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Arduino Impulse to DTMF Converter

based on recommendation Atmel AVR-314
Implemented for Arduino mini @8 MHz
Works also @16 MHz, ATMEGA 328 and 168
uses Timer2, hardware PWM pin 3, needs PWM low pass filter (RC)
code and pictures can be found here:

works with an old telephone W48, FeTAp 611, Model T29