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 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.
Circuit diagram :
Control Interface via PC Keyboard Circuit Diagram
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 in the same way; for example to scan through a number of TV channels in a cyclical fashion. To do this, you could, for example, shunt the ‘next channel’ button using one of the 4066 switches, which itself is activated by a 1-Hz square wave generator.
In the schematic only switches A and B of the CD4066 are connected to the keyboard. You can, of course, use all four of the switches and if you need more than four you can use multiple CD4066 ICs. The indicated wiring between the keyboard IC and the 4066 is an example only, and each ‘typed’ letter has to be determined by the user for the specific keyboard that is used. It is important that each CD4066 switch is always connected between a row- and a column connection. The output signal from the sensors has to be suitable for the CD4066 and the power sup-ply voltage of 5 volts used by the keyboard. The power supply for the CD4066 may be obtained from the keyboard.
Author : Jacob Gestman Geradts - Copyright : Elektor
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