This is an amplified telephone line tap, which is designed to be minimally invasive. The interconnect to the phone line has a large input impedance, and is electrically isolated from the main electronics of the circuit. I designed this for use with a DTMF controlled relay driver, which did not come with an interface for direct connection to the phone line. When properly set up, the output audio from the tap is very clear, and the tap causes no disturbance to the phone line, and is essentially undetectable.

The LM386 audio amplifier is set up for a voltage gain of 200, and the output is capable of driving an 8 ohm speaker. The transformer used is from an analog modem. Depending on the application, it may be necessary to connect the circuit ground to the electrical ground of the building to reduce noise pickup (this is not necessary with batteries, but some DC power supplies will cause problems).