How does the low air warning switch mechanically operate, and why does it require an electrical connection to function?

1. It is a solar-powered sensor that detects air density inside the cab
2. It is a pressure-sensitive diaphragm switch that opens or closes electrical contacts, completing a circuit to power the dash light and buzzer
3. It uses engine oil pressure to complete a mechanical warning circuit
4. It uses high-pressure air to sound a mechanical whistle directly from the tank