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I don't understand how can a non magnetic object like wood be detected using a stud finder

For instance, stud finders find wood from dielectric constance, but I don't understand the princible.

The basis of my question is to understand how sensors can detect non-magnetic objects in order to build a paintball ball counter.

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  • \$\begingroup\$ Don't stud finders detect the nails used to attach the studs? \$\endgroup\$
    – PeterJ
    Mar 19 '14 at 23:46
  • \$\begingroup\$ please provide an example and/or more details \$\endgroup\$
    – kimliv
    Mar 19 '14 at 23:47
  • \$\begingroup\$ @PeterJ , no they use the dielectric constant of wood and a magnetic sensor which I don't know the principle of. \$\endgroup\$
    – Napster
    Mar 19 '14 at 23:49
  • \$\begingroup\$ Is your actual question more to do with what the dielectric constant actually is? You talk about magnetic stud finders ("using a magnet and a sensor") in the body of your question, which have nothing whatsoever to do with internal capacitor stud finders (which your title mentions). Please figure out what you want to know about and adjust your question accordingly, else it may be closed due to being unclear. \$\endgroup\$ Mar 19 '14 at 23:52
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    \$\begingroup\$ What exactly are studs? \$\endgroup\$
    – Andy aka
    Mar 20 '14 at 8:31
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There are several types of capacitor-type stud finder, see the excellent Wikipedia article for more information.

The basic principle is that the capacitance of a capacitor of fixed geometry is proportional to the dielectric constant of what is between the plates. The drywall is a constant and can be calibrated out, leaving the difference between wood (dielectric constant of maybe 2 or 3) and air (dielectric constant of 1).

The seminal patent was US 4099118, which discloses a 3-plate capacitor design allowing the edge of the stud to be detected by differential capacitance.

enter image description here

That's the sensing principle. There are a number of ways of measuring capacitance, often making a ramp or an oscillator incorporating the unknown capacitance and measuring the time for a given voltage change or the oscillator frequency is an easy way, but there are others.

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  • \$\begingroup\$ Can I use a magnet and a coiled sensor to detect the change in the electric field? \$\endgroup\$
    – Napster
    Mar 22 '14 at 22:10
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    \$\begingroup\$ I can think of ways but nothing as simple as that outlined in the (expired) patent I referenced above. \$\endgroup\$ Mar 22 '14 at 23:01

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