GPIO: Displaying currency exchange rate on 7-segment indicators

The Hardware

• Configure the LTPS as shown on the Tibbit layout diagram below.

• Assemble the shift register chain and 7-segment indicators according to the wiring diagram below (wires connecting the 2nd, 3rd, and the 4th resistor blocks to their respective indicators are not shown).

Note: When the power is first applied the indicators may display a random pattern.

Wiring diagram (click to enlarge)

Tibbit configuration (click to enlarge)

74HC595 pinout

Node.js Application

• The app uses the Request package to fetch data from fixer.io, the Express package to serve static files, and socket.io to facilitate a link between the onboard app and the web interface.
• The application requests USD exchange rates for a number of currencies. Requests are made every ten minutes. Fixer.io updates the rates daily, around 4 pm CET.
• By default, the USD/EUR rate will be displayed on the indicators.
• The App's web server listens on port 3000.

Configuration and Installation

• Define the required configuration in the LTPS Web Interface
• Login to the LTPP3 board from the SSH client
• Install NPM and other tools as needed.
• Install the app:

git clone https://github.com/tibbotech/gpio-indicators.git
cd gpio-indicators
npm install .

• Launch:

node rates

Controlling shift registers

The code that controls 7-segment indicators is found in /modules/indicate.js.

Comments in the code explain how it works:

const gpio = require("@tibbo-tps/gpio");

class indicator {
    constructor(socket, length){
        this.length = length;


        this.digits = {
            1: [0,1,0,0,1,0,0,0],
            2: [0,0,1,1,1,1,0,1],
            3: [0,1,1,0,1,1,0,1],
            4: [0,1,0,0,1,0,1,1],
            5: [0,1,1,0,0,1,1,1],
            6: [0,1,1,1,0,1,1,1],
            7: [0,1,0,0,1,1,0,0],
            8: [0,1,1,1,1,1,1,1],
            9: [0,1,1,0,1,1,1,1],
            0: [0,1,1,1,1,1,1,0],
            N: [0,0,0,0,0,0,0,1], // Dash symbol
            B: [0,0,0,0,0,0,0,0] // Blank symbol
        };

        // Sets up pins
        this.dataPin = gpio.init(socket+"A");
        this.dataPin.setDirection("output");
        this.dataPin.setValue(0);

        this.clockPin = gpio.init(socket+"B");
        this.clockPin.setDirection("output");
        this.clockPin.setValue(0);

        this.latchPin = gpio.init(socket+"C");
        this.latchPin.setDirection("output");
        this.latchPin.setValue(0);
    }

    indicate(number){
        var inst = this;

        // Converts number to the array of signals to be sent
        const numberToSignals = function(number){
            var output =[];
            number
                .toString()
                .split("")
                .forEach(function(current, index, array){
                    if(current !== "."){
                        var symbol = inst.digits[current];
                        if (symbol === undefined){
                            symbol = Array.from(inst.digits["N"])
                        }else if(array[index+1] === "."){
                            symbol = Array.from(symbol);
                            symbol[0] = 1;
                        }
                        output.unshift(symbol);
                    }
                },[]);

            // crops number to the first "length" digits, if needed
            output = output.slice(-inst.length);

            // pads the number with spaces if it's shorter than 4 digits
            while (output.length < inst.length){
                output.push(inst.digits["B"])
            }

            return output.reduce(function(prev, current){
                return prev.concat(current)
            });
        };

        var signals = numberToSignals(number);

        // Sets ST_CP (latch) to LOW
        // This operation sets shift registers into "programming" mode.
        // When latch is LOW, shift register doesn't change output states,
        // but reads and "remembers" data from DS pin.
        inst.latchPin.setValue(0);

        signals.forEach(function(value){
            // sets value to be pushed into the register on DS pin
            inst.dataPin.setValue(value);

            // sets SH_CP (clock) to HIGH and then to LOW
            // on rising edge of the clock shift register reads state from DS pin, and prepares it for setting on Q0 output.
            // Each of the previously SET values will be shifted to the next pin.
            inst.clockPin.setValue(1);
            inst.clockPin.setValue(0);
        });

        // then all signals are sent, sets ST_CP (latch) to HIGH
        // If latch is HIGH, all the read values will be simultaneously set to the outputs.
        inst.latchPin.setValue(1);
    };
}

module.exports = indicator;

Web interface (click to enlarge)

Web Interface

The web interface files are found in the -/static folder.

• The web interface app requests data from fixer.io independently from the onboard app.
• The Angular toolset is used to display the exchange rates table and provide interactivity.
• The Socket.IO library is used to detect the board's status (the table will be hidden if the board is offline) and send the currency data to the board.