Motion Detectors

Motion detectors come in several styles. The three most common types of motion detectors use infrared, microwave, or ultraviolet light for detection. Each of these motion detector sensors works differently. You won't find much information about ultraviolet motion detectors because they're used for special purposes.

Of the three types of motion detectors, the infrared is the most common and simplest to understand. Essentially, infrared motion detectors are passive. They don't send out a signal of any kind. The motion detector detects body heat as someone comes within range of the motion detector. Infrared sensors are the right choice when cost is a concern and you can limit the possible heat sources the motion detector encounters. For example, an infrared motion detector might not be the best choice for a patio that receives more than a little sunlight.

Microwave motion detectors rely on transmitted energy. Microwave motion detectors transmit a high-frequency signal and wait for the signal to return. It samples the signal and looks for any change in the frequency. The Doppler Effect lets the motion detector sense motion. As an object moves within the microwave field, the frequency changes slightly and the motion detector and detect the change.

The advantage of using a microwave motion detector is that you can send the signal omnidirectionally (in all directions) and this motion detector doesn't care about heat or other forms of interference. However, a microwave motion detector can't detect movement when someone maintains the same distance from the sensor because it works on the Doppler Effect.

Some vendors combine infrared and microwave motion detectors to create motion detectors with the best features of each technology. Even though these motion detectors cost most, they're well worth the extra money. The additional flexibility lets the motion detector perform additional tasks, such as recognizing that your pet isn't an intruder.

One way to protect your house and still allow airflow in the summer is to use a combination of drop pins and multiple motion detectors. You drill holes into both sides of the window at an angle. When you open the window, you insert the drop pins to make it impossible to move the window any farther.

You place the drop pins in such a way that you need a magnet to remove them. The window also has two magnets. The first is in the closed position and the second is three inches up so that the motion detector matches either the closed position or the drop-pin position.