The Model D-24A Normally Closed Wall-mount Transmitter has a normally closed input circuit that operates in conjunction with an externally mounted normally closed magnetic or mechanical contact, window foil, or security screens. When the external circuit opens for at least one-second, 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.