Secret Diagram

Control Interface via PC Keyboard

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