatively longer range of control system. Using DTMF based remote control circuit we can control (switch on/off) any type of device by keying the number allocated to it. Since the keyboard used with the transmitter is similar to those used in basic version of DTMF telephones, the circuit implementation becomes easier. The entire DTMF based remote control system is composed of two subsystems which are described briefly in paragraphs given below.

Circuit Description of DTMF Based Remote Control System

Code generator and transmitter

This part of DTMF based remote control system produces the code for control and transmits to the respective gadget requesting operation as deigned to. FIG.1 gives the clear view of the code generator part. It is a standard DTMF generator built around IC1 (UM91215B). IC1 generates the DTMF signal in correspondence to the number entered from the keyboard. The generated signal is then fed to an FM transmitter. Large numbers of FM transmitter circuits are available in the online, we can make use of any of them. Therefore for simplicity of the entire circuit, the part of transmitter is not included here. One must not forget that the operating range of the system depends on the range of the transmitter used.

circuit diagram of dtmf coder

Receiver and decoder

Fig. 3 gives the schematic diagram of receiver and decoder for DTMF based remote control system which is here refer as master circuit. Here again, a common FM receiver kit replaces the receiver portion which is easily available in the market. The output of the FM receiver is a carbon copy of the DTMF signal keyed in at the remote transmitter end. This output of FM receiver is then fed to DTMF decoder IC2 (CM8870P) which gives binary number as an output corresponding to the signal received from the transmitter. Thus obtained -bit binary output is fed to IC3 (CD4067), which is a -line to 16-line decoder IC. One of the outputs of IC3 will go high, depending on the binary input fed in. IC3 produces a predetermined sequential output which activates IC4 (CD4067) via two flip-flops IC5 (a) and IC5 (b) connected to its output terminal. Two extra outputs of IC3 are used for activating/deactivating IC4. In similar way, the outputs from IC4 are used to switch the state of toggle flip-flops, which in turn control the devices using suitable interface (relay driver opto-coupler etc.) circuits.

dtmf based remote control system

In Fig.4 and fig.5, we can observe two toggle flip-flops wiring using IC6 (CD4013). IC6 circuit should be repeated for other pairs of outputs of IC4. The circuit we have presented here can control up to 10 devices using outputs of IC4.

Device controlling circuit2Device controlling circuit1

Operation of the Circuit DTMF based Remote Control System

The entire circuit of DTMF based remote control system bears the ability to control 100 different gadgets (switch on and switch off). To understand the control mechanism, we can assume all the 100 channels are grouped in 10×10 matrix format. That means, there are 10 locations, with each location having 10 outputs to control up to 10 gadgets connected to it.

For clear understanding, lets go with an example, if we want to switch on gadget numbered 5 at location 3. For this operation the particular numbers to be entered in the sequence are; 3 # 5 *, in that order. The buttons marked # and *, generally available on all the keyboards, are used to activate and then deactivate a particular location.

As we receive the binary code for decimal number 3, pin 6 of IC3 of decoder circuit goes high, making the set input (pin 8) of IC5(a) high. This cause pin 13 of this flip-flop to go high, thereby making pin 5 (data input) of IC 5 (b) also high. Then on receiving the next digit #(corresponding to decimal digit 12), the D flip-flop configured around IC5 (b) is clocked with the help of pin 19 of IC3 and in turn IC5 (b) is activated and its output at pin 2 enables IC4 by taking its pin 15 active low.

The next keyed in digit 5 will make the corresponding output of IC4 (pin 4) to go high. This positive going pulse can then be used to trigger IC6 (CD4013) configured as toggle flip-flop. In the circuit, the gadget to be controlled is connected to the flip-flop via relay driver, opto-coupler etc, as required. Receipt of the next input * (corresponding to decimal digit 11) will then make pin 20 of IC3 high to reset IC4 and hence disable IC4. However, this will not change the state of the gadget under control. Thus whenever * button on the keypad is pressed (transmitted), all the locations get reset/deactivated but the existing latched state of the devices (“on or off”) at all the locations remains unaffected.

To switch on gadgets at other locations the numbers on the keypad should be pressed (transmitted) as per the sequence mentioned earlier. Now to switch off the gadget, the above switching process, i.e. transmit 3 # 5 * for switching off (in fact, toggling the existing state) device/gadget 5 at location 3, must be repeated. The sequence should be strictly followed, otherwise false switching may occur and control mechanism fails. The schematic diagram in Fig. 3 shows the wiring for location 3.

The circuit in Fig. 3 needs be repeated for each location with only a slight change pertaining to the output pin of IC3, which is to be connected to ‘set’ pin 8 of IC5 (a). For locations 1 through 10, IC3 pins 8, 7, 6, 5, 4, 3, 23, 22 and 21 respectively should be connected to pin 8 of IC5 (a). For two units located in close proximity to each other, a common FM receiver may be used.

One must remember that the binary equivalent code for “0” in the keypad equals decimal “10”. To avoid confusion, we can assume “0” on the keypad as “1”. For the same reason, (pin 9) output of IC3 and IC4 should never be used; instead pin 21 representing location/device 10 may be used.

Construction and Testing

The circuit can be built on a vero-board. However, an actual-size, single-sided PCB comprising circuits in Figs 1 and 3, except the FM transmitter and receiver modules, is shown in Fig.6. The component layout is shown in Fig.7. Circuit of IC6 has been repeated on the PCB to enable control of up to 4 devices.

long rang remote pcb

Fig: 6 Solder Side PCB Design

long rang remote solder

Fig: 7 Components Side PCB design

The DTMF generator and FM transmitter should be housed in metal box with the keyboard suitably placed. Be careful to watch the supply voltage of FM receiver module if it shares a common supply with the system. After finishing the construction, switch on the transmitter and connect a small speaker or amplifier to the output of FM receiver. Then press any key on the keyboard and at the same time tune the FM receiver until a clear tone is heard and the signal received LED starts glowing. Always tune the FM receiver where there is no signal from any radio centre. If the output of the FM receiver is too low, a low-power, compact amplifier may have to be used between receiver and decoder. The system is now ready for use.

 PARTS LIST OF DTMF BASED REMOTE CONTROL SYSTEM

Resistors (all ¼ W, ± 5% carbon, unless stated otherwise):

R1 = 150 Ω

R2, R6 = 1 KΩ

R3, R7 – R11 = 10 KΩ

R4 = 120 KΩ

R5 = 220 KΩ

Capacitors
C1 = 100μF, 25V electrolytic

C2, C3 = 0.1μF, ceramic disc

C4 = 1000μF, 25V electrolytic

Semiconductors
IC1 = UM91215B DTMF dailer

IC2 = CM8870P DTMF decoder

IC3, IC4 = CD4067 Demultiplexer

IC5, IC6 = CD4013 Dual D Flip-Flap

ZD1 – 3.3V, zener diode

LED1 – 5mm Red LED

Miscellaneous
SW1 – On/Off switch

– Keyboard

Xtal-1, Xtal-2 = 3.579542 MHz crystal

– +9V DC power supply or battery

– TRANSMITTER Antenna

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