Rab18’s Weblog

The next stage of the process involved connecting the picaxe project board to the protocircuit. The protocircuit is powered via the project board’s 4.5v battery pack.  A positive and ground is wired to the protocircuit from the project board. The four switches that connect to ground will be replaced with out- put pins 4,5,6,7 from the project board. Out-put pins 0,1,2,3 from the project board are connected to pins 6, 8,9,12 of the four digit display.  Now I know this circuit works I can start designing the layout of my PCB . It also allows me to start experimenting with programming.

The four inputs a, b, c and d on the 4511 connect to ground via four black cables (these can be seen in the previous entry) this gives the inputs a low reading. When these cables were removed the voltage pulls up through the 10k resister to the 4.5v and gives a high reading (1). Using these cables I was able to work out using a truth table how to count 0 to 9.

This protocircuit setup has moved on from the last protocircuit. On the previous circuit I was able to display three digits and count 0 to 9. The problem I had with this circuit was the numbers were not being displayed clearly.  Other LED’s were lighting up causing the display to read unclear numbers. This is due to the electrons building up at the field effect transistor. This can be rectified by using 10k pull up resistors.

On this protocircuit I have used 10K pull up resistors on each of the inputs to the 4511 BCD. This enables the electrons that have not past through the field effect transistor to pass up through the 10K resistors. When the voltage to the inputs is connected to ground it give no pulse (0), when the connection to ground is disconnected it pulls up through the 10K resistors to the 4.5v giving the inputs on the 4511 a positive charge (1). This gives a much clearer display of the numbers.

Initially the aim is to just get some numbers displayed on the four digit seven segment display. Using the circuit diagram from circuit wizard I was able to setup this protocircuit (See link for more info on protoboard). I also used the data-sheet for the four digit seven seg display. This enabled me to identify what each of the 12 pins on the display do. Using this data-sheet and the information I already had I was able to calculate what resistors were required between the seven pins from the 4511 and the seven pins on the display.

Calculations for Resistors.

IF=20ma    Max  Voltage 2.5  V/I = R   2.0/0.02= 1oo ohms resistor.

There is a 10% forgivness with this sum, so I selected 120ma resistors between the seven outputs of the 4511 and the seven pins of the display, the other five pins are connected to ground. These pins power the four digits and the dots between the numbers.

The 4511 is designed to be used with a common cathode display. The aim of this circuit is to count 0 to 9 on each of the four digits of the display.  The diagram from circuit wizard demonstrates the possibility of using four switches as a means to input the 4511. An open switch represents 1 and a closed switch represents 0. Using circuit wizard as a means to experiment it allowed me to formulate a truth table. This can be used to identify the inputs needed for 4511 to display each number.  

Using this truth table as guide I was able to send high (1) and low (0)  binary code to 4511 BCD which then displayed the numbers on the display. I was able to do this using wire switches on the protocurcuit.