dlkosmas Posted November 13, 2003 Report Share Posted November 13, 2003 Does anyone know how to connect a dtmf transceiver to the telephone line? Quote Link to comment Share on other sites More sharing options...
Alpha Posted November 14, 2003 Report Share Posted November 14, 2003 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 HellierDownunder Quote Link to comment Share on other sites More sharing options...
dlkosmas Posted November 14, 2003 Author Report Share Posted November 14, 2003 The connection you mention worked for a dtmf Decoder(Receiver) but not for a Tansceiver.Dont know the reason yet,but as far as i have searched i saw that you need to connect it to the phone line through a DAA chip which is FCC aproved( Data Access Arangement).Any ideas? Quote Link to comment Share on other sites More sharing options...
Staigen Posted November 15, 2003 Report Share Posted November 15, 2003 What is a DTMF tranciever used to? Just curious!//Staigen Quote Link to comment Share on other sites More sharing options...
dlkosmas Posted November 16, 2003 Author Report Share Posted November 16, 2003 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 :) Quote Link to comment Share on other sites More sharing options...
Staigen Posted November 17, 2003 Report Share Posted November 17, 2003 I know what DTMF is, i have known it for about 30 years now, what i was curious about was what a TRANCIEVER is used to//Staigen Quote Link to comment Share on other sites More sharing options...
dlkosmas Posted November 17, 2003 Author Report Share Posted November 17, 2003 It is a Transmiter-Receiver and it is Transceiver, not transciever. Quote Link to comment Share on other sites More sharing options...
Staigen Posted November 18, 2003 Report Share Posted November 18, 2003 Ok, i am not so good at the english language, but you did not answer the question! And its transmitter, not transmiter//Staigen Quote Link to comment Share on other sites More sharing options...
dlkosmas Posted November 18, 2003 Author Report Share Posted November 18, 2003 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. Quote Link to comment Share on other sites More sharing options...
Staigen Posted November 19, 2003 Report Share Posted November 19, 2003 Ok, BTW, you can connect the transformer directly to the phone line via a relay or something, thats the way modems are connected.//Staigen Quote Link to comment Share on other sites More sharing options...
Alpha Posted November 23, 2003 Report Share Posted November 23, 2003 Have a look on the Net for the MT 8870A DTMF Transceiver by Mitel Corp. Look for a data and applications info sheet, this whould help yo a lot.Alpha Quote Link to comment Share on other sites More sharing options...
dlkosmas Posted November 24, 2003 Author Report Share Posted November 24, 2003 That's the one i have.unfortunately the datasheets and application notes dont provide information for connecting it to the telephone line. Quote Link to comment Share on other sites More sharing options...
Alpha Posted November 28, 2003 Report Share Posted November 28, 2003 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.)Alpha Quote Link to comment Share on other sites More sharing options...
rob Posted December 28, 2003 Report Share Posted December 28, 2003 wow all this DTMF stuff sounds cool. DAM IT dont side track me from another project !+"I before E eccept after C" hehe ;) Quote Link to comment Share on other sites More sharing options...
Tsilipe Posted March 12, 2004 Report Share Posted March 12, 2004 I have a question how can i create DTMF tones with a MCU like 90S4433 or any other of the Atmel family, and how can i receive with a MCU with out any other component directly from the MCU with only a typical parts for the corect working of the device. Quote Link to comment Share on other sites More sharing options...
MP Posted March 12, 2004 Report Share Posted March 12, 2004 Bascom AVR compiler has the dtmf output built in and also the A to D in your other post. The compiler is free to code up to 2K of the memory. You can download it from www.mcselec.comMP Quote Link to comment Share on other sites More sharing options...
h_urain Posted March 28, 2004 Report Share Posted March 28, 2004 Does anyone use CM8880 to dial through phone line ? Can you help me ???? If you can, please post your circuit at least the part you connect CM8880 to telephone line ? thanks Quote Link to comment Share on other sites More sharing options...
MP Posted March 28, 2004 Report Share Posted March 28, 2004 It is my understanding that this device is used to decode DTMF you "receive" so that it is usable. "Sending" DTMF is just a matter of sending tones. You would do this with a tone generator type of circuit.MP Quote Link to comment Share on other sites More sharing options...
h_urain Posted March 28, 2004 Report Share Posted March 28, 2004 I don't think so. As fact, CM8880 is a DTMF transceiver, not like 8870 just decode DTMF. I don't know whether I should connect CM8880 to telephone line directly Quote Link to comment Share on other sites More sharing options...
MP Posted March 28, 2004 Report Share Posted March 28, 2004 You are correct. But there is not a different schematic for each operation. I have attached the data sheet. The information is there for both types of operation.MPCM8880.pdf Quote Link to comment Share on other sites More sharing options...
MP Posted March 28, 2004 Report Share Posted March 28, 2004 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. MP Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.