Invention Lab — Powered by RadioShack®

This was easily a weekend project, and RadioShack had all the components. A phone line gives about 48vdc, with red being positive. Some older houses have mismatched wiring with green positive and some newer have digital signals carrying multiple lines on the same two wires. This project targets those with plain copper wire, on a single line, looking to feel like a fancy corporation with the phone gadgets. The action of ‘hold’ on a phone line allows the disconnection of all handsets, but keeps the line open and the end-user able to receive whatever we pipe in. A 330ohm 1/4 watt resistor in parallel with the phone line, acts as an active handset and allows enough current through to prevent the phone company from hanging up the call. With the line still open, we are able to inject linear audio from any source. This audio is heard by the end-user, and by us if our handset is still active.

Converting linear audio to ‘phone grade’ audio requires an isolation transformer between the audio source and phone line. A 47uf electrolytic cap gives plenty of filter buffer between the linear source and input. The transformer is non polar, though be sure the coil connections are paired as on the schematic. A 3.5mm headphone jack is our audio input source. Any form factor input connector can be used, though the most common connector for audio these days is the 3.5mm; a jack was used to simplify the design and keep all components contained. - ted

http://t.markson.us/projects/