remote control using telephone line

[summersnow]

remote control using telephone

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 connect 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

 

[banjo]

telephone remote control circuit

Looking at your schematic, I do not see the power connection to the top of the relay. Pin 1 of the relay should connect to a positive power source. The required voltage depends on the coil voltage of your relay. If you use the 10K resistors and then place a short circuit from the collector to emitter of T1 does the phone line go off-hook? If it does not, then something is wrong with the relay or its power source.

 

[summersnow]

remote control using phone

actually,i do connect a power source to the relay..and it works fine just as what i mention in the previous post..Besides the power source of the relay,do u have any idea on why the circuit cant works?basically, the TR2 is connected to the DTMF decoder...

 

[banjo]

remote control by telephone

Ok, I am a little confused. What does item #1 mean? Are you saying that the phone line is captured immediately at the first ring without the relay being energized? If this is the case, then I think you need to either reduce the value of the series capacitor on the ring detection leg or add additional resistance to only that portion of the circuit.
Off-hook detection by the phone company is performed by sensing a drop in the DC voltage across the phone line. This DC voltage drop is accomplished by the low resistance of R2. When you increase R2 to 10K, there is insufficient load across the phone line, the DC voltage does not drop low enough, and the phone company never recognizes that you went off-hook.
Please try the following:
A: Remove the series capacitor from the ring detection opto-isolator circuit.
B: Use 330 < R2 < 10K and attach both transformers and op-amp.
C: Ring the phone and then have the microcontroller turn on the relay.
If the circuit works and the op-amp can play speech back the phone line, then the problem is isolated to the opto-isolated ring detector.

Again, if I am understanding your statement #1 correctly, it would appear that there is not enough resistance in the opto loop and then charging current of the series capacitor is too large. This large charging current is fooling the phone company's off-hook detection and they are removing the ring tone because they believed you picked up the phone.

 

[summersnow]

telephone line off-hook detection opto

Hi Banjo,

Ya..Your understanding of the first statement is correct. May i know which capacitor you mean that i need to remove?The first one which is connected to the RING or the one at the output of the optoisolator?

After modifying the circuit,i found out that the circuit works when i connect both transformers and op-amp with a single resistor 1kohm but the RING which connected to the relay leave unconnected instead of connecting it to the GND.Is it possible that i leave it unconnected like that?I tried to ask around and there are some friends who point out that the RING shoudn't be connected to the GND. But,i wonder if not connecting it to the GND where should i connect ..Can you please help me in this?

 

[banjo]

control with phone line circuit

The capacitor I was referring to is C5. Based on schematics that others have posted to your other query, this value is too large. You can either decrease the size of C5 or increase the value of R2.
In my opinion, the NC, or normally closed pin of the relay should not connect to anything. It should be open. While this may look like you have a circuit net that goes no where in the on-hook case, this is NOT true. Your schematic does not show the phone connector. In the on-hook case, the circuit connected to the phone line should only be the opto circuit with C5 on the RING side and R2 on the TIP side. Everything else is disconnected via the relay.
Draw a little ac_generator symbol between the RING and TIP nets to symbolize the phone company and you will see why the NC pin of the relay should be open.

Also, remember that your schematic is a simplification of the actual circuit. The real circuit you are testing has several connections not shown on the schematic. Make sure that your construction is good and that you have followed the schematic. Transformers wired in series rather than parallel or swapping the primary and secondary windings on one of the transformers will cause lots of weird problems.