How Do You Convert An Outside Light To A Motion Sensor?

How often have you come home at night, fumbled for the door key, and wished you’d left the light on? Or had to negotiate an icy sidewalk in the dark? Or felt apprehensive about strange noises outdoors?

A motion detector that automatically switches on the outdoor lights is the perfect convenience to welcome you home at night, usher you up the walk (no need to fear tripping on a baseball bat!), and ease your concern about intruders entering your yard. It switches on the lights exactly when and where you need them. It’ll even spot the furry critter that raids your garbage can at night!

In this article, you’ll read about how motion detectors work, see the best places to put them, and learn how to make a safe and trouble-free installation.

How Do Motion Detectors Work?

Motion detectors are small electronic eyes that detect infrared waves, that is, the heat waves that objects radiate, even at night. When the detector sees an object move across its field of view, especially warmer objects such as people, animals, and cars (see the illustration above), it electronically flips a switch that turns on the lights. The lights remain on anywhere from 1 to 20 minutes, depending on how you preset the timer. Then it automatically shuts the lights off unless the detector continues to sense movement.

As you can see in the illustration above, a motion detector has tunnel vision. Its field of view is limited to a wedge-shaped area about 100 degrees wide and not much beyond 50 ft. So you have to aim it right at the zone you want to cover.

And since the detector senses the motion of animals as well as people, it’ll react to the movement of the family dog standing about 15 ft. away, as readily as to the approach of an adult some 45 ft. away (see illustration). In fact, a detector will sometimes react to wind-blown leaves on a bush or to a passing car.

Nuisance “trips” like these that fool the detector and cause lights to come on when you don’t want them can be irritating to both you and your neighbors. In fact, many homeowners won’t install a motion detector, believing it to be uncontrollable and too much hassle. However, you can solve most unwanted switching by adjusting the detector’s sensitivity and aiming it accurately (as you’ll see the demonstration later).

Manufacturers connect motion detectors to lights in a variety of ways, as we illustrate in Fig. B.

Position detectors and lights to cover approaches to your house, commonly used areas and dark, shadowy pans of your yard. Floodlights are the most versatile and cast the most light.

Most often, the detector is connected to a waterproof cover plate along with one or two light bulb sockets (Fig. B, No. 1). You can conveniently replace an existing outdoor light fixture with one of these, as our photo series demonstrates. Yet you can still operate the lights in the normal way; the manufacturer’s instructions tell you how to shut off the detector when you don’t want it to operate. In addition, motion detectors sense daylight and automatically shut themselves off, so they won’t switch on lights when you don’t need them.

You can buy motion detector/light units at any hardware store or home center. Prices vary, depending on the number of extra electronic features, the quality of the electronics and the number and types of light fixtures included (Fig. B). Usually, the more expensive ones with longer guarantees will better withstand the rigors of weather.

Where Are The Best Places To Put Motion Detectors?

For convenience, position motion detectors to cover the walks leading to your front and back doors and the driveway, so the lights will come on when you come home at night (Fig. B). They’ll also light up the walks and front steps for visitors. For further convenience, use them on patios, decks and any potentially hazardous locations such as around stairways and swimming pools.

If improved security is a priority, aim motion detectors to cover all the approaches to your house, including fence gates and patio doors. Also aim them into the darker areas of your yard, areas near dense trees and bushes where intruders might hide. (A friend of mine aims one into his side yard to foil mischievous kids who previously used it as a nighttime pathway.) While good night lighting doesn’t guarantee safety, it’s one of the best low-cost steps you can take to discourage unwanted intruders.

Ideally, it’s best to mount motion detectors 6 to 10 ft. above the ground, and position them so that most movement will occur across the sensitivity zone rather than directly toward the detector. Obviously, you can’t always do this if you use existing light locations. One solution is to install a motion detector that you can mount some distance away from the lights themselves (Fig. B, No. 3). The wires that connect the detector to the light are low voltage and not dangerous, so you don’t have to enclose them in metal or plastic conduit.

A Primer About Installing Motion Detectors

Motion detectors are easy to install, but each type and brand has a few different details. So read the installation instructions supplied by the manufacturer, and use these step-by-step demonstration photos and expert tips as a general guide.

