DTMF Based Programmable FM Remote with 89C51

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.

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