Normally open, normally closed or antivalent

Normally open (NO) is the most common switching variant. Similarly to a light switch, the contact closes when the sensor detects the measurement object in its detection range. The output is through-connected and referred to as a 'high signal'. Normally open switches are identified using the abbreviation 'NO'. They have the following switching symbol:

The normally closed variant interrupts the contact when a measurement object is detected within the detection range. The output is interrupted and referred to as a 'low signal'. Normally closed circuits have the great advantage in sensing technology that broken contacts are reliably detected. This has been recognized as being a decisive factor especially since the introduction of the Machinery Directive in 2012. Normally closed switches are identified using the abbreviation 'NC' (or 'R' in German-language contexts). They have the following switching symbol:

Sensors that provide both normally closed as well as normally open functionality are referred to as being antivalent. Such devices have two output conductors that can be connected simultaneously.