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Hi,
You can connect the DTMF output to a Telephone line as shown in the diagram, it is assumed the DTMF out put is from a low impedance source, you may have to make some changes in the circuit if it is not so.
Regards, Laktronics
Hi,
What do you mean by ' in digital form'? Is it available as one pin output of a microcontroller or two pins, since each DTMF digit is a combination of two frequencies. Is it square wave or sine wave, and of what amplitude? Have you checked it using a scope? Give all details.
Regards, Laktronics
1- the ucontroller(ATmega8) generate DTMFon 1 pin ...(pwm)
2- i filter it to get the analog dtmf for using it to dial a number ...(via capacitors and resistors)
3- the amplitude is less then 0.3 volts
4- i dont want to use a transformer
5-how can i amplify this signal
Hi,
The telephone lines are problematic because the battery positive at the exchange side is EARTHED, so, a transformer isolation is generally called for. If your Micro circuit ground is not connected to safety earth, may be, you could connect the output with both leads capacitively coupled. The voltage level of 0.3V may be OK since the DTMF receiver on the exchange side will be able to handle a large dynamic range and the level will be well within its range. So you can connect your signal in the place of the DTMF generator shown in the schematic given above, provided your output impedance is not very large, depends on your filtering components.
Even with a transformer coupling, you need to take permisson from the Phone company to connect devices to the phone line (see the correction done on my earlier schematic.). Again the schematic given does not have scheme to put the line in Off-hook state and you need to use it with a parallel phone while testing, and additional circuits for the same could be added afterwards. You do not need a bulky transformer in this case as it does not carry heavy currents, a small size audio transformer used in MW radios could be tried for testing.
Regards, Laktronics
Hi,
Never mind pressing help, having already done this much why don't you connect it to a phone line using the above circuit in parallel with a telephone, lift the hanset and dial and see what happens? You should hear the DTMF tones from the hanset earphone and a ringback tone after successful dialing. I think as per telephone specs, the tones are specified 20 db below speech level of 0 db and your output should be quite acceptable. Just try once and see.
Regards, Laktronics
i connected the circuit as u said above.
i hear the tones BUT i did NOT have a successful dialing .....why???i dont know!!!!
the important tip that i forget is that my dtmf is square wave ....
i need ur help ..thanks
Hi,
The sequence you have to follow is:
1. Lift the handset, you will hear dial tone,
2. Before the dial tone stops, start dialing. The DTMF should last for at least 100 msec., then may be 100 msec gap ( I do not exactly remember now ), please check Application notes from Mitel or other DTMF IC Companies.
3. If the exchange has accepted your first digit, the dial tone should stop, and you should continue with dialling.
4. After the last digit dialled, you should hear ringback or busy tone on handset, you should keep the telephone OffHook all the time.
At which stage of the above sequence your dialing is failing?
I hope you have produced the correct pair (TWO) of tones for each digit. Square wave as such may not matter since the telehhone line will smoothen it out, but, correct frequency pair is important. Also. if you have a DTMF receiver IC like MT8870 or similar, you can make a circuit of that and connect your output to the IC and see if its digital output is correctly coming.
What respose you get from telephone line when you are dialling?
Regards, Laktronics
i did what u said above but didnt have a successful dialing .
when the ucontroller starts to dial i both hear the free tone sound and the ucontroller dialing tone !!
i think my output doesnt have a suffiecaint amplitude ???
Hi,
I still feel, your amplitude is not the problem.
1. What is the per digit duration of DTMF signal in your case?
2. What interdigit gap you leave between digits?
3. Are you sure you have used correct frequency pairs for each digit?
Now, coming to the amplitude, why is the amplitude so small? Will it not improve if you pull up the port bit using say 1K to 2.2k resistor?
Check all these and if still not ok, check using a DTMF receiver IC like MT8870 and feeding the signal directly to its input, one digit at a time.
Regards, Laktronics
well .......
the gap is 100msec and digit duaratin is 50msec
i am not sure that freq. pairs are correct cause i generate the freq. with a command in BASCOM AVR compiler
when i write dtmfout 2,50 i mean that generate the dtmf of number two with 50msec duration .
Hi,
Please make a set up with a DTMF receiver IC like MT8870 and check first if it is able to show each digit, and confirm if your program is correct. Also if you want to try a brute force technique, just increase your DTMF on to 200 millisec and interdigit gap to 200 millisec. and try directly on the phone line. If it does not work do a thorough testing of your signal with MT8870.I can assure you that tip and ring has nothing to do with your problem, they are just designations of the two leads of the phone lines.
Regards, Laktronics
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