How do automatic systems work?

An object disrupts or enters the proximity sensor zone beams out from a sensor tap. The infra red sensor detects the object and sends an electronic signal to the solenoid valve (usually inside a control box mounted under the sink) via its signal wire. The solenoid valve opens up to let water supply through it, and water is dispensed or fed to the user via the flexible hose connected to the tap itself. More info and visual diagram can be found here: http://www.autotaps.com/how-automatic-tap-work.html