Build a COS Detector
By Kevin Custer  W3KKC

You may need a way to detect the presence of a carrier on a receiver if your radio does not have a cos output.  There are several ways to accomplish this.  You could build a COS detector like the one below or you can fiddle around in the radio to see if you can find a way to detect the presence of a carrier.  Since many of you don't want to open an expensive rig and prod around to find a cos,  this circuit may help.

This simple circuit measures the noise on the discriminator audio, when the noise level falls the circuit enables the COS output.  C1 along with the 47K resistor make up the high pass circuit. These values may need changed depending on how the circuit is driven. This circuit was intended to be used on a radio that has a discriminator or raw audio output like the one sometimes available for a packet connection.  Possibly a larger C1 will need to be used if the circuit is driven from the speaker connection.  Avoid making C1 too large or voice talk off will occur.  If using this circuit on a speaker connection, be aware that you should terminate the speaker leads of the radio with either a speaker or a dummy load of 8 ohms.  A dummy speaker load will allow quieter operation as the squelch on the radio needs turned all the way open so audio is present at the speaker at all times or the circuit will not work correctly.  Avoid setting the audio level too high on the radio.  Positive logic is produced, meaning a voltage is present on the COS output when receiving a signal below (quieter) than threshold of squelch control R1.  VCC is nominally 12 volts DC.