More and more level sensing of brake fluid, window washer fluid, and water cooling fluids are controlled by Reed Sensors.
A float, with a magnet mounted in it, is generally placed in the container. The Reed Switch is placed either inside or under the container for float detection.
In the past, automotive manufacturers used the Reed Switch in the brake fluid application in the following manner: when the container is full the float opens the Reed Switch. When the liquid level drops, the float goes down and activates the Reed Switch. A lamp is then activated on the dashboard. Nowadays, automotive manufacturers use the Reed Switch in reverse order. When the container is full, the magnetic float, actuates and closes the Reed Switch. When the level of the float drops, the Reed Switch opens.
The change in monitoring the opening instead of the closure has the advantage that a malfunction of the switch can be detected much easier. The brake fluid reservoir will be filled before the cars leave the factory. During the filling process the lamp in the dashboard switches off when the container is full. This confirms that the Reed Switch and the float are working fine.
If the on-board computer on the automobile can electrically detect a level sensor, then an advanced level sensor can be used. This sensor has more electronic components than a Reed Switch. It is made with a printed circuit board on which a resistor is mounted in series that protects the Reed Switch, and a second resistor is mounted in parallel so that the computer detects that the sensor is connected and in place.