SPL-TO is an optical tamper sensor that detects any attempt to dampen the
piezoelectric transducer of the siren by using PUR foam.
The principle of operation of the sensor is similar to that of the infrared
barrier. A transmitting element and a receiving element are installed
on the electronics board, with infrared pulsed beam being transmitted between
the elements. If the transmission is disrupted by an obstacle, which may be the
expanding assembly foam, the sensor opens contacts of the relay, thus
triggering an alarm.