This circuit goes between a telephone and a
headset. The project has a headset cord and plug (cut from a cheap cell phone
headset) on the input and a suitable jack on the output. The speaker signal passes
through this circuit, but the microphone signal is passed straight from the plug to the
This circuit is specifically designed for
ultra-low battery consumption. It uses a special control circuit (Q3, Q4 and !5) to
regulate the power amplifier's quiescent current. Also, effort was spent to design
an auto-on circuit (Q10 through Q14) that draws only 1.5 uA, yet has 15mV sensitivity.