The simplest aftermarket alarms are one-piece units with a siren and control module. The most common type of sensor is a shock sensor and two wires (12-volt constant power and ground) which are connected to the car's battery. This type of alarm is triggered by vibration transferred to the shock sensor, or by voltage changes on the input (the alarm assumes that a sudden change in voltage is due to a door or trunk being opened, or the ignition being turned on); however, it is very prone to false triggers on late-model vehicles with many electronic control modules, which can draw current with the ignition off.