I'm looking to build a motion sensor which detects if an object is in the monitored area. Doing some researched I found out that I can use PIR motion sensor for this but it seems the area it monitors is pretty small.

I need something that can monitor about 3 feet x 1 feet area. Is there something else that I can use for this?

My goal is to light up some LED lights when an object is detected in that area.

Thanks in advance.

  • What kind of object? What are its dimensions compared to the area it's in? Is it an enclosed area or an unbounded area within a larger space?
    – Majenko
    Commented Aug 15, 2015 at 9:58
  • I wanted to mount on my garage's ceiling and monitor when a car is in the monitored area
    – ed1t
    Commented Aug 15, 2015 at 19:13
  • I would strongly suggest a ultrasonic ping sensor. It is low cost (under $3 on E-Bay) and will give you an accurate reading. Here is an example: instructables.com/id/… I would use an Arduino Nano on e-by if you want to keep total cost under $5. Commented Aug 16, 2015 at 22:54

2 Answers 2


A standard PIR motion sensor will work fine. It can monitor motion in a very room. You just need to make sure the PIR sensor is far enough away.

Here is one article that states it should work up to 10 meters away.


Note that the device that the PIR sensor is mounted on must be stationary. It will not work on a robot of anything that is moving.

  • 1
    Also, note that the object sensed by the PIR must be hot, and that object must be moving, not stationary. The PIR works by sensing changes in infra-red light between two overlapping fields of view. That's why you can sit in a room with a PIR and it's normally off. As soon as you stand up or otherwise move it'll activate.
    – Majenko
    Commented Aug 15, 2015 at 9:58

It sounds like what you want is an ultrasonic distance ("ping") sensor over the area where the car is.

When the car isn't there it will measure the height of the garage (say 3m). With the car there it will measure the distance from the ceiling to the top of the car (say 1.5m).

Ultrasonic sensors have a limit to their range. The cheap ones are usually around 2m, so may not work for you, or you may get away with it if it will return "infinite" for anything outside its range, in which case anything that is not "infinite" will be something present under the sensor.

  • What's the coverage of a ping sensor? can it monitor specified area or it's just one straight line ?
    – ed1t
    Commented Aug 16, 2015 at 18:44
  • It's roughly a straight line, but a car is a pretty large object - bigger than the 3' by 1' area you mention. If it's there it'll be filling the area, so even with a 1µm wide laser as the sensor it'll still be present.
    – Majenko
    Commented Aug 16, 2015 at 19:02
  • What if I wanted to monitor a specified area rather than a straight line ? 3' by 1' was something I specified so that it can be placed in the middle of the garage.
    – ed1t
    Commented Aug 16, 2015 at 19:18
  • Pressure sensor under the floor? Array of ping sensors? Grid of infra-red beams intersecting the area? For these kind of things it can often be easier to sense an entry or exit to an area rather than the area itself.
    – Majenko
    Commented Aug 16, 2015 at 19:29

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