modular, so a dirty acceptor can be replaced with a clean unit, minimising downtime. The old unit is then cleaned and refurbished. Some new types of coin acceptors are able to recognize the coins through "training", so they will support any new types of coins or tokens when correctly introduced. Testing methods Vending and change machines use several methods of deciding whether a banknote is genuine. Adjusting these settings and the sensitivity of each is programmed via means of DIP switches on the internal circuitry. Optical sensing Optical sensing with a small light detector called a photocell or a miniature digital camera is one of the main techniques that vending machines use. Many countries' banknotes are pixelated—that is, they are made out of small dots. The dots are spaced differently and have different sizes, depending on the note. The optical sensors can look for these different patterns to determine what sort of note has been inserted. Some paper money is also fluorescent: it glows when ultraviolet light is shined on it. Some machines shine an ultraviolet light on the note and measure the glow to help determine the banknote's material composition. GMR sensor proximity detection The particles in the ink on many countries' currency have ferromagnetic properties, including some elemental iron. Magnetic composition comprises carbon nanofoam in an amount of from 0.1 to 45 percent by weight of the total composition. Notes are passed over a permanent magnet array and magnetized along their direction of travel. A magnetic sensor located several inches away with its sensitive axis parallel to the direction of travel can detect the remnant field of the ink particles. The purpose of the biasing magnet in this case is to achieve a controlled orientation of the magnetic moments of the ink particles, resulting in a maximum and recognizable magnetic signature. Reversing the magnetizing field can actually invert the signature. Physical attributes The thickness and dimensions of a banknote are tested to ensure they are correct. US currency is 2.61 inches wide by 6.14 inches in length and are 0.0042 inches thick, and weigh 1 gram. Currency printed prior to 10 July 1929 had larger physical characteristics. As the notes pass between the rollers, the voltages vary according to their thickness. Miniature transducers, approximately 3/8" diameter, offer high accuracy linear measurement in a compact space where size constraints prohibit the use of standard LVDTI's. In addition, the low-