A stud finder (also stud detector or stud sensor) is a handheld device used to determine the location of wood and metal framing studs used in light-frame construction after the walling surface has been installed.
There are two main types of these devices: magnetic stud detectors and internal capacitor stud finders.
Magnetic stud detectors
Stationary magnetic stud detectors use a small (stationary) magnet to detect the nails or screws placed into studs during the manufacturing of the wall. It is the "pull" of the metal fastener on the magnet that alerts the user to the (possible) presence of a stud. Since the amount of "pull" is greatest when the screw or nail is very close to the magnet, the ability for the user to feel the pull drops off quickly as the metal fastener gets deeper into the wall board. For example, the metal screws that hold sheetrock or drywall to the studs are just under the surface and therefore have the greatest effect on the magnet. On plaster walls, the metal fasteners are buried under plaster that can be up to 1/2" thick. This greatly reduces the effective pull on the magnet and thus the ability for the user to "feel" the presence of said fastener. It is for this reason that stud finders consisting of nothing more than a stationary magnet, will have limited use much beyond sheetrock. It should also be noted that increasing the size and/or strength of the magnet will have little effect on extending the performance any further.
Moving magnet stud detectors are an enhancement involving a neodymium magnet that is suspended in a housing such that it is free to move in response to hidden metal fasteners. The strength of this rare earth magnet, along with the ease of movement of said magnet, allow this type of magnetic stud finder to extend its range of detection to include all types of construction. What this means to the user is that fasteners buried deep under plaster or tile, can be located with this type of device, since it is not dependent on the operator to "feel" the attraction between the metal fastener and the magnet. The magnet is suspended in such a way that it always sits in its "home" position until it is moved directly over a metal fastener or metal stud. Once the magnet is in the vicinity of the metal, it is pulled towards the wall at a rate of acceleration that is proportional to the distance between the magnet and the metal. On walls having fasteners that are shallow such as sheetrock, the magnet moves towards the wall with such velocity that it makes a distinct thud sound when it hits the wall. As the metal fastener gets deeper, the thud becomes more of a click since the speed of movement is reduced.
Internal Capacitor Stud Finders
Internal capacitor stud finders use an internal capacitor plate to detect changes in the dielectric constant of the wall as the user moves the finder over the surface of the wall. A change in the dielectric constant indicates a dense object behind the wall, normally a framing stud. Internal capacitor stud finders are the most common.
Internal capacitor stud finders are divided into three classes:
- Edge finders detect the edges of the stud or other target behind the wall. The sensor in an edge finder detects when it's directly over a change in density, such as the edge of a stud. Edge finders have to be moved from both right and left directions to find both edges so the center can be determined. The first internal capacitor stud finders were all edge finders, and most inexpensive models still are.
- Center finders detect the center of the stud. These tools register the wall’s dielectric constant from multiple sensors, and use the different readings to determine the location of the target center. Unlike edge finders, these stud finders need to be moved from only one direction to indicate the center of the stud. These kinds of stud finders are also called "one step" finders.
- Instant stud finders have multiple sensor plates, and do not need to be moved from one location to another to detect a stud. The instant stud finders sense multiple regions of a wall simultaneously and can sense the center of the stud, the edges of the stud, and regions without studs simultaneously. They use an algorithm to analyze the readings from the multiple sensor plates to determine the precise location of any studs. Because the instant stud finders use multiple readings to determine the location of studs they are less vulnerable to construction anomalies (such as uneven paint, wall textures, wallpaper, uneven plaster, etc.) that can confuse center finders and edge finders.
Internal capacitor stud finders also can come with other features that locate metal and live AC voltage.
The internal capacitor stud finder was first designed by Robert Franklin in 1977. After he had patented the device, he tried to sell the rights to produce it to several major tool companies, all of which turned down the idea. He took the design to Zircon Corporation, an electronics manufacturer in Campbell, California, who agreed to build and market the device. Zircon was the sole producer of internal capacitor stud finders until 1998, when the original patent expired, allowing other companies to manufacture it as well. Other developments and improvements in stud finders remain under patent protection.
- US patent 4099118, Robert C. Franklin and Frank I. Fuller, "Electronic wall stud sensor", issued 1978-7-4
- Short range micro-power impulse radar with high resolution swept range gate
-  - Instant Stud Finders