In most cases, you’ll simply replace an existing fixture with the new one, as this demonstration shows. However, if you have to run a new electrical cable and install a switch, the job can get complex. Electrical cable run outdoors must be encased in approved conduit and weatherproof electrical boxes (Fig. C). If you’re unfamiliar with conduit or the rules for running new circuits, call in a licensed electrician.

Connect wire to a waterproof surface box from the wall side with plastic-sheathed cable OR from the outside with cable run inside approved conduit.

Working with old electrical boxes can be tricky too. Sometimes they won’t contain an equipment ground wire (bare copper or one with green insulation) or other grounding means such as a metal conduit. The National Electrical Code requires that all exposed metal parts of lights be grounded, so check with your electrical inspector to determine if you have to run a new cable with an equipment ground.

Turn off the electrical power at the main panel and unscrew the old fixture.

In addition, many electrical boxes, such as the shallow one in Photo 2, no longer meet the minimum size required by code.

Test the wires to make sure the current is off. Rub one lead of a voltage tester (which can be bought at a hardware store or home center) against the ground wire and rub the other lead against the hot wire (black) first, then the neutral wire (white). If the tester lights up in either case, the power is still on. Shut it off at the main panel before continuing.

How Do I Calculate Adequate Electrical Box Size?

Follow these guidelines for calculating box size:

  1. Add the number of hot and neutral wires coming into the box (two in Photo 4),
  2. one wire for all the ground wires combined (one in Photo 4),
  3. one for all the cable clamps combined (this plastic box has one), and
  4. two wires for any fixture connected to the box (no matter how many lights are on the fixture)/li>
  5. To find the minimum box size, multiply the above sum by the space required for the largest wire, 2.00 cu. in. for the 14-gauge wire in our example (2.25 cu. in. for 12-gauge wire).

This method sometimes overestimates the minimum box size, but it simplifies the calculation.

Wires entering box (neutral and hot) 2
Ground wires (combined total will count as 1) 1
Clamps 1
Detector and lights (count as 2) 2
Total 6
  • 14-gauge wire requires 2 cu. in. of space per wire.
  • 2 cu. in. /wire X 6 wires = 12 cu. in.
  • This box volume = 16 cu. in., so it is large enough.

Insert a replacement electrical box (or surface-mounted waterproof box, Fig. C) if the old box is too small. Plastic boxes have the size in cubic inches stamped on the plastic.

To install this light, it was necessary to replace the old box with a larger one.

Mount the fixture according to the manufacturer’s directions. Run the wires through the rubber gasket. Then connect the neutral wires (white), hot wires (black), and the ground wires (green or bare copper) with wire connectors.

Here are 8 more motion detector installation tips

  • The cover of an outdoor electrical box must be waterproof. Seat the rubber gasket carefully, and if you lay it against a rough surface (Photo 5), caulk it as well.
    • Screw the fixture in place, making sure the rubber gasket seals the edges of the box so moisture can't get in. Add silicone caulk if necessary.
      • See Fig. C for details on how to install a surface-mounted weatherproof box instead of the flush-mounted one we used.
      • Moisture can seep into the detector and light sockets and ruin them, so locate the fixture undercover or buy one that has bulb seals (Photo 6) and angle the lights downward.
      • Heat from a light bulb can confuse the detector. Keep the bulb and detector- as far apart as possible (Photo 6).

      Direct the motion detector at the zone you want to be covered. Screw in the light bulbs and point them to the area you want lit. Keep the bulbs as far away from the motion detector as possible. Turn the power back on.

      • Adjust your sensor to avoid nuisance trips from normal passing traffic, animals, pools of water, air conditioners, heating vents, and trees and shrubs (Photo 7).

      Test and fine-tune the motion detector using the sensitivity and timer controls as described in the manufacturer’s instructions.

      Cover a portion of the lens with plastic tape to narrow the field of view if necessary.

      • Apply for an electrical permit from the local department of inspections, and check for special local rules. Have your worked checked by an inspector when you're finished.
      • Caution: Do not let your ladder or body touch overhead power lines. They could deliver a lethal shock.