‰  NOW 30 WPM  ‰  TEXT IS FROM OCTOBER 2018 QST  PAGE 41‰

RIGHT CHANNEL CONTROL TONE IS USED FOR FSK, YOU MUST ENSURE THAT THE FLDIGI
RTTY DIDDLE OUTPUT IS ACTIVE.  THEN, THE RIGHT CHANNEL CONTROL TONE WILL BE
ACTIVE FOR AT LEAST ONE BIT OF EACH CHARACTER, AND THE DIDDLE FEATURE
ENSURES THAT CHARACTERS ARE CONSTANTLY GENERATED WHEN IN TRANSMIT MODE.
SO, IF WE ADD A 250 MILLISECOND RE TRIGGERABLE TIMER HANG TIMER TO THE
OUTPUT OF THE CONTROL TONE DETECTOR, WE CAN USE HANG TIMER OUTPUT TO DERIVE
A PTT SIGNAL FOR FSK.  THE FSK DRIVER TRANSISTOR, Q2, CAN BE A 2N3094 FOR
LOW VOLTAGE APPLICATIONS.  IF THE FSK CIRCUIT FROM THE TRANSMITTER IS HIGH
VOLTAGE, THEN USE AN MPSA42 FOR Q2.  FOR FLEXIBILITY, THE USER CAN SELECT
VIA SW3 EITHER THE LEFT CHANNEL OR RIGHT CHANNEL AUDIO OUTPUT FROM THE
SOUND DONGLE TO KEY THE TRANSMITTER.  COMPARATORS U2A AND U2B DETECT
POSITIVE AND NEGATIVE PEAKS, RESPECTIVELY, OF THE AUDIO CONTROL TONE.
RESISTORS R33 R36 ESTABLISH BIAS VOLTAGES ON THE COMPARATOR INPUTS SUCH
THAT WHEN NO AUDIO CONTROL TONE IS PRESENT AT THE INPUT, BOTH COMPARATOR
OUTPUTS ARE PULLED HIGH VIA R37.  THE COMPARATOR OUTPUTS ARE BOTH OPEN
COLLECTOR, WHICH MEANS THAT THE OUTPUT IS THE OPEN COLLECTOR OF AN EMITTER
‰  END OF 30 WPM TEXT  ‰  QST DE W1AW  ƒ