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Opto-isolated PTT/CW interface

Serial communication: avoid the pitfalls

Opto-isolated PTT/CW interface
My opto-isolated interface allows you to transmit PTT (Push To Talk) or CW (continuous wave or Morse) command signals from a computer to a transmitter, while keeping the computer circuits isolated from the radio circuits. The electrical isolation between radio and computer makes it possible to avoid interference from the computer being conducted via the serial interface to potentially cause interference in the receiver.
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Component List
R1 = 1kOhm 0.25W
R2 = 1MOhm 0.25W (optional)
C1 = 10nF
LED = LED, red
Optoelectronics (see text)
version with phototransistor: 4N25, 4N26, 4N27, 4N32, 4N33, 4N37, CNY17-3, SL5500, TIL111
version with optoMOS : LCA110, OMA160 (CLARE)
6-pin DIL socket
S2 = pushbutton (optional)
S1 = 3-position slide switch
K1 = mono 3.5mm jack socket (preferably isolated)
K2 = 9-way sub-D socket
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