Door Sensor Secrets: What They Are, How They Work & 5 Unexpected Uses

Posted July 24th, 2013 by SimpliSafe

entry sensor on door frame

One of the many uses for a door sensor.

Entrances and exits are your home's weakest points—and to a burglar, an unsecured door or window is like a big "WELCOME" sign. That's why door sensors—also known as "entry sensors," "window sensors," or "contact sensors"— are some of the most popular home security devices. The earliest electronic home security system was entirely made up of door sensors—when someone opened a door or window, it triggered a big vibrating bell in a central part of the home. Luckily, technology has improved since then, and homeowners are faced with a wide array of choices. Here's everything you need to know about today's door sensors: how they work, how to use them, and how to design the best setup for your home.


Almost all door and window sensors use a "reed switch" to determine when a protected area has been breached. Reed switches were invented in Bell Telephone Laboratories in 1936, and are used in everything from pedal keyboards to laptops (ever wonder how your computer knows to go to sleep when you close the lid? That's a reed switch working!). A reed switch consists of a set of electrical connectors placed slightly apart. When a magnetic field is placed parallel to the electrical connectors, it pulls them together, closing the circuit.

Door sensors have one reed switch and one magnet, creating a closed circuit. If someone opens an armed door or window, the magnet is pulled away from the switch, which breaks the circuit and triggers an event. Depending on your setup and what mode your system is in, this could be a discreet text, a chime alert, or a full-blown alarm.


Because the technology involved in an entry sensor is so elegant, it can be used in a lot of creative ways—especially if you can program different responses for alarm events. Some security companies, such as SimpliSafe, allow customers to decide whether a triggered sensor means a full-on alarm response, an alarm response with a detailed event report sent to your email or phone (a Smart Alert), or a private text or email, without a public alarm event (a Secret Alert). SimpliSafe calls their door sensors "entry sensors" because they can be used in a variety of situations.

Here are some ways to take advantage of those features and get the most out of your entry sensors.

  • Doorbell:

    Your SimpliSafe entry sensor doubles as a doorbell: If your system is off, it will still sound a chime if someone triggers a sensor. Great way to know when your husband has finally brought home dinner—or if your toddler has managed to open a window in his room.

  • "Virtual Bouncer":

    Entry sensors don't just keep out the outside world — they can warn you about breaches within your home, too. Stick one on the entrance to the game room or the door to the liquor cabinet. Rig it up with a Secret Alert so that it only tells you if the seal is broken — so you can crack down on those rule-breakers.

  • Spy:

    Savvy SimpliSafe employee Ryan K. positioned his TV right where he likes it, stuck on an entry sensor, and then set up a Secret Alert. "Now I know if anyone moves it," he says. I wish I'd done something similar with my mini-fridge in college.

  • Party Fails: 

    Want guaranteed entertainment at your next house party? Put an entry sensor on a medicine cabinet in your bathroom and set up a Secret Alert. You'll get a text message every time someone peeks, and you'll finally know which of your friends can't control their curiosity.

  • Protecting Your Outdoor Stuff:

    Entry sensors are also great for protecting outdoor equipment that you can't keep inside your stronghold—an in-window air conditioner, for example. Stick one half of the sensor on your AC unit and the other on the windowsill, and anyone who tries to nab it will be held accountable. TIP: Although entry sensors are electronics and can't get wet, wrap one in saran wrap or a plastic bag and it will still work just fine.


  1. If I crack my window to let in a breeze while I'm away, will my entry sensor go off?

    For extra flexibility with door and window positioning, just install a second magnet. That way the entry sensor has two "safe" positions—say, "window closed" or "window open 6 inches"—and if it's moved from either of those it will trigger an alarm.

2. Will my entry sensor work with my unusual door/window?

Before deciding what sort of door sensor to buy, it pays to check its technical specifications to make sure it fits where you want to put it. A SimpliSafe Entry Sensor's flexible design ensures that it will work with nearly any type of door or window, including sliding doors, double doors, and swinging windows. Click here for specialized entry sensor installation tips. (For certain situations—such as doors with very deep frames—a motion sensor might provide better protection.)

3. Should I install my entry sensors in a certain direction?

SimpliSafe entry sensors work just fine upside down, sideways, diagonally—you name it. As long as the magnet is less than two inches from the sensor, it makes no difference how the sensor is positioned.

4. Is there anywhere I shouldn't install an entry sensor?

Certain metals can, over time, affect the magnets within the sensors. If you are looking to put an entry sensor on a metal object, a thin plastic buffer placed between the metal and the sensor will ensure it remains unaffected.

5. Could an intruder disable my entry sensor with his own magnet?

Although technically possible, this scenario is very unlikely. "The burglar would need an expensive, powerful magnet," says SimpliSafe Customer Service Manager Ryan K., "and they'd need to know where the entry sensor is inside of the door. If someone was going to do that, it would be someone who had already been in your home and seen where things are." Even if a burglar tried, he'd likely trip the alarm in the process, because reed switches are very sensitive. As Deves, a professional alarm system servicer and SimpliSafe customer, points out, "it is very easy to disturb the reed switches and cause an alarm when you attempt to use a secondary magnet from outside—it usually disturbs the magnetic field and causes an alarm."

Have you found creative uses for your entry sensors? Share in the comments!