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You can use a smartphone as a simple, short-range magnetic anomaly detector—but not as a general-purpose metal detector. If your phone has a magnetometer, an app such as phyphox can show how the local magnetic field changes near a magnet or many iron and steel objects. It usually will not detect a gold ring, copper wire, or aluminum can just because those items are metal, and it is not a practical way to search for buried treasure.
The simplest build needs only a compatible phone and a magnetometer app. A homemade plastic or wooden probe can make sweeping steadier, but it does not increase the sensor’s range or sensitivity.
What this phone-based detector can do
A smartphone magnetometer measures the local magnetic field. A nearby permanent magnet or ferromagnetic object can change the field at the sensor, producing a visible change in the app’s readings. The phone is sensing a magnetic-field anomaly—not identifying metal in general.
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That makes this a useful construction-site or workshop demonstration for locating a loose steel screw, nail, or other magnetic object at close range, provided the surrounding area is not too magnetically noisy. It is not a substitute for a conventional detector, which uses purpose-designed coils and electronics to find a broader range of targets and to search the ground.
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Many phones have a magnetometer, but not all do. Phyphox lists supported sensors and notes that sensor availability depends on the device: phyphox supported sensors. The app is listed for Android and iOS; the available readings and interface can vary by phone.
What you need
- A smartphone with a working magnetometer.
- A magnetometer app that displays numerical readings or a live graph. This guide uses phyphox because it exposes sensor data rather than only showing a vague “metal detected” gauge.
- A small steel screw or nail and a small magnet for controlled tests.
- Optional comparison objects: aluminum, copper, brass, or a gold-colored item.
- Optional ruler or tape measure to record your own test distances.
- Optional phone holder made from cardboard, wood, PVC, acrylic, or plastic, plus tape or nonmagnetic fasteners.
Keep steel brackets, magnetic clasps, magnetic phone mounts, MagSafe accessories, metal-heavy grips, and metal-bodied power banks away from the sensor during testing. A holder makes the sweep more repeatable; it does not amplify the phone.
Check for a magnetometer and find its position
- Install and open phyphox, then choose its magnetometer or magnetic-field experiment. Confirm that numerical readings and a live graph appear. Exact labels can vary with the app version and device.
- Hold the phone still and move a small magnet slowly around the back, without letting it strike or cling to the phone. Watch for the largest change in the reading to estimate where the sensor is located.
- Mark the approximate location on a removable case or a piece of tape, then take the magnet away. Do not calibrate with the magnet still nearby.
- If no magnetic-field values appear, check the app’s sensor list and try a second reputable sensor app. Test with a known magnet. If the phone still shows no clear response, use a different phone rather than buying parts for an unsupported device.
The sensor is not necessarily in the center of the phone, and its location differs between models. A University of Jyväskylä teaching guide discusses variation in smartphone magnetic-sensor placement: magnetic-field guide.
The Tool Desk
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1. Choose a clean test area
Remove magnetic cases and accessories. Move away from cars, steel tables, metal shelving, loudspeakers, laptops, chargers, power supplies, and loose tools. Do not set the phone on a metal table. Keep the phone’s orientation fixed while you take readings; turning it can change the axis values even when the surroundings have not changed.
2. Record a baseline
Place the phone in the orientation you plan to use and observe the readings for about 10–30 seconds. Note the normal variation, not just one instantaneous number. If the reading jumps around, relocate and check for nearby interference before proceeding.
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Do not use a universal threshold such as “50 µT means metal.” The Earth’s field, phone orientation, nearby objects, sensor characteristics, and app processing all influence the readings. A relative change from a stable baseline is more useful than a number borrowed from another phone.
3. Test known targets
- Start the live graph and bring a steel screw or nail slowly toward the marked sensor location.
- Note whether the graph changes clearly and, if you are measuring range, record the distance at which you first see a repeatable change.
- Repeat with a magnet. Keep it controlled and do not leave it attached to the phone.
- Test aluminum, copper, brass, and a gold-colored object separately. Record both positive and negative results; do not assume every metal should register.
- Move each object away and repeat the approach to see whether the signal returns and changes again consistently.
Your distances apply only to your phone, target, orientation, and test environment. They are not a general performance specification.
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Hold the sensor at a consistent height and move the phone slowly in parallel passes. If the graph shows an apparent hit, go over the same spot again and approach from another direction. A repeatable change is more meaningful than a single spike. Nearby steel, electronics, and changes in phone orientation can all create misleading readings.
Build 2: Make a simple phone probe
For easier sweeping, attach the phone securely to a short shaft or handle made from wood, PVC, cardboard, acrylic, or plastic. Position the marked sensor near the end of the probe and keep the phone in the same orientation from pass to pass. Tape, zip ties, or nonmagnetic fasteners can hold the phone in place.
Before using the probe, test the empty holder. A steel screw, magnetic clasp, or metal shaft close to the sensor can overwhelm the target’s signal. The holder should be mechanically rigid but magnetically quiet. It only helps you keep height and orientation consistent; it does not make the phone detect deeper objects.
