Hi all
I'm an Electronics Student.

I have the same question as "vik1501" and i don't understand the schemtaic given there
ref : http://www.electro-tech-online.com/viewtopic.php?t=8534

I have a dtmf dialer and have successfully generated dtmf tones using a microprocessor. Plz suggest me a way to interface it to the Telephone line.

Can i get "that" schematic in some easily recognizable form.? :?:

Vinoth

 

You have to put a DC load on the line to get it "off hook". You will have to determine what that is, I don't remember. This schematic should do the job.

Attached Images

line_interface.jpg (15.2 KB, 1078 views)

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Hi russlk
I couldn't get proper output when i implemented the schematic. Whats wrong..? I think the phone line loads the dtmf dialer - is that possible ? or am i wrong. Any other suggestions for the circuit?
Thanks
Sraj

 

What value of RLOAD did you use? It should be just enuf to bring the line off hook.

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hi russlk,
I used 330 ohm resistor to bring the line off hook.I think this is enough.I also checked this by dialing to our telephone number.I unable to know where is problem.
I request you give me any more suggestion..........
sraj

 

330 ohms is too low. Try 39K and if that does not work, use a lower value, for instance: 10K.

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The load resistor must be under 1000ohm (Hungary).

 

I'm from India..! 8)

 

Heres one i used for the UK. It worked fine.

The "IN" is where you put your signal (audio). I used a TBA820M el-cheepo audio amp just before this input with a 1uF coupling capacitor in series with the signal to block DC hitting the coil.

You need a signal of about 700mV pk-pk maximum.


The Coil is a 600:600R line transformer.


The circuit is NOT approved for use in the UK.

Attached Images

digital_phone_book_v1.7.jpg (12.4 KB, 946 views)

 

 

Thank you Sebi for the diagram. :P
But my problem is that i'm using an RPS (5 volt) for Vcc supply to other components and ICs of my circuit and i have to use the same reference point for measuring voltages at any node of the circuit. :shock: But i cannot ( i suppose and i'm told not to ) connect any of the two conductors of the Tel. Line to the reference terminal of the RPS. :cry: So my output is not properly referenced..!
The circuit given by the above link uses the voltage in the TEL line in some form to power up the IC 91214 (i.e. does not use an external power supply). This might again create a new problem wherein different stages in the circuit being referenced to different voltages..! (i'm not sure)
One more problem is with the R-Load. I tried to use larger resistances as Russlk had suggested (like 1K or 2.2 K) but they get really heated up...as the off-hook voltage doesn't come down very much.. It stays up there at 32 volt ( on hook voltage 38 volt approx ). So i'm back to using 330 ohm resistor as S'raj has done.
So please help me in this regard also .. :roll:
Thanx a lot for showing so much interest.
R.Vinoth

 

What is the general range of amplitude trim resistor?

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The range of amplitude trim resistor i used is 2 to 10 Kohms. I found it didn't affect my output much..!! :? Wonder what for it is connected.. :?: :?: i'm ignorant..if anybody knows pls reply..

 

Hi every1
I found a way out of the Rload crisis. I have connected a 2H inductor in series with the 330 ohm R-load and the Relay, so that the DTMF output is not loaded by the Relay "part" of the circuit. And the good news it that the signal is available in a slightly distorted and attenuated form when connected to the Tel. line but the sad news is that waveform is not recognized as a valid DTMF tone .... :cry: :cry: I donno why
What 2 do.. :?:
Any ideas :idea: are welcome
Thank you all
R.Vinoth