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I am a completely beginner. Could anyone give some suggestion to my enquiry??

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ponyta

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If I have a telephone placing on a table.
I would like to detect whether a person have picked up the phone or not.
If he/she pick up the phone, it will call two different computer to run the program.

May I know which type of sensor I should use in detecting this case?
The sensor should not be seen by the person who pick up the phone.

Thank you very much.
 
Is this an isolated desk phone not to be connected to the phone company, or is this a regular phone making a normal call?
 
The phone company (in the US) does this by connecting a 48V battery through a relay coil to the phone. A hung-up phone is an open circuit (for DC). When the phone is off-hook, the phone has a low DC resistance (several hundred Ohms). Current flows through the series circuit from the positive pole of the 48V battery, through the relay coil, through one leg of the wire line to the residence, through the phone, and back along the other wire back to the negative pole of the 48V battery at the central office.

The relay coil "pulls-in" when it detects the loop current (>50mA). You could do the same with a DC power supply (12V to 48V), and a sensitive DC relay (one that pulls in with less than 100mA). You could reduce the loop-current by reducing the power supply voltage and then substitute an opto-isolator for the relay. The LED emitter in the opto would detect the current flow when the phone is off-hook (10mA to 20mA) , and the output of the relay or opto-isolator could switch a pin on a computer's serial or parallel port.

Do you also want to do something with the audio that comes out of the phone?

Please use a title for your posts that deals with the specific topic (you are no longer a beginner).
 
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