|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
Membrane keyboard. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer Everyone who has ever been involved in the creation of equipment with a large number of switching elements knows how complex and low-tech it is. So. on the panel of a modern tuner-amplifier, the number of switches reaches a dozen, and in electronic examiners, automatic generators of telegraph code signals and consoles of personal computers, it often reaches several tens and even hundreds. Creating a compact, reliable and easy-to-manufacture keyboard is a significant challenge. Meanwhile, there are very simple designs of switching units that can significantly simplify the manufacture of the keyboard. One such design is the so-called membrane keyboard. It consists of. three main elements (Fig. 1): substrate 1, gasket 2 and metallized membrane 3. The substrate is a printed circuit board on which fixed contacts are formed. Movable contacts are formed by metallization on the membrane 3, made of thin - 0,1...0,2 mm - dielectric (for example lavsan) metallized film. The entire structure is fixed by a clamping frame 4 made of sheet dielectric or metal.
Key markings or corresponding pictographic signs are applied to the outer side of the membrane. A gasket with holes under each key will be placed between the substrate and the membrane, allowing the movable and fixed contacts to close when the membrane is pressed. The thickness of the gasket that determines the gap between the contacts is usually chosen in the range of 0,3 ... 0,8 mm. The gasket can be made from any insulating sheet material. Such a keyboard is characterized by a closing force of about 0,5 ... 2 N. contact resistance 0,1 ... 50 Ohm; it agrees very well with the electronic control units of the equipment. As can be seen from the figure, the keyboard can be made very thin (less than 2 mm) and, if necessary, glued to the front panel of the device. Sealed design of the keyboard ensures the reliability of the contact groups in various operating conditions. Although the contactor can consist of independent contact pairs, its advantages are most pronounced with matrix addressing of keys, when the metallization on the membrane and substrate is made in the form of stripes-lines common to several contacts at once. Consider the features of an alphanumeric membrane keyboard designed to introduce a standard set of characters into a telegraph signal generator or microcomputer. The keyboard has 79 pairs of contacts and, together with the electronic unit - keyboard controller - generates a standard seven-bit binary code of Russian and Latin alphabet characters, as well as service character codes in accordance with the KOI-7 table. To control the correctness of the transmission, the controller generates one bit of the addition of the number of bits to an even number. A drawing of a printed circuit board-substrate made of foil fiberglass with a thickness of 0,5 ... 2 mm is shown in fig. 2, a. The location of the keys and the distance between the centers of the key pads in a row and between rows is best chosen close to standard. In addition to the keyboard pads, there are square pads on the edge of the board, through which the membrane conductor lines are led out in the assembled contactor. The membrane in the area of square areas is tightly pressed against the substrate.
The membrane is cut from an aluminized lavsan film 52 µm thick. With a solution (10%) of caustic soda, using a brush, excess metallization is etched from the film and only line conductors are left (shown in black in Fig. 2, b). The gasket with a total thickness of about 0,2 mm is made of two layers of flat phototechnical film. The gasket has round holes with a diameter of about 18 mm. Under the elongated keys ("Space", etc.), the holes in the gasket are made in the form of slots. The width of the spacer should be such that it covers only the field of keyboard (round and rectangular) pads on the substrate. Key markings can be applied to the outer side of the membrane, protecting it with an additional layer of transparent lavsan film. Sticky film for pasting book covers is suitable for this purpose. The keyboard parts are superimposed one on top of the other, aligned and compressed into a package by a frame, under which a strip of foam rubber 1 ... 2 mm thick is laid. In this case, the membrane conductors are connected to the square pads of the substrate. Mounting pads with holes are provided on the substrate to connect the keyboard to the electronic unit. To reduce the oxidation of contacts during operation, it is desirable to assemble the keyboard in a dry room. Before assembly, the working surface of the substrate should be polished with abrasive paste or chalk, thoroughly rinsed with ethyl alcohol or acetone, and, if possible, the pads should be coated, for example, with Wood's alloy. Small unevenness of the membrane can be corrected by heating the assembled keyboard to 100...150 °C in an oven. To seal around the perimeter of the assembled keyboard, you can apply Elastosil glue or SB-1 silicone paste. The codes of the symbols depicted on the keys are generated by the controller (its diagram is shown in Fig. 3), which sequentially interrogates all the keys at a frequency of about 80 Hz. To do this, the controller provides a counter DD2, DD3, counting the pulses of the clock generator, collected on the Schmitt trigger DD1.1 and operating at a frequency of about 20 kHz. The number written in the counter determines the address of the key in the keyboard matrix, i.e. the number of the horizontal (connected to one of the inputs A-E of the DD6 multiplexer) and vertical (connected to one of the outputs 0-15 of the DD5 decoder) lines, at the crosshairs of which there is a closed pair of contacts of the pressed key.
