I successfully built and programmed a Sure 3216/arduino uno combo to scroll "ON A CALL" on the display, and now would like to interface it to my Plantronics HL10 handset lifter. I currently have a in-call indicator light that flashes a bunch of lights that nobody understands, and thought about tying it into this. the ICI light is connected via stereo 3.5mm headphone jack to the HL10 lifter attached to my phone and Bluetooth headset. I stripped the wires for the phono jack and measured it, hoping to see +voltage when the lifter lifted (thus powering the light). What I saw instead, was a constant +7 and +12 volts on two of the 3 wires (third being ground) at all times, even when the light was off. There was some fluctuation of about a volt on the 12v line, which makes me think that perhaps instead of the logic being in the light, it's in the lifter and it just sends pulses to the light, kind of like an SPI interface or something.
Since this seems too difficult for me to figure out how to interface/power with, my next thought went to the handset cable itself. Would it be possible to use an optoisolator or something to interface with the handset, so that when a voltage came through to the headset (plantronics Bluetooth setup), that it would trigger the arduino to turn on?
A bit of Googling decoded your question, the Sure 3216 is a display and the Plantronics HL10 is a servo motor to mechanically lift a phone handset. Instead of assuming everyone knows what these are, include links to the products, or even better, to technical data sheets and you will get more replies.
I didnt find any data sheets for the HL10 so I still dont know what youre talking about, but lets try a different approach.
Most wired phones still use 2-wire POTS (Plain Old Telephone System). The Central Office (CO) or Private Branch Exchange (PBX) supply a dc battery voltage thru a series load. The phone is normally open circuit. When the phone goes off hook aka "seizes the line" it draws current and the PBX or CO can sense this, to stop the ringing for incoming calls or listen for dialling for outgoing calls.
If you have a POTS-like line, you can insert an opto-isolator in the line to detect when the phone is off-hook for any reason.
Put LED of an opto-isolator inside a bridge rectifier, maybe some capacitor and zener to ensure theringing voltage (90 v ac) dont hurt your circuit. Put it in series with one wire of the POTS line.