The photo cell is meant to keep the light from activating in daylight hours. Having a switch on the circuit enables you to prevent it from activating at other times, but must be ON in order for the light to function. Motion detectors primarily trigger on motion going across their view, but have much less sensitivity to motion going directly towards them. Harsh, direct sunlight on the motion detector lens will shorted it's life.

Problem is if the on switch is on the hall light stays on all day and night and we don't want that.

Hard to imagine why someone would want it wired that way. Some motion detectors are controlled by a switch to: 1) turn on (independent of detecting motion), 2) set it in motion detecting mode, and 3) turn it off. That's how mine work.

It could be wired so a switch would leave it on, or the motion sensor, and/or photo cell, could be faulty.

@toriat -- We have several outdoor md (motion detector) lights. I don't understand jrb451's first sentence, but after that, right on. These lamps have a few modes, and like many electrical devices with various options, any button, toggle, or switch (in order to save space and cost) work in various, multiple ways.

The lamp and MD unit itself will usually have adjustments for "sensitivity" to light (you want it to come on at dusk-like low light or only darkness) and to motion sensitivity. Too sensitive and it might come on every time an insect flies past. You can also, at the lamp MD unit select for how long the lamp stays ON (2 mins., 5 mins.) and there is a TEST mode that is used for when you want to adjust the position of the sensor. A person can put TEST mode on, and walk around and see how the sensor works at whatever angles of approach (light will come on only momentarily).

On the inside of the dwelling, the MD light is connected to an on off switch, and this also has various MODES. Turn the switch ON (say, if only one light switch, in the UP position. This turns the MD Light ON. ON in this scenario means that the MD sensor is ON. Usually the light comes on, then goes OFF until motion is detected. Then the lamp stays on for whatever time has been selected on the unit, or stays on if there is continued motion (say a person, perhaps you, walking around outside).

For our front step lights, we leave it on MD mode. However, if I want to leave the lights on for arriving company, I can override the MD sensor, and have the lights STAY ON. Light products differ so you can read a set of directions (online) for your lamp, or experiment to figure out how it works. For our lights, it goes like this to take it out of MD mode and put it into STAY ON mode: flip the switch off, then switch on again within 5 seconds. Of course, flipping the switch DOWN or OFF results is NO LIGHT coming on. You may want it like this for the upcoming meteor showers ;)

The best MD units (most nowadays) RE-SET after a power outage or interruption. We had an older set that would stay ON after a power bump. Expensive if you have incandescent bulbs and don't catch it. We replaced that unit.

WHY is it that these units sometimes fail? I've experience with at least two reasons.

a) The UV light and weather can wear out some of the parts. That happened to our favorite 15 year old Journeyman set. I loved that floodlight unit.

b) The plastic lens of the MD sensor seems to get either cloudy with UV damage, or dusty. One of our lights (14 yr old or more) is misbehavin' because of this sort of aging. I don't think there's a way to get behind the MD plastic to clean it.

c) The Light Sensor (like a little eye) can get dirty, dusty, or UV damaged and doesn't read the light level correctly. One of our older lights comes on in broad daylight.

So, there you go. Nothing lasts forever, especially if out in the weather. I hope this helps, and if not, there's lots of info online about MD sensing lighting. They are great devices and some of our floodlights coming on at 2 AM in February gave me a heads up on a beautiful visiting coyote searching the snow for mice or rodents.

Thanks for the detailed info. One problem is the light came with the building and we don't know how old it is. Plus there are no directions. I'm going to have a motion detector put back by the garage so I'll probably just buy a new unit and start from scratch.