The Model D-21A Normally Open Wall-mount Transmitter has a normally open input circuit that operates in conjunction with an externally mounted normally open magnetic or mechanical contact. When the external contact closes, the transmitter automatically transmits a two-second digitally coded signal to a companion standard digital receiver. Pressing the test pushbutton on the front of the transmitter will also cause the transmitter to send a signal. Each transmission causes the red LED indicator on the front of the transmitter to light, showing that the transmitter is sending a signal and that the battery is in good condition.