How to detect a valid connection in a circuit?

[motoshan]

Hi all, Once again im back with a new question :smile:
My first post in this forum was regarding the DTMF circuit, I had built that circuit and its working perfectly.
I would like to know that: is there any mechanism/technique to detect whether the input to decoder ic is connected to the phone or not?
My requirement is:
if there is no valid connection(ie phone earphone jack unplugged) detected with the input of decoder, I have to do something.

 

[chuckey]

Without your circuit its impossible. The way I would do it would be to have a transistor with a 10 K collector load, then feed the jack from the Vcc via a 1M ohm resistor and another 1 M ohm resistor to the base of the transistor. With nothing plugged into the jack, the transistor should take current and its collector would be low , if something is plugged into the jack with a DC resistance of less then 10K, the transistor would be off and its collector would be at Vcc. The reason for the high resistances is to stop them interfering with the existing circuitry.
Frank

 

[motoshan]

Without your circuit its impossible. The way I would do it would be to have a transistor with a 10 K collector load, then feed the jack from the Vcc via a 1M ohm resistor and another 1 M ohm resistor to the base of the transistor. With nothing plugged into the jack, the transistor should take current and its collector would be low , if something is plugged into the jack with a DC resistance of less then 10K, the transistor would be off and its collector would be at Vcc. The reason for the high resistances is to stop them interfering with the existing circuitry.
Frank

thanks for the reply chuckey !!!

hxxp://www.circuitsgallery.com/2012/07/dtmf-dual-tone-multi-frequency-decoder-circuit-schematic-using-m8870-ic.html

pls see the above link, which has the circuit diagram im following....

pls explain the idea with a circuit if posdible.. since im not an electronics expert

thanks in advance

 

[motoshan]

Im still waiting for a reply . . .. :-(

 

[godfreyl]

My requirement is:
if there is no valid connection(ie phone earphone jack unplugged) detected with the input of decoder, I have to do something.

The easiest option is to use a jack socket that has a built-in switch. See here.

 

[motoshan]

The easiest option is to use a jack socket that has a built-in switch. See here.

Thanks for the reply, But...
Using a 3.5 mm Jack socket wont do the job for me.
My DTMF circuit is ready , And the input to the circuit is the 3.5 mm Jack.The jack will be connected to a mobile Phone.
When the user unplugs the jack, I need to reset/turn off the DTMF decoder Ic

 

[motoshan]

Im still waiting ...... . . . .

 

[motoshan]

Im still waiting ...... . . . .

Not found any solution yet !!

 

[BradtheRad]

A telephone line has -60 V on it normally. Is that the place you want to check for a connection?

It is at a very high impedance (at the phone company). If your equipment puts a low impedance on it, then it drops to zero. It is what happens when you lift the receiver. It tells the phone company you wish to make a call.

Your monitoring circuit can check the voltage on the incoming wire (referenced to the incoming neutral wire). This voltage should drop if your equipment is drawing even a small amount of current.

 

[motoshan]

A telephone line has -60 V on it normally. Is that the place you want to check for a connection?

It is at a very high impedance (at the phone company). If your equipment puts a low impedance on it, then it drops to zero. It is what happens when you lift the receiver. It tells the phone company you wish to make a call.

Your monitoring circuit can check the voltage on the incoming wire (referenced to the incoming neutral wire). This voltage should drop if your equipment is drawing even a small amount of current.

Thanks for the reply.instead of using traditional telephone line im using a mobile phone. I want to detect whether the cable is disconnected or not from the mobile phone