Hardware-only Transmit Switching
As of software version 201710081, the Portsdown transmitter can be configured so that transmission is enabled by switching the state of a GPIO Pin. This means that it can be used as a simple (single configuration) stand-alone transmitter with no touchscreen or computer required.
First of all, you must configure and test your transmitter from the Console. Make sure that all the settings are as you want them and that the desired receiver can see demodulate your signal with no problems. Then, you can configure the keyed transmit mode.
Keyed Transmit Mode
In keyed transmit mode, the Portsdown system is keyed to transmit when a GPIO pin is raised from 0v to 3.3v. An LED connected through a current limiting resistor (470 ohm?) to ground is illuminated when transmitting. By default, the key signal is input on pin 12 of the GPIO connector. This input is also known as WiringPi 1 or BCM 18. The default pin for the indication LED is pin 7 (WiringPi 7 or BCM 4). These pins are marked as 12 and 7 on the Digithin connector on the Portsdown GPIO Extender Card.
If you want to use different pins, refer to this chart https://wiki.batc.org.uk/PTT_and_band_switching#Summary_of_GPIO_connections to look up the WiringPi Numbers and then edit line 223 of rpidatv/scripts/startup.sh.
/home/pi/rpidatv/bin/keyedtx 1 7
The WiringPi number for the switch is first, followed by the WiringPi number for the indicator.
Once you have the hardware in place, you can set up your Portsdown to boot-up into Keyed Transmit Mode. From the console, go to the System Setup Menu, and select option 1, Autostart. Then use the arrow keys to highlight "Keyed_TX_boot" and then (very important) press the space bar to select it. Then press enter.
You can now test the streaming from the console by selecting "Shutdown and Reboot Options". Then select Option 2, "Reboot Now". Test that the transmission starts and stops in response to the switch and that the LED works. If you are happy with the setup, you can disconnect your PC and simply control the transmitter from the switch.