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Motion Detectors are an integral component of home automation systems.  Wireless motion detectors provide the added convenience of installation in places where electrical power is unavailable.

Passive Infrared (PIR) sensors are perhaps the most commonly found detector types due to their a wide sensing area and excellent motion sensitivity.

Wireless open/close sensors provide information on the status of doors, windows, cabinets, etc.  One of the most cost-effective sensor implementations utilizes a simple reed switch & magnetic contact.  Both devices are installed in close proximity to each other on the sides of a door or window; spatial separation forces the opening/closing of the reed switch, triggering an alarm when doors or windows are opened.
Ambient light photo sensors detect the amount of light present within an environment, sending a signal proportional to the amount of light detected to control backlighting and basic on/off light control.

This specific reference design utilizes a dual-purpose IC from Silicon Labs which offers ambient light sensing and proximity sensing capabilities in a single device. This exceptionally small integrated sensor provides ease of implementation as it eliminates the design effort required for separate solutions.
Microphones convert sound in the form of acoustic pressure fluctuations into electrical signals. In home automation, microphones are used in communication systems, entertainment systems, glass breakage detectors, knock sensors, and noise canceling systems.
Wireless temperature and humidity sensing has become one of the most popular solutions in home automation.  Applications extend beyond HVAC to include the control of window shades and vent coverings.
A Water / Leak Sensor is ideal for detecting the presence of water on a floor or in a false ceiling. Leaded wires can be used to reach areas far away from the sensing circuit since you are just measuring impedance of the liquid.  Tap water tends to conduct at several 100kohm so it’s very easy to cover a wide area with a few of these sensors circuits.  With this specific reference design, it’s simple to create a water detector powered by 2 AA batteries (batteries circuit not shown).