This unit also senses dusk and dawn, and sends a signal that is one higher than the one programmed. For example, if the unit is set to A1, it will send 'A2 ON' at dusk and 'A2 OFF' at dawn.

To program the unit code:

1. Press and hold the unit button (under the batter compartment lid) until the red light blinks twice, then release the button.

2. Press the button the desired number of times for the unit code you want (once for unit code 1, twice for unit code 2, etc.). The light blinks each time you press the button.

3. About 2 seconds after your last press, the light confirms your entry by blinking the number of times you pressed the button.

To program the house code:

1. Press and hold the house button (under the batter compartment lid) until the red light blinks twice, then release the button.

2. Press the button the desired number of times for the house code you want (once for house code A, twice for house code B, etc.). The light blinks each time you press the button.

3. About 2 seconds after your last press, the light confirms your entry by blinking the number of times you pressed the button.

To check the currently set code, press and release either button. The light blinks the appropriate number of times for the code set.

For testing purposes, you can press the house button to send the appropriate 'on' signal, and the unit button to send the appropriate 'off' signal.

NOTE: The MS12A only senses motion (and sends the corresponding X10 signals) at night. If you would like to modify the detector so that it also senses motion during the day, check out Tom's Home Automation web page. Tom describes how you can hack the device (which voids any warranty) to disable the dusk/dawn circuitry. I used his instructions and my MS12A works great here in my office.