Do I need door contacts or motion sensors?

Many sites need both. Contacts and motion sensors do different jobs.

What is better for perimeter protection?

Door and window contacts are often the stronger starting point for perimeter awareness.

What is better for internal catch zones?

Motion sensors are usually better for that job.

Can a good alarm use only motion sensors?

Sometimes, but many sites lose useful perimeter information if they ignore contacts completely.

What is the biggest sensor-planning mistake?

The biggest mistake is assuming one sensor type should handle every part of the building.

Door Contacts vs Motion Sensors

Door contacts and motion sensors do different jobs. Contacts tell the site that a door or window has opened. Motion sensors tell the site that movement has happened inside a protected space. Many useful alarm systems need both.

Sensors

Contacts versus motion in simple terms

Sensor type	Main job	Usually strongest for
Door or window contact	Detect opening at the perimeter	Front doors, rear doors, windows, roller doors, cabinets
Motion sensor	Catch movement inside the protected area	Hallways, office interiors, stock rooms, internal routes

When each matters most

Contacts are especially useful where the owner wants to know that a door has been opened at all, even before someone walks through the building. Motion sensors are useful where an internal catch zone matters. In many better systems the site uses contacts on the vulnerable openings and motion sensors on the internal path someone would actually take after entry.

Where contacts and motion sensors usually go

Part of the building	Contact usually goes here	Motion sensor usually goes here
Home	Front door, rear door, laundry door, garage-entry door	Hallway, stair path, internal travel route after entry
Small office	Front office door, rear staff door, server-room or store-room door	Reception-to-corridor path, office hallway, internal route to valuables
Warehouse	Office entry, side personnel door, rear service door, selected roller-door path	Office corridor, mezzanine stair path, stock-room or dispatch route
Clinic or pharmacy	Front and rear doors, dispensary or restricted-room door	Reception path, corridor to consulting rooms, dispensary approach

Worked examples

Worked example

A home front and rear door plus hallway

Situation: A home has a front door, rear laundry door, and one main hallway that an intruder would use after entry.

Solution used: Contacts on the front and rear doors plus a PIR in the hallway rather than choosing only one sensor type.

Why this was chosen: Contacts tell the owner which opening was used, while the PIR confirms internal movement on the route that actually matters. The two layers answer different questions and work better together.

Installation notes: This type of layout is usually stronger than putting PIRs in multiple rooms while leaving the perimeter reporting vague.

Worked example

A small warehouse office and roller door

Situation: A small warehouse has an office door, a rear personnel door, and one roller door that becomes the obvious after-hours attack point.

Solution used: Contacts or opening detection on the office and personnel doors, an appropriate trigger path for the roller door, and motion coverage on the office or internal passage route.

Why this was chosen: The roller door may justify a contact or perimeter trigger, while the office and internal passage still benefit from motion. That is why many commercial alarms need both layers rather than choosing only contacts or only PIRs.

Installation notes: This is one of the clearest examples of why sensor choice should follow door type and travel path rather than one blanket rule.

What usually works

For many homes and businesses, the best result is not choosing contacts or motion. It is choosing where the contact gives perimeter awareness and where the PIR gives internal confirmation. That is why a front and rear door plus one well-placed hallway PIR often outperforms a more random detector layout.

If pets, roller doors, detached buildings, or staff-only rooms are part of the site, the balance between contacts and motion may change, but the main idea stays the same: perimeter information plus internal catch zones.

What to be careful with

Do not use only motion if the owner really wants perimeter awareness.
Do not use only contacts if the internal catch zones also matter.
Think about pets, air movement, and actual internal travel paths when placing PIRs.
Do not put the PIR where it is easy to install if that position does not actually watch the route an intruder would use.

Relevant SecurityWholesalers Categories and Products

These detector examples and alarm branches are useful when the sensor mix is the main buying decision.

Hikvision AX PRO wireless PIR - Useful internal motion reference.
Hikvision AX PRO alarms - Useful sensor ecosystem reference.
Dahua AirShield wireless door detector - Useful door-contact reference.
Dahua AirShield - Useful wireless sensor branch.

Sources and Further Reading

Frequently Asked Questions

Do I need door contacts or motion sensors?
Many sites need both. Contacts and motion sensors do different jobs.
What is better for perimeter protection?
Door and window contacts are often the stronger starting point for perimeter awareness.
What is better for internal catch zones?
Motion sensors are usually better for that job.
Can a good alarm use only motion sensors?
Sometimes, but many sites lose useful perimeter information if they ignore contacts completely.
What is the biggest sensor-planning mistake?
The biggest mistake is assuming one sensor type should handle every part of the building.