Build a Digital Voltmeter

Introduction

In this example you combine the MCP3008 ADC with a 4-digit 7-segment display module to build a digital voltmeter. You connect the MCP3008 ADC to the Raspberry Pi hardware using SPI peripheral. You connect the 4-digit 7-segment display to the Raspberry Pi hardware using the I2C peripheral. A potentiometer connected to one of the analog inputs of the MCP3008 ADC provides a variable input voltage. You measure this voltage and display it on the segmented display.

You can replace the potentiometer with an analog temperature sensor or a photo-resistor to build a digital thermometer or a digital light sensor.

Prerequisites

It is helpful to complete the following examples

Required Hardware

To run this example you need the following hardware:

Connect MCP3008

Connect MCP3008 to the Raspberry Pi hardware as seen in the following circuit diagram. To simulate a variable voltage applied to CH0, use a 10 kOhm potentiometer connected to CH0.

The potentiometer (POT), in this example, is a three-terminal device with terminals 1 and 3 comprising the end points of a resistor embedded in the POT. The second terminal is connected to a variable wiper. As the wiper moves, the resistance values across terminals 1 and 2 and terminals 2 and 3 change. In this circuit, POT acts as a variable voltage divider. As you move the knob of the potentiometer, the voltage seen at terminal 2 changes between 3.3 Volts and 0 volts.

Connect the Display

To connect the display to Raspberry Pi hardware, you need to do some light soldering. Since you use the I2C interface of the display in this example, solder jumper wires to the SDA, SCL, GND and VCC pins of the display. You can also choose to solder some straight male headers to be able to mount the display on a breadboard for prototyping. Before going any further, follow manufacturer's hardware assembly guide.

Connect the display to the Raspberry Pi hardware following the circuit diagram below.

View Available SPI Channels

Create a raspi object and inspect the AvailableSPIChannels property of the raspi object.

clear rpi
rpi = raspi();
rpi.AvailableSPIChannels

If the SPI peripheral is not enabled, you will see an empty cell array. To enable the SPI peripheral use the raspi-config utility that comes with the Raspbian Linux image. AvailableSPIChannels property of the rpi object should yield 'CE0' and 'CE1' SPI channels listed as available.

Scan I2C Bus

Once the display is connected to the Raspberry Pi hardware, test the presence of the display on the I2C bus.

for i = 1:length(rpi.AvailableI2CBuses)
    scanI2CBus(rpi, rpi.AvailableI2CBuses{i})
end

The scanI2CBus() method scans the given I2C bus and returns a cell array of device addresses on the bus. If you connected the display to Raspberry Pi hardware correctly, the snippet of MATLAB code above should return a device address of '0x71' for one of the I2C buses available on the Raspberry Pi hardware. '0x71' is the factory default address of the display.

Read Voltage

Create an MCP3008 object to read the voltage value at Channel 0 of the ADC.

clear mcp3008
mcp3008 = raspi.internal.mcp3008(rpi, 'CE0');
for i = 1:20
    voltage = readVoltage(mcp3008, 0);
    fprintf('Voltage = %0.2f\n', voltage);
    pause(0.2);
end

Try playing with the POT knob while executing the loop above. You should observe that the voltage value printed on the MATLAB prompt change.

Test the Segmented Display

Create a segmented display object connected to the appropriate I2C bus on the Raspberry Pi hardware.

clear sd
sd = raspi.internal.segmentdisplay(rpi, <I2C bus>, <I2C address>)
clr(sd);  % Clear screen
print(sd, '112');
print(sd, 'dead');
print(sd, 'beef');

Substitute the correct I2C bus and I2C address for your segmented display.

Display Voltage Measurements

Read the voltage at Channel 0 of the MCP3008 ADC and display it on the segmented display every 0.1 seconds.

delay = 0.1;
adcChannel = 0;
clr(sd);
for i = 1:N
    voltage = readVoltage(mcp3008,adcChannel);
    print(sd,sprintf('%0.0f', voltage*100));
    pause(delay);
end

Summary

This example showed how to use a MCP3008 ADC and a segmented display connected to Raspberry Pi hardware to build a digital voltmeter application.