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DTMF Based Programmable FM Remote with 89C51

By Ashutosh Bhatt December 20, 2012

This is the ultimate industrial remote control with 16 different channels and a very high range of more than 1 Km. Let us see the main features of this remote control system:

  • Single chip DTMF based FM transmitter with 16 different channels.
  • Programmable, multi functional, DTMF based FM receiver because of 89C51.
  • Perfectly suits with any kind of industrial remote control application because of very long range and FM transmission.
  • Maximum 16 different functions or devices can be controlled from safe and remote place.

 

Basically DTMF encoder and decoder chips 91214B and MT8870 are used to transmit and receive 16 different codes. These codes are transmitted via FM transmitter and received by FM receiver. Received codes will be decoded and given to 89C51 and it will perform the particular task.

Here the system is installed in one particular industrial application that is laser cutting machine. The machine is used to cut small pcs of iron from one big iron sheet. There are two stepper motor one is for horizontal cutting and second is for vertical. From 16 we need only 8 (or 9) channels for this particular application.

The complete project is divided in two parts (1) transmitter part (2) receiver part

 

Transmitter part generates different DTMF codes and transmits it via FM transmitter over 100-102 MHz channel. With suitable antenna like wheep, loop or for the highest range Yagi-uda one can easily transmit signal to 500 mts. 

Receiver part does so many functions. It Receives and demodulates the FM signal, then it decodes the codes and then performs the particular task. The most advantageous thing is that it has programmable micro-controller so one can easily update/modify its functionality depending upon his requirements.

 

Transmitter Part:-

The circuit diagram tab 1 shows arrangement for 9 channel DTMF transmitter

Connections:- whole circuit is given power from 9 VDC battery. A zener diode D1 is connected across the supply terminals of IC 91214B. The crystal X1 of 3.59 MHz with two capacitors C1 & C2 is connected with crystal terminals. 9 different push button micro switches are connected in 3 by 3 raw column structure. All three raw terminals and three column terminals are connected with raw (R1, R2, R3) and column (C1, C2, C3) pins of IC. The output of IC is directly given as input to FM transmitter.

Operation:- to understand internal function of 91214B one has to refer its datasheet. Initially when one switch on the supply all the raws are on high state and column are on low state. Each raw is assigned one particular frequency from lower group and column from higher group.  Please refer the table.

 Raw / Column

 Assigned frequency in Hz

 R1

 697 (lower group)

 R2

 770 ( ” )

 R3

 852 ( ” )

 R4

 941 ( ” )

 C1

 1209 (higher group)

 C2

 1336 ( ” )

 C3

 1477 ( ” )

When we press any key the raw and column are connected and the two frequencies one from lower group and second from higher group will be combinely generated and that is known as Dual Tone Multi Frequency (DTMF) code. (if you connect a head phone at the output terminal of IC you can here a sweet tone). This code is transmitted by FM transmitter.

I have connected nine keys and only eight of them are utilized (one is kept for future use) for eight different functions. As we have already gone through the operation of laser cutting machine our four main functions are to move laser in horizontal and vertical plane in forward as well as reverse direction. So four keys are utilize for these purpose. Next four keys are used to set the displacement of laser in horizontal and vertical plane. One can increase or decrease this displacement with the resolution of 1 cm. Please refer the table.

 Switch

Function 

 SW1

 move laser forward in horizontal plane

 SW2

 move laser reverse in horizontal plane

 SW3

 Move laser forward in vertical plane

 SW4

 move laser reverse in vertical plane

 SW5

 increase the displacement by 1 cm in horizontal plane

 SW6

 decrease the displacement by 1 cm in horizontal plane

 SW7

 increase the displacement by 1 cm in vertical plane

 SW8

 decrease the displacement by 1 cm in vertical plane

Receiver Part:-

The block diagram of 9 channel DTMF based programmable receiver is given below.

Programmable FM Receiver

The major components are DTMF decoder IC 8870, 89C51 micro controller and current driver chip UNL2008A.

Connections:-  The output of FM receiver is connected with input of DTMF decoder chip 8870. The four digit output D0-D3 is connected with four pins of port P1 (P1.0 – P1.3). All six LEDs LED1 to LED6 are connected with port P0 as shown. Port P2 is utilized to drive two stepper motors through current driver IC UNL2008A.

Operation:- FM receiver will demodulate the code and give to 8870 chip. 8870 has to decode the encoded hybrid frequency code. It will generate four bit digital output depending upon which key is pressed at the transmitter side. Please refer the table.

 Raw

Column 

 D3

D2 

D1 

D0 

 R1

  C1 

 0

0 

0 

1 

 R1

 C2

 0

0

1

0

 R1

 C3

 0

0 

1 

1 

 R2

 C1

 0

1

0

0

 R2

 C2

 0

1 

0 

1 

 R2

 C3

 0

1

1

0

 R3

 C1

 0

1 

1 

1 

 R3

 C2

 1

0

0

0

 R3

 C3

 1

0 

0 

1 

   

This four digit output is directly given to 89C51. It will collect this code and start comparing it with inbuilt code. When it founds perfect match it will switch to that subroutine and perform that particular task. As already explained in transmitter part there are eight different functions there are eight different subroutines and there LED indications. Please refer the table.

 

 Subroutine

 LED

Function 

 Forward1

 LED1

 rotate first stepper motor in clockwise direction for desired  rotations 

 Reverse1

 LED1

 rotate first stepper motor in anticlockwise direction for desired rotations 

 Forward2

 LED2

 rotate second stepper motor in clockwise direction for desired rotations 

 Reverse2

 LED2

 rotate second stepper motor in anticlockwise direction for desired rotations 

 Incr1

 LED3

 increase the rotation of motor 1 by 7.2 degree

 Decr1

 LED4

 decrease the rotation of motor 1 by 7.2 degree

 incr2

 LED5

 increase the rotation of motor 2 by 7.2 degree

 decr2

 LED6

 decrease the rotation of motor 2 by 7.2 degree

 

 

 

Circuit Diagrams

DTMF-TX-1


Filed Under: Electronic Projects
Tagged With: 89c51, dtmf, fm, microcontroller, remote
 

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