A raspberryPi running from an external 12v source requires a 5v converter. We chose a converter which included an enable pin to shut off the output because we wanted a way to send a hard reset from a rPi GPIO pin (simulated by the logic train) to toggle the "ENable" pin on the voltage converter (simulated by the LED) to toggle the converter output off. When the "EN" pin is set to a LOW state, the converter disables the 5v output, causing the rPi to loose power forcing a hard reset. But a 5 volt HIGH signal is needed from the 12 volt source to turn the converter on and so the 12v source is used to charge a capacitor and a 5v zener is used to clamp the voltage across the capacitor to 5 volts thus the circuit is converter is enabled and the rPi will boot up if the 12v source is still good. A hard reset is triggered when the dedicated rPi GPIO pin is set to logic state HIGH. The 5v source will have a small ~7 mS delay on the restart caused by the RC delay.
