RF module based Remote control

        Remote controls are  part of an electronics device used for operating the device remotely. Remote control has continuously evolved and advanced over recent years.The technologies like bluetooth connectivity, motion sensor enabled capabilities and voice control have been included to remote controls.
      
        Here we are going to build a remote control. This remote control could be used for controlling  robo cars, helicopters or for switching home appliances.Normal remote controls use IR transmission,but in this case RF  transmission is used. Transmission through RF is better than IR (infrared) because of many reasons. Firstly, signals through RF can travel through larger distances making it suitable for long range applications. Also, while IR mostly operates in line-of-sight mode, RF signals can travel even when there is an obstruction between transmitter & receiver. Next, RF transmission is more strong and reliable than IR transmission.

• Component required :

1. RF  module:                                                                                                                                         For the transmission of control signals (here DTMF signals) we need a RF transmitter and a receiver. FM transmitter and receiver pair or any kind of RF TxRx module will do. In this remote control ASK modulation based RF transmitter & receiver pair are used for transmitting signals .Get  information about ASK module : click here .                                                                  .
2. IC UM91214b( DTMF TONE GENERATOR):                                                                       This is a single single chip IC with an on-chip oscillator for a 3.58Mhz crystal.This is capable of generating Dialing Pulse(DP) and Dual Tone Multi Frequency(DTMF).But we don't need to be bothered about DP generation.All you need is just to generate DTMF signals.A keypad interface is necessary to generate different DTMF signals.The advantage of using UM91214b is its inbuilt keypad interface facility. You just have to connect the  keypad pins to UM91214's Row & Column pins.         Download UM91214 datasheet from here.

             A simple circuit of Um91214 for the purpose of generating only dtmf  is shown:

        Whenever a keypad button is pressed, a DTMF tone is generated on pin7 accordingly. To check whether DTMF tone is being generated or not, simply just connect a 8ohm speaker,press a button and hear the tone.

       

      3. KEYPAD :

          DTMF keypad is available in market. You can make one for yourself. It is easy.the Interconnections of
          a  4x4 keypad is shown here:

         

        

N.B. : UM91214 is compatible for 4x4 matrix keypad.But pressing 16th button won't generate any DTMF tone,it is for redialing,we don't need  this function,we can omit the column for ABCD.

  
      4.   A 3.58MHZ crystal :

  

      5.   DTMF decoder IC:

            Upon receiving the DTMF tone from the RF receiver we need to decode it to binary or bcd. A commonly used DTMF decoder ic is cm8870 or MT8870 or XX9370. IC XX8870 takes DTMF tone as input, decodes the tone to binary and outs it to its pin11 to pin14. Download datasheet of MT8870 from here. 

Circuit diagram for cm8870 is shown bellow:

          In this picture pin11 to pin14 (named as D0,D1,D2,D3) are  output pins,if the DTMF tone for key1 is pressed output will be 0001(pin14=0,pin13=0,pin12=0,pin11=1) 

Key Tone	Output Logic
	D3(pin14)	D2(pin13)	D1(pin12)	D0(pin11)
1	0	0	0	1
2	0	0	1	0
3	0	0	1	1
4	0	1	0	0
5	0	1	0	1
6	0	1	1	0
7	0	1	1	1
8	1	0	0	0
9	1	0	0	1
0	1	0	1	0
*	1	0	1	1
#	1	1	0	0
A	1	1	0	1
B	1	1	1	0
C	1	1	1	1
D	0	0	0	0

          

         N.B. For desired output, the input gain of CM8870 has to be adjusted. It can be done by varying the resistances connected to pin2 and pin3.

• Circuit diagram of the transmitter part of the remote control :

• Circuit diagram of  receiver part of the remote control :