I have successfully tested the PTT from Direwolf using the GPIO pins on the raspberry pi, connect via an optocoupler. This circuit works both the Yeasu 8900 and Baofeng UV5R and should work with other radios.
This removes the need for a VOX circuit, which removes the long tail on each tranmission. Most VOX circuits typically leave the transmitter keyed for some time after the audio ends causing the response to be missed.
The circuit uses a PC817 optocoupler which keeps the radio electrically isolated from the Raspberry Pi. The secondary circuit board is audio transformers to keep the audio electrically isolated as well. This avoids any ground loop interference between the radio and the Raspberry Pi.
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