Programmable number lock system

System:

In this section we will be explaining the hardware design these units individually. Each unit description is accompanied by its circuit diagram.

KEYBOARD UNIT :

The keyboard used in this system is an encoder type. It uses IC 74147 priority encoder to convert the entered the digits to its corresponding binary codes. IC 74147 has a active low inputs as well as active low outputs (for more details please refer the corresponding data sheets). Each of the pins are activated using push to on switches. These switches ground the encoder inputs to activate that particular number giving the equivalent BCD code. This is then given as input to IC 7404 which is an inverter. To avoid loading of 7404 by the later stages transistor switches are provided to each of its outputs. The circuit diagram of the keyboard is as given in the next section.

DISPLAY UNIT :

The logic used here is shift and display i.e. when the next number is entered the previous number is shifted to the left by one unit and then the next number is displayed. To achieve this we have used IC 7496 which is a 5–bit shift register (for more details please refer the corresponding data sheets). The data lines from control section are given serially to the registers as shown in the circuit. The clock to these registers is given by the program every time a button on the keyboard is pressed. The shifting used here is parallel shifting. The outputs of the registers are given to CD4511 BCD to seven segment decoder. The seven segment display used are common cathode type since the output of the decoder is high when activated. This section consists of a reset switch which is used to reset the display.

Control unit:

The program codes for this project is given in the next page