How a door frame metal detector works (DFMD working principle)
A door frame metal detector, also called a DFMD or walk-through metal detector, has transmitter coils in one side panel and receiver coils in the other. The transmitter sends a pulsed or continuous electromagnetic field across the archway. When a person carrying metal walks through, the metal disturbs that field: eddy currents form in the metal object and create a small magnetic field of their own, which the receiver coils pick up. The control unit compares the signal with the sensitivity you have set and, if it is above the threshold, sounds an alarm and lights the zone bars where the metal passed.
Because the reading depends on the size, shape and type of metal, a good DFMD can be set to ignore small items such as a belt buckle or coins while still alarming on a knife or a gun. That is why sensitivity is set at the site, with the guard, using the items people normally carry.
| Single zone | Alarms when metal passes, without telling where. Cheapest, mostly for low-risk events. |
|---|---|
| 6 zone | Shows roughly whether the metal is at head, waist or leg level, so the guard knows where to check with the handheld detector. |
| 18 / 24 zone | Finer location on left, centre and right as well as height. Common in malls, hotels, offices and hospitals. |
| 33 zone and above | Pinpoints the metal more precisely and handles several items at once. Used at airports, metro and high-security government sites. |
Prices for 6, 18 and 33 zone models are on our door frame metal detector price list, and the multi-zone DFMD price guide compares them.
A door frame metal detector (DFMD), also called a walk-through metal detector, alarms when a person carries metal through it. Here is what matters when you choose one.
Number of zones
Multi-zone detectors split the frame into zones from head to feet and show which zone found the metal. Common options are 6, 18, 24 and 33 zones. More zones help guards find the item faster with a handheld detector.
Sensitivity settings
Good detectors let you set sensitivity so that small items such as keys and coins can be ignored while larger objects still alarm. This keeps queues moving without lowering security.
Counters and display
Look for a clear alarm light and sound, a people counter and an alarm counter. These help you understand footfall and alarm rates at each entry.
Indoor or outdoor
For outdoor use, choose a weather-protected model or place the detector under a canopy. Rain and direct sun can affect electronics over time.
Placement tips
- Keep the detector away from large moving metal such as lift doors, sliding gates and vehicles.
- Keep it away from heavy electrical equipment and power cables that cause interference.
- Fix it firmly to the floor so vibration does not trigger false alarms.
- Leave space after the frame for a guard with a handheld detector.
Pair it with a handheld detector
A walk-through detector tells you that metal is present. A handheld detector finds exactly where it is. Every screening point should have both.
Frequently asked questions
What is the working principle of a door frame metal detector?
Coils in one side panel create an electromagnetic field across the archway and coils in the other panel measure it. Metal carried through the arch disturbs the field, and when the change is above the set sensitivity the detector alarms and shows the zone where the metal passed.
How many zones should my door frame metal detector have?
Six zones are enough for most offices, schools and events. Malls, hotels and hospitals usually choose 18 or 24 zones, and airports, metro and high-security sites use 33 zones or more.
Can a door frame metal detector be used outdoors?
Only models built for outdoor use with a weatherproof rating, or with a canopy over the unit. Standard indoor models should be kept out of rain and direct sun.
Why does the metal detector alarm for everyone?
Usually the sensitivity is set too high, or there is interference from nearby metal doors, motors, escalators or another detector placed too close. We set the sensitivity and position at site to avoid this.
Our engineers can visit your site and recommend the right option. Call +91 97119 00801 or request a quote.
