control interface via pc keyboard

Posted on Feb 5, 2014

One of the more difficult aspects when making a control or security system that uses a PC (a burglar alarm using a PC, for example), is the connection of the sensors to the computer. In addition to typically requiring specialist interface expansion boards, the writing of the program that includes interrupts is often also an insurmountable obstacle. But when only a simple system is concerned

control interface via pc keyboard
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consisting of, for example, four light barriers or, if need be, trip wires giving a digital on/off signal when uninvited guests enter, then a much cheaper but nevertheless effective interface is possible. For this interface we use an (old) computer keyboard. This contains as many switches as there are keys. These switches are scanned many times per second in a matrix in order to detect the potential press of a key. The number of columns is usually eight (C0 C7 in the schematic); the number of rows varies for each type of keyboard and can range from 14 to 18 (R0 R17 with the H T82K 28A keyboard encoder mentioned in the example). To each switch there is a single column and a single row connection. The intention of the circuit is that sensor A will push` the letter A, when it senses something. This requires tracing the keyboard wiring to figure out which column and which row is connected to the A key. One of the four analogue switches from the familiar CD4066 CMOS IC is then connected between these two connections; that is, in parallel with the mechanical A key on the keyboard. When the Control-A input of the CD4066 is activated by sensor A, the letter A will be sent to the computer by the key-board. The PC can then act appropriately, for example by entering the alarm phase. The system is not limited to (burglar) detection using a PC. The remote control of a TV set or other electronic devices can also be operated with a 4066...

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