Detect amplifier keying line (PTT) with Arduino

Use an Arduino to detect transmit/receive state.

Using an Arduino Uno to detect the transmit state of a radio. The amplifier keying line (PTT) is grounded on transmit. The Arduino sees it as a switch being pulled low. The keying line is connected to digital input 2 (D2) and ground. The digital input uses the builtin pullup resistor. The sketch is the Arduino digital | input pullup example. Max polls the serial data from the Arduino and displays the state of the keying line.