A tamper contact indicates if the device cover is open or closed and is often used in burglary and robbery alarms and restricted access systems. The tamper contact is marked as TMP tamper in the diagrams, instructions and directly on the devices.
Fig. 1. Tamper contact in the window opening sensor JA-60NB by Jablotron
The tamper contacts are available in many versions. As an example, in INT-TSG-BSB keyboard, the tamper contact is an optical IR sensor, whereas in KRM-511 combination lock by ATLO it is a photoresistor detecting changes in light and activating alarm. The most common solution used by the manufacturers is a monostable switch operating as a mechanical contact. It is the simplest and most cost-effective solution.
NC (Normally Closed) is a type of contact closed in normal operation. In tamper contacts, it means that the voltage flows through the TMP contacts until the case is opened (tamper).
Fig. 2. NC contact
NO Normally Open is a type of contact open in normal operation without any voltage flowing through TMP contacts until the case is opened (tamper).