TOUCH_TONE_ENCODER

Published:2009/7/14 8:19:00 Author:May | From:SeekIC

Uses 555 timers to generate Touch-Tone frequencies in pairs using two of seven possible frequencies, under control of standard 12-button pad. Adjust R10 so low-group oscillator reads 941 Hz at pin 3 of U1 when * key is pressed. Frequencies of 852, 770, and 697 Hz will then be correct within 2 % when 7, 4, and : are pressed, if 1 % resistors are used and 0.047-ptF capacitors are tantalum or Mylar, Automatic push-to-talk control uses U4 connected as 1-s mono MVBR driving relay K1.-H. M. Berlin, Homebrew Touch-Tone Encoder, Ham Radio, Aug. 1977, p 41-43.



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