To poll the keyboard, the decoder of the four least significant bits of the address DD5 alternately sets a low level on one of the lines of the keyboard membrane, and the multiplexer DD6, in accordance with the value of the three most significant bits of the address, connects one of the lines of the substrate to the input S of the trigger DD4.2. If a pair of contacts, the address of which is recorded in the counter, is open, the output of the multiplexer will be set to a high voltage level, therefore, the state of the trigger will not change. As soon as a closed pair of contacts is found during the polling process, a 6 signal will appear at the direct output of the DD0 multiplexer, which will set the DD4.2 trigger to a single state. At the same time, in the current polling cycle, capacitor C1, charged to the power supply voltage, will discharge through transistor VT4. At the same moment, the buffer register DD8 remembers the code corresponding to the pressed key [1]. To convert the address of a key into a standard code, a permanent memory device DD7 with burnt jumpers was used [2]. It stores a table of correspondence between the key address coming from the keyboard controller counter, the KOI-7 code and the value of the parity bit. The use of ROM for transcoding allows you to connect keys in the matrix arbitrarily, based on ease of installation. As soon as the DD4.2 trigger is set to state 1, a low voltage level at the DS0 input of the DD8 register will allow the key code to be written to it. After writing the code, a high level will appear at the INT output of the DD8 register - the OBF signal - signaling the need to transfer the code from the keyboard controller to the information receiver device. In turn, the information receiver reads the key code via the DO-D7 lines and, upon completion of the operation, issues an "Accepted" pulse to the controller, indicating the possibility of receiving the next code. This kind of asynchronous information exchange is called handshake exchange. In order to prohibit changing the code at the output of the controller until it is read by the receiver, the low level of the "Ready" signal is fed through the diode VD2 to the input of the inverter DD1.2 and does not allow the next code of the pressed key to be accepted until the information receiver responds with the STR signal ( "Received"). The method of dealing with the "bounce" of contacts in the controller is completely identical to that described in [3]. As already mentioned, the key code table is stored in the EEPROM. To simplify the formation of codes for the upper and lower registers of the keyboard in the storage device, there are two areas (pages) selected by the value of the bit address A7, i.e., the state of the trigger DD4.1. The first of them contains a table for uppercase characters, and the second one for lowercase characters. The trigger switch occurs after pressing the HP and BP keys, respectively. The keyboard has function keys 1-16 and cursor keys, the codes of which can be assigned when programming (burning) the PROM. For burning, you can use a hand-held programmer [4], in which you should remove the capacitor shunting the power outputs of the programmable microcircuit, and increase the number of switches that set the address to eight. In addition to those mentioned, the keyboard controller can generate special control codes within 00H-1FH, while pressing the "U" key and one of the alphabetic keys. In this case, the code table for the keys is switched by bit A8 of the PROM. In conclusion, it should be noted that the membrane keyboard, made in amateur conditions according to the described technology, has a relatively low wear resistance due to the extremely thin aluminum coating of the membrane, therefore, during intensive use, the membrane has to be replaced periodically. Literature
Author: D. Lukyanov, Moscow; Publication: N. Bolshakov, rf.atnn.ru
A New Way to Control and Manipulate Optical Signals
05.05.2024 Primium Seneca keyboard
05.05.2024 The world's tallest astronomical observatory opened
04.05.2024
▪ Livescribe 3 pen for digitizing handwritten notes ▪ New camera series from CANON ▪ Electric capsule to stimulate the stomach and improve appetite ▪ New camouflage material against thermal cameras
▪ section of the site Videotechnique. Article selection ▪ Article Fearing the Abyss of Wisdom. Popular expression ▪ How much does a tridacna clam shell weigh? Detailed answer ▪ article Blueberries are undersized. Legends, cultivation, methods of application
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page