offhook the telephone -- ckt.

[ronydc]

dear friends,

can some one suggest me the circuit tht can offhook the telephone connected to it when i give some signal ( say+5v.) to it.

my intention is ti make the telephone offhook by the microcontroller.

please help.

-- Rony.

 

[betwixt]

Use an opto-coupler to activate a constant current load across the line.

Whatever you do, DO NOT connect anything grounded to the phone line, both wires are above ground potential and can carry voltages in excess of 125V when the exchange (switch unit) puts ringing signal on the lines.

Brian.

 

[ronydc]

can some one .. show me the circuit .? i remember thr was a 2 transister and few resister circuit i saw in past.. which was making the phone offhook whn some 2 ends are shorted.. ( but i don hv it now..)

some one plz guide me wid the circuit...

 

[betwixt]

You could try the circuit attached.

Seize the line by energizing the relay so the contacts close.

Note there is no direct connection between the ring indicator (output of the opto), the audio or the relay coil to the phone lines. The 180Ω resistor should be rated at 1 Watt.

I missed out a diode when I drew the diagram, please add a reverse diode across the LED in the opto-coupler. A 1N914 will do.

The values are different from one country to another but I think the values I used should work in most places.

Brian.

 

[ronydc]

dear friend betwixt,

thx for the ckt.
in my project here, i hv to make a motor run via the command form the telephone line.
my project is intended to be connected in parallel to a normal telephone.
when some one will ring the number on which this project is connected, it will start counting the rings,
and at the 6th ring, it will offhook the telephone line and at that instant, the caller party will dial a say 3 digit code ( dtmf) and if the code is correct, it will run a motor.
( the code comperision and giving the signal to make the motor on will be given by the micro controller) -- and thats not the issue for me.

in my development,
i hv almost made a ring detector, and i have a circuit already designed for -- "telephone offhook detector" but the offhook detector i have has the work to make an led on when ever the telephone is offhook.while in my project i need to -- make the telephone "offhook"
and i am confused -- how to do it.
i think shorting the line with a resisrot of abt 180 ohms .. makes the tel line to produce the offhook condition .?

plz enlighten my mind..

-Awaiting rony.

 

[betwixt]

For just ring detecting you can use the components before the bridge rectifier in my schematic. Phone lines normally have about 50V DC across them when the phone is 'on hook' but when ringing, an AC voltage of up to 75V is superimposed. It is very important to understand that this is a dangerously high voltage to connect to other circuits, that's why the opto-isolator is used. The capacitor is to block the DC and the back-to-back zener diodes are to desensitize the opto so it doesn't respond to voice or tones on the line.

You can just connect 180Ω across the line to make it look 'off hook' but the best way to do it is to draw a constant current. Using just a resistor will reduce any audio on the line and the current will also depend on the line condition. If you use the transistor as a constant current load, it will draw the same current whatever the line is and the audio will not drop.

Brian.