‰  NOW 25 WPM  ‰  TEXT IS FROM OCTOBER 2018 QST  PAGE 41‰

GROUNDED TRANSISTOR.  IF THE OUTPUTS OF SEVERAL OPEN COLLECTOR OUTPUT
DEVICES ARE TIED TOGETHER, THEN ANY ONE OF THOSE DEVICES CAN PULL THAT PART
OF THE CIRCUIT TO GROUND.  BUT THE CIRCUIT WILL ONLY GO HIGH WITH THE AID
OF A PULL UP RESISTOR WHEN ALL THE TRANSISTORS ARE INACTIVE.  WHEN THE
CONTROL TONE AMPLITUDE INCREASES TO SOMEWHAT GREATER THAN 500 MV AC P P,
THE COMPARATORS WILL BEGIN TO DETECT THE POSITIVE AND NEGATIVE PEAKS OF THE
AUDIO SIGNAL, WITH A PULSING LOW OUTPUT.  HOWEVER, FOR CONSISTENT OPERATION
OF THE FOLLOWING FILTER AND DETECTOR, THE CONTROL SIGNAL SHOULD BE AT LEAST
1R5 V AC P P.  THE OUTPUT SIGNAL FROM THE COMPARATORS PASSES THROUGH A
FILTER FORMED BY R38 AND C12, AND THE VOLTAGE ACROSS FILTER CAPACITOR C12
IS MONITORED BY U3A, ONE ELEMENT OF A 556 TIMER CHIP, WHICH IS USED AS A
VOLTAGE DETECTOR AND LATCH.  R39 AND D1 IN PARALLEL WITH R38 IMPROVE THE
SYMMETRY OF THE ON/ OFF TIME OF THIS FILTER/DETECTOR.  WITHOUT R39 AND D1,
‰  END OF 25 WPM TEXT  ‰  QST DE W1AW  ƒ