I built this circuit to interface a conventional cordless phone to a desktop computer to use with my Skype service.
With a headset plugged into the phone, I'm hands free and not
tethered to my computer.
This circuit is only an audio interface so calls must be dialed or answered using your computer directly. The circuit connects to the mic input and line output of the computer sound card. I have used the interface quite a bit and I am satisfied with it's performance.
I used a +12 and -12 volt power pack I had on hand to power the circuit ICs and to supply 24 volts to the land line phone that is connected to the circuit. The 24 volts is used to create the line current when the connected phone is in use. (off hook)
I have a computer / speaker headphone switch pictured below to switch the computer sound card audio output to the my computer speakers or to the input of the phone interface circuit.
I used a Midcom 671-8005 coupling transformer that I removed from an old modem. I needed a transformer designed to work with a dc current applied to it and this transformer primary coil is rated 0 to 100 mA dc.
I choose resistor values that gave me an off hook line current of about 30ma with the 24 volts from my power pack using a South Western Bell model 439 phone I had on hand. The minimum off hook current for the old style phones with carbon transducers to work properly is about 23ma.
Circuit board populated and waiting for wires to be soldered in place.
Checking the etching progress of the circuit board. I warm the etchant with an aquarium heater to speed the etching process.
I did the circuit schematic and board drawings with Eagle layout software. After completing the board layout I turn off the display grid, and the visible layers I don't want on the printed version. Then I export the image as a 300dpi bit map file, open it in Paint Shop Pro and modify the board layout till I am satisfied with the result.
I found that I get the best printout quality if I don't resize the image but instead use the print scaling feature during the printing process. Scaling the printout to about 24% gets me in the ball park. I do test printouts to make sure I have the scaling correct using components to check against the printout.