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netduino2 and FM-radio

netduino2 FM-radio audio

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#1 torgro

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Posted 18 September 2013 - 08:58 PM

Hello members,

 

Proud owner of some new netduino2 cards for my project.

 

Project description:

I am building an IR-motion detection system. Each system consists of one netduino2, an IR-sensor and an a very cheap off the shelf FM-radio (walki-talki). I have the IR-sensor nailed I think, however I am kind of struggling with connecting the netduino to the FM-radio. The goal is to have several systems that communicate through the FM-radio using DTMF-tones. I have built my own "packet/protocol" that I will use when I send packets. WI-FI is not an option as i need range (mote than 1000m).

 

Problem:

The FM-radio have a microphone. I need the netduino to send "DTMF-tones" to the microphone. The microphone have to pins (grd and +). I guess I should remove the mic on the radio to remove background noise, however can I connect the netduino to the radio and create DTMF-tones? I have started to look in to PWM, however I cannot find any examples of anyone doing DTMF-tones. Any help would be most welcome.

 

Cheers



#2 Stanislav Husár

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Posted 19 September 2013 - 02:37 PM

Hi, I am not sure, if you could generate DTMF tones using PWM. PWM generates square wave, but DTMF tones use sine wave. Also, if you need two frequencies, you should use two PWM.



#3 OleNorway

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Posted 20 September 2013 - 07:44 AM

Not sure about the square waves either, but it might be worth a try. Two PWMs, with "isolating" diodes maybe?

 

                       GND

                           |

                           |

                    Radio Mic  

                           |

               ______|_____

              /                      

          Diode              Diode

              |                       |

              |                       |

              |                       |

         PWM1             PWM2

 

/O



#4 CW2

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Posted 20 September 2013 - 12:18 PM

I am not sure, if you could generate DTMF tones using PWM. PWM generates square wave, but DTMF tones use sine wave. Also, if you need two frequencies, you should use two PWM.

If the PWM frequency is high enough (think of audio sampling rate), it can be used to generate close enough sine wave, for more accurate output a smoothing circuit (RC filter) can be added. Although it is technically possible to combine two PWM channels to mix the frequencies, it is much easier to simply drive one PWM with the superposed wave data. In principle it is basically just replaying wave samples (usually at fixed rate); you can calculate DTMF tones by 'sampling' the superposition of 2 sine waves and sending the values to PWM channel...
  • Stanislav Husár likes this





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