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remote control using telephone line


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My project works like this.

Once the circuit detect a ring tone, it will off hook after 3 ringing tones(controlled by microcontroller).When the telephone is off hook, the speaker will play the message recorded in the speech IC.

The problem i face here is once i connected the the whole circuit, the circuit cant off hook.I tried to change the resistor and the connection and it still cant works.

I wonder whether the problem is caused by the resistor at the transformer so i tried to change the value of the resistor.This is the result that i get when i make the changes.

1. When both of the transformer is connected with R1= 10kohm and R2=330ohm, the circuit off hook directly once the ringtone detected
2. When both of the transformer is connected with R1=10kohm and R2=10kohm, the circuit cant off hook when the microcontrolller send logic high to the base of the transistor.
3. When the part of the TR1 and the op-amp is not connected to the circuit, with R1=10kohm and R2=10kohm, the circuit cant off hook when the microcontrolller send logic high to the base of the transistor.
4. When the part of the TR1 and the op-amp is not connected to the circuit, with R1=10kohm and R2=330, the circuit is able to off hook when the microcontrolller send logic high to the base of the transistor.

The result 4 works but what i need is i need the opamp circuit to amplify the message recorded at my speech IC..My question is how can i combine the whole circuit to makes it off hook?

Please help me in this..Thanks in advance

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You must not connect one side of a phone line to ground, the phone line must remain balanced to avoid mains hum.

To go off hook then use two relay contacts to connect a transformer winding between the phone line's tip and ring wires. The transformer winding must be able to handle the telephone line current of up to 50mA. Remove R3 since it reduces the off-hook current and attenuates the audio.

Why do you have two transformers? Use one transformer and use a "hybrid" circuit to separate transmit and receive audio.

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Hi audioguru,

Thanks for the reply. The reason i use two transformer is because TR2 will be connected to DTMF decoder and in this way, it is used to electrically isolate the phone line with the DTMF decoder. For TR1,it is used to allow an audio signal pass unmodified from the primary to the secondary while blocking DC voltage and the radio frequency interface.

If i not connecting one side of the phone line to ground,May i know how can i connect it to the relay to make the telephone off hook?Can you please suggest any option to me?

I not really understand why we need to connect two relay contacts to connect to the transformer to go off hook. Can you please explain more about this?

Thanks

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Telephone circuits use a single transformer and a "hybrid" electronic circuit to separate transmit and receive audio. Look in Google for Telephone Hybrid Circuit.

A phone line must remain balanced to avoid mains hum. Therefore two relay contacts must be used to connect and disconnect a transformer winding to the phone line. The DC current from the phone line in the transformer winding causes the line to go off hook and stop the ringing.

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  • 15 years later...
On 3/6/2007 at 3:47 PM, summersnow said:

My project works like this.

Once the circuit detect a ring tone, it will off hook after 3 ringing tones(controlled by microcontroller).When the telephone is off hook, the speaker will play the message recorded in the speech IC.

The problem i face here is once i connected the the whole circuit, the circuit cant off hook.I tried to change the resistor and the connection and it still cant works.

I wonder whether the problem is caused by the resistor at the transformer so i tried to change the value of the resistor.This is the result that i get when i make the changes.

1. When both of the transformer is connected with R1= 10kohm and R2=330ohm, the circuit off hook directly once the dzwonki mp3 detected
2. When both of the transformer is connected with R1=10kohm and R2=10kohm, the circuit cant off hook when the microcontrolller send logic high to the base of the transistor.
3. When the part of the TR1 and the op-amp is not connected to the circuit, with R1=10kohm and R2=10kohm, the circuit cant off hook when the microcontrolller send logic high to the base of the transistor.
4. When the part of the TR1 and the op-amp is not connected to the circuit, with R1=10kohm and R2=330, the circuit is able to off hook when the microcontrolller send logic high to the base of the transistor.

The result 4 works but what i need is i need the opamp circuit to amplify the message recorded at my speech IC..My question is how can i combine the whole circuit to makes it off hook?

Please help me in this..Thanks in advance

I have the same problem. How did you fix it?

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