Patent GB-1347774-A (GB)
Title: Solid state switches
IPC: H03K17/97
Filed: 1971-10-26
Abstract: 1347774 Keyboard switches ILLINOIS TOOL WORKS Inc 26 Oct 1971 [26 Oct 1970] 49730/71 Heading H3B [Also in Division G4] A keyboard switch comprises a pair of saturable magnetic cores 12, 14, Fig. 1, mounted in a fixed position in the switch body 10, six magnets 16, 18, 20, 22, 24 and 26 being mounted on the legs 28a and 28b of a keystem 28 to be moved past the cores when the cap 32 is depressed. The body 10 may be a single piece of polycarbonate, while the keystem is of a magnetic material such as steel having three sides and a top. The key cap is normally biased upwardly by spring 30, the upper position being set by tangs 28c on the keystem. The saturable cores may be of a barium ferrite, and each carries primary and secondary windings 36, 38 whose coupling is varied by the position of the permanent magnets. Each core may have several secondaries. The magnets are separated by non-magnetic spacers 17, 19, 23 and 25, which cause the coupling between the windings to vary as in Fig. 3, when the key is depressed. This Figure plots the output obtained from the winding 38 of each core when its other winding 36 is fed with A.C. Initially both cores are saturated and the output is a minimum, the magnet spacing being such that the upper core 12 completes its desaturation first. When the key has been fully depressed the output is again a minimum. The output from the second core 14 is fed to a level detecting circuit (below) having a threshold level 64, and operates a circuit which strobes the output from the first core 12 to utilization apparatus. The polarity of the magnets is given in Fig. 2A (not shown). Coding arrangement.-Fig. 4 shows the four cores associated with the 7 and 9 keys of a keyboard. The cores are fed with A. C. on windings 36, and the cores 12 also provide signals for the output gates 62; the 7 key for example has its core 12, linked by windings which operate the 4, 2 and 1 gates. The cores 14 have only one secondary winding whose output is rectified and operates a level detector 48 which in turn has its output differential to control a monostable pulse generator 54. A gate 56 receives the output from the generator 54 and also a signal from the cores 12, and its output controls gates 62. Suppose the 7 key is depressed; when the level 64 of Fig. 3 is exceeded in line 41 the level detector and pulse generator will provide a signal to gate 56, and since this gate also has a large signal on line 58 it will operate to enable the output gates 62. The level detector will provide a further signal when the key is nearly fully depressed, as the signal on line 41 drops below the threshold level 64, Fig. 3. To avoid spurious outputs the pulse generator must therefore respond only to pulses of one polarity so that it is not operated by this further signal. On release of the key there are-also two signals generated by the level detector; as seen from Fig. 3, when the core 14 returns to the threshold level 64 and gives the first of these signals, the output from core 12 is still very small, and hence gate 56 is not operated. The final signal occurs when the signal from core 14 drops below the threshold 64 (corresponding to a displacement of about À08 inch), but is ignored by the pulse generator 54 because it is of the wrong polarity. The characteristics of Fig. 3 show that when the keyboard is in use one switch may start to be depressed before a previous one is fully released without producing false signals, providing a "roll over" capability for fast operators.