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How to read and calibrate the signal
Some apps show the field along three axes, often labelled X, Y, and Z, as well as a total magnitude. If the app gives the three components, the resultant magnitude is:
B = sqrt(Bx² + By² + Bz²)Here, B is the resultant field and the readings are commonly expressed in microteslas (µT). A target may cause a clearer change on one axis than in the total magnitude, so inspect both when troubleshooting.
For a simple comparison, calculate or observe the difference from your baseline:
delta = B - B_baselineIf direction is not important, use the size of that difference:
abs_delta = abs(B - B_baseline)In practice, phyphox can display, graph, and export sensor data; its official site describes its sensor experiments and analysis features: phyphox. A rolling average can help advanced users account for slow drift, but a baseline that adapts too quickly may absorb a large stationary target. For a beginner, a fixed, freshly recorded baseline and repeated passes are easier to interpret.
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What different materials are likely to do
| Target | Likely response | Why |
|---|---|---|
| Permanent magnet | Often strong | It produces its own magnetic field. |
| Iron or steel | Often detectable nearby | Ferromagnetic material can alter the field measured at the phone. |
| Stainless steel | Variable | Magnetic behavior depends on the alloy and its condition. |
| Aluminum | Usually little or no useful response in this static phone test | It is not ferromagnetic. |
| Copper | Usually little or no useful response | It is not ferromagnetic. |
| Brass | Usually little or no useful response | It is not ferromagnetic. |
| Gold | Usually little or no useful response | A basic magnetometer is not a general-purpose precious-metal detector. |
These are practical expectations, not guarantees for every object or phone. Size, shape, orientation, distance, alloy, and environmental interference matter. A failure to detect copper or gold does not mean the project is broken; it reflects what a phone magnetometer measures.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Optional advanced project: external sensor or coil with Bluetooth
If you want custom graphs, alarms, or data logging, a separate electronics project can send readings from a microcontroller to a phone:
External sensor or experimental coil → microcontroller → Bluetooth Low Energy → phone graph or alertPhyphox provides an official Arduino BLE library for sending microcontroller data to the app. Its repository lists supported boards, including ESP32 and several Arduino models; check the current documentation for the exact board, firmware, and library requirements: phyphox Arduino BLE library. The phone may visualize external readings, but this is not simply an extension of the phone’s built-in sensor.
Phyphox also documents educational experiments in which a coil’s magnetic field is measured by a phone, including smartphones as ammeters and an LC-circuit experiment. Those demonstrations do not, by themselves, make a proven buried-metal detector. A DC-powered coil acts mainly as an electromagnet. Reliable passive-target detection requires a designed and tested circuit that accounts for coil geometry, current, target material, distance, orientation, and what the electronics measure.
For a BLE build, keep the sensor away from motor noise and switching regulators, use a stable battery-powered setup, route wiring consistently, and establish a baseline with the complete rig assembled. If phyphox cannot see the device, check the board’s firmware and BLE settings, phone Bluetooth permissions, and the library’s current compatibility documentation. Exact wiring, pins, and sampling settings depend on the chosen board and circuit, so do not copy them from a different hardware revision without checking its documentation.
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Troubleshooting
The app has no magnetic-field reading
The phone may lack a magnetometer, the wrong experiment may be open, or the app may not be receiving the sensor. Check the app’s sensor list, try another reputable sensor app, and test with a known magnet. If there is still no clear response, switch to a compatible phone.
The graph is unstable
Look for a magnetic case or mount, nearby charger or power bank, steel furniture, vehicle bodywork, speakers, tools, or changing phone orientation. Remove accessories, move to a wooden or plastic surface in a less cluttered area, keep the phone still, and record a new baseline.
A target appears only from one direction
That can happen because a magnetometer measures a vector field and the target’s contribution depends on orientation. Compare the individual axes, approach from more than one direction, and repeat the test with the phone held consistently.
It responds to a nail but not aluminum or copper
That is an expected limitation of the phone-only method. It detects field changes, not all metal objects. Do not interpret a nonresponse to a nonmagnetic metal as proof that the phone or app is faulty.
You see anomalies near a building or vehicle
Steel structures, pipes, reinforced concrete, electrical equipment, and vehicles can affect readings. Move to an open area and compare a control sweep with a sweep over the suspected target. A field anomaly alone does not prove what caused it.
Safety and when to use a real detector
Do not use a phone or homemade detector to identify hazardous objects, locate utilities, or decide that an area is safe to dig. Contact the appropriate utility service before any excavation, and obtain permission before searching private property. Follow local rules for parks, beaches, archaeological sites, and public land; avoid disturbing infrastructure or protected sites.
Keep exposed wiring, conductive parts, batteries, and capacitors away from the phone unless the electronics are properly assembled and insulated. Avoid shorted capacitors, high-current coils, loose lithium cells, and magnets that could damage or interfere with the phone. A conventional metal detector is the better choice for outdoor ground searching, nonferrous targets, greater depth, discrimination, ground balancing, or waterproof use.
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