Wireless communication using DTMF and RS232 protocol

[Shaheers]

Due to high price of Wirless modules I am going to design a cheap solution that is
to convert rs232 signals to dtmf tones and send it over air and in reverse process retrieve it back, i have no idea how to send digital signals over air using easy to find components, I need help me on it

uC---->RS232------->DTMF------->AM/FM over air (Tx)=========== Rx---->DTMF decoder---->uC

 

[GUMY]

I don't exactly understand how do You want to transfer DTMF signal? As audio signal or over radio transmission? First You say that Wireless modules are expensive, but then You write "AM/FM over air (Tx)".

 

[TommiRouvali]

Due to high price of Wirless modules I am going to design a cheap solution that is
to convert rs232 signals to dtmf tones and send it over air and in reverse process retrieve it back, i have no idea how to send digital signals over air using easy to find components, I need help me on it

How about PMR radio and AFSK? I have been working with them and have partial(sorry) blog post on subject. Maybe you can find some ideas from it? **broken link removed**

Please check that it is legal to transmit AFSK on PMR frequencies in your country.

 

[Shaheers]

I don't exactly understand how do You want to transfer DTMF signal? As audio signal or over radio transmission? First You say that Wireless modules are expensive, but then You write "AM/FM over air (Tx)".

Yes u r right, but the RF modules use in embedded application are costly not ordinary FM/AM Rx Tx.

How about PMR radio and AFSK? I have been working with them and have partial(sorry) blog post on subject. Maybe you can find some ideas from it? **broken link removed**

Please check that it is legal to transmit AFSK on PMR frequencies in your country.

Here is a block diagram of

 

[betwixt]

Before elaborating on how to do it, do you realize that using standard DTMF equipment you can only send at about 50 Bauds. It is VERY slow and can only send four bits per tone pair.

Brian.

 

[Shaheers]

Before elaborating on how to do it, do you realize that using standard DTMF equipment you can only send at about 50 Bauds. It is VERY slow and can only send four bits per tone pair.

Brian.

Yes i know its slow but I just realize that its very very slow,

Perhaps it will work or not, for sending and receiving commands and status to general equipment like switching and data longing sensors eg. fan speed, temp. relay
To make it fast, i just get a new idea, that is to eliminate the slow DTMF and use some other sources
here is a block diagram.


I will thankful for your expert suggestion

 

[betwixt]

That's a much better solution if you really want to use audio tones. What you propose is the same as is used for radio teleprinters and RTTY mode. However, I suggest you look at using ICs designed for dial-up modems which already use this technique and are optimized for highest speed. You should be able to manage 1200 bauds easily and maybe 2400 if you are lucky.

The 'old' way of doing this is to use a tunable oscillator (even a 555 will do) and the data is used to change it's output frequency, alternatively a frequency divider switching to divide by 2 for one of the tones. The receiver would then use a PLL (NE565 or XR2211) to see if one of the tones was in range or not. This method makes the circuit simpler and avoids the two filter stages in the receiver.

Brian.

 

[Shaheers]

I am afraid to say that my propose not exactly to use audio tones, but is to make a digital wireless link by means of any thing that can we use instead of RF modules, in simpler way I want to do it using without RF modules, My poor knowledge says that audio tones is a way, because such a technique is used in ADSL. and I found it simpler then using QAM or other digital modulations.

 

[betwixt]

You still need 'wireless' at some point to jump the gap between transmiter and receiver so whichever modulation you use will still involve some RF modules or circuits.

Regarding ADSL, forget that straight away. The speed of data, unless you move to very exotic modulation methods, decides the bandwidth you need and ADSL uses far too wide a range of frequencies to be easily transmited. It also uses a system similar to QAM and requires a two-way link to work properly.

Using plain tones is easiest but will never reach the speeds you can achieve with a direct digital wireless link. The reason is that the length of each bit of data needs to hold enough cycles of audio tone to be recognized at the decoder. Even the best designs will not be able to decode a bit based on less than one cycle of audio tone so you have to use higher audio tones to achieve faster data rates. The problem is that higher frequencies also mean you need more modulation bandwidth.

Brian.