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DTMF transceiver connection

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Hi there,
connecting a DTMF device to an active phone line can be tricky. For starters, you've got arond 50 volts DC on the line when it's idle (phone is 'on hook') then you've got around 80 to 110 volts AC when the phone is ringing (interrupted) and around 5 to 12 volts when the phone is picked up 'off hook' to make or answer a call.

Many DTMF decoder chips (eg: the MC 145436P by Motorola) uses only 5 volts DC to power it up. So your device must have at least one low voltage DC supply independent of the phone line for it to work safely (no house fires from bodgy circuitry!) and reliably.

You can try connecting the DTMF chip via a 600 ohm/600 ohm transformer specially designed for phone line interfacing. One side of this transformer (known as the 'primary winding') will conect via 2 capacitors (around 0.22uf at 400 volts DC each - usually mains grade power filter caps set in epoxy, often called 'greencaps' or by the more modern named 'MKT' version).

The other side of the transformer winding (known as the secondary winding) an be connected to the input of the DTMF chip via a smaller capacitor (a 0.1uF/100 volt will do) and your circuit ground connection. It is also advisable to position 2 'back to back' diodes across the secondary, to combat the possibility of high voltage ring signals from damaging your DTMF decoder chip. There are a number of similar interface circuits around the Net, so have a good look. Epanorama is a good place to begin your search.

Austin Hellier

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In a word,DTMF are those musical tones you hear when you are dialing a telephone number.Every DTMF signal is a mixture of two tones with different frequencies(one HIGH and one LOW freq.).With a DTMF transceiver you can decode( DTMF to binary) DTMF signals and encode(Generate).Thus,connecting it to a telephone line wil enable you to dial a number.More information you can find,well,in Google :)

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Well,a Transceiver can transmit DTMF tones and receive,as well.
You enter the number in a binary form and you get the DTMF tone in the output.Also,if you use it for receiving DTMF tones,you get the binary form of the number that you,or someone else send through the telephone.It has registers that control its functions and you need a MCU to control it.

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Just use it the way I've described it to you up above in the first response to your question. I can't see why the transformer coupling won't work, whether it's the MT 8870 or the MC 45436 chip, or it's a transceiver (both ways) device. You may have to identify a pin on the chip which alternates between transmit and receive, in order to enable both functions (eg: this pin may have to be taken high to +5 volts for the chip to transmit, and then be taken low to 0 volts - for it to receive - something like that.) Look on the data sheet for an 'enable' pin (sometimes called the CE or chip enable pin.)


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Further to my last reply:
You should not connect anything "directly" to the telephone line. You should use the small transformer which is common for telephone line connection. In most places, if your circuit causes a problem with the telephone company's equipment, you are liable for the repairs.


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