Alarm motion detectors: which to fit and where

Part of the Costa del Sol security guide

The short answer

An alarm is only as good as its detectors and where they are placed. In a flat, one photo detector in the main through-route, contacts on the doors and, if you have large windows, a glass-break detector is usually enough. In a villa you add the outside: façade or garden detectors that warn you before anyone reaches the door. And if you live with a dog or a cat, the detector is chosen and positioned so it ignores them. At Dartel Solutions we fit this with Ajax, with no monthly fees, in Málaga and across the Costa del Sol. Phone and WhatsApp: +34 672 81 57 89.

The types of detector, one by one

Not every "motion sensor" does the same job. These are the ones we use and what each is for (figures from the manufacturer's data sheets):

Detector	What it watches	Where we put it
Indoor PIR (MotionProtect)	The body heat of a person moving in a room, up to 12 m	Living room, hallway, main through-route
Dual technology (MotionProtect Plus)	PIR + microwave: it only alarms when both detect	Sunny kitchens, rooms with a fireplace or sudden temperature changes
With photo (MotionCam HDR)	Like the PIR, but when triggered it sends a series of up to 5 photos to the app	The route any intruder would have to take
Curtain (CurtainProtect)	A thin plane (6° horizontally) in front of a window or terrace door	Large windows, so you can arm the alarm while you are at home
Outdoor (MotionProtect Outdoor)	Garden or façade, up to 15 m and 90°, with two sensors that must agree	Garden, side path, driveway
Outdoor with photo (MotionCam Outdoor)	As above, with up to 5 infrared photos for night-time	Villas and houses with a terrace or garden
Glass break (GlassProtect)	The sound of breaking glass, up to 9 m and 180°	Living rooms with large windows, shop fronts

A glass-break detector does not detect movement: it tells you the moment the glass breaks, before anyone is inside. That is why it is combined with motion detectors, not used instead of them.

Where does each one go?

The installer's rule is to count the ways in, not the square metres:

The through-route. The hallway or landing that anyone coming in has to cross, whether they enter by the door or a window. That is where the photo detector goes: one device covers several entry points.
Rooms you can reach from outside. A living room with a terrace, a ground-floor bedroom, a kitchen with a door to the patio.
Doors, with a magnetic contact. A motion detector does not replace the contact: the contact alerts when the door opens, the detector when someone is already inside.
The outside, only if there is land. In a villa, knowing someone is crossing the garden buys you minutes you do not have once they are inside. In a flat it makes no sense.

Height: the manufacturer specifies 2.4 m for indoor detectors, with a clear view and the detector looking across an intruder's path rather than straight at it. Any lower and pets walk into the sensitive zone.

What causes false alarms

The Ajax manual is clear about where not to put an indoor PIR detector:

Outdoors. Indoor detectors are not built for it; that is what outdoor ones are for.
Where direct sunlight hits the lens.
Facing heaters, radiators or anything whose temperature changes quickly.
In draughts: next to fans, open windows or open doors.
Looking through glass. A PIR detector does not register movement behind a window.

In a tricky room (a kitchen with afternoon sun, a living room with a fireplace), the dual-technology detector uses microwave to confirm what the PIR sees and cuts those alerts. The photo detector covers the rest: if it goes off, you look at the image and know within seconds whether someone is there.

What if I have a dog or a cat?

You can have an alarm with pets, as long as the detectors are chosen and placed properly. According to the Ajax manual, its indoor detectors ignore animals up to 20 kg and 50 cm tall, provided they are mounted at 2.4 m and the sensitivity is set correctly: on high they ignore cats up to 25 cm, on normal small dogs up to 35 cm and on low animals up to 50 cm. What goes wrong in practice is rarely the detector, it is the furniture: if the cat can climb onto a cupboard inside the field of view, it will trigger. That is why we look at the room before deciding where each one goes. In the garden, the outdoor detectors we install (MotionProtect Outdoor and MotionCam Outdoor) are pet-immune up to 80 cm in height, according to their data sheets.

Motion detectors and presence sensors are not the same thing

If you are searching for a "presence sensor", you may not want an alarm but home automation: lights that come on as you walk in, air conditioning that switches off when the room is empty. An alarm PIR only sees movement; a millimetre-wave radar presence sensor such as the Aqara FP2 detects a person even when they are sitting still (the manufacturer says so) and needs mains power. It is for automating the house, not for replacing the alarm. More in home automation.

How many detectors does a home need?

It depends on the entry points, not the size. As an installer's guide:

Flat: one photo detector in the through-route, contacts on the front and terrace doors, and glass break if there are large windows. That is the basis of the Ajax alarm kit for a flat.
Villa or detached house: the above on each floor, plus curtain detectors on large windows and one or two outdoor detectors in the garden or side path. That is the basis of the Ajax alarm kit for a villa.
Shop or business: photo detector in the through-route, contacts on door and shutter, glass break on the shop window.

The price depends on the number of detectors and the property; we break it down in how much does an alarm cost in Málaga. If you are weighing up brands, we compare Ajax and Dahua (which also makes alarms) in Ajax or Dahua: which alarm to choose.

Who gets the alert when a detector goes off?

With an Ajax alarm with no monthly fee, the alert goes to your phone and to anyone you add, with the photo if the detector takes one. If you want a monitoring centre to verify the alarm and call the police, that service is provided by a registered private security company, not by us: we explain it in monitored alarm or phone alerts?.

Frequently asked questions

How high should an indoor motion detector be mounted?

At 2.4 m, according to the Ajax manual, with a clear view and looking across the intruder's path.

Can I have an alarm with a dog?

Yes. Ajax indoor detectors ignore animals up to 20 kg and 50 cm tall when installed correctly, and the sensitivity is set to the size of your pet. Keep furniture they can climb out of the field of view.

Can a motion detector see through a window?

No. A PIR detector does not register movement behind glass. To get a warning before someone gets in, you use outdoor, curtain or glass-break detectors.

What is the difference between a motion detector and a presence sensor?

A motion (PIR) detector picks up someone moving and is the one used in alarms. A radar presence sensor also detects a person who is not moving and is used for home automation.

Why does my alarm go off for no reason?

Almost always because of placement: direct sun on the lens, a heater or radiator opposite, a draught, or a pet climbing onto furniture. A photo detector lets you check straight away.

Do you install detectors across the Costa del Sol?

Yes, from Nerja to Sotogrande. See security alarms in Málaga.

Want to know which detectors your home needs?

Send us the floor plan or a few photos and we will tell you how many detectors you need, which type and where, with an itemised quote. Site visit and quote with no obligation. We speak English, Spanish, Russian and Ukrainian.

Ask for a quote on WhatsApp See Ajax detectors in the shop

Sources: MotionProtect product page and manual (ajax.systems/products/motionprotect and ajax.systems/support/manuals/motionprotect), MotionProtect Plus product page (ajax.systems/products/motionprotectplus), MotionProtect Outdoor product page (ajax.systems/products/motionprotect-outdoor), dartelsolutions.com product pages (CurtainProtect, GlassProtect, MotionCam HDR, MotionCam Outdoor) and the Aqara FP2 product page (aqara.com/en/product/presence-sensor-fp2). Checked 23-10-2026.

Motion detectors for your alarm: which one to fit and where it goes