I am a newbie to Arduino and created my first project. The project is using a Sharp short range sensor and a 7 seg LED board (http://embedded-lab.com/blog/new-version-of-max7219-based-4-digit-serial-seven-segment-led-display/) to count objects.

Everything works fine when on USB power to my computer or if I use a 5v 1A wall adapter with a USB cable. The problem is when I use an external power adapter or use a DC power supply to the Vin. The board powers up and I measured the output voltage on the 5v pin and is measuring around 4.8v.

The Sharp sensor is connected to the A0 Analog in and I see the voltage fluctuate when an object passes, but the LED is not displaying a value when on the external power. The display is connected to GRD, 5v where DIN, CLK, and LOAD pins of the display are connected to pins 7, 6, and 5 as the instructions stated.

I tried two different Arduino boards and several different AC to DC power adapters ranging from 9v to 12v plus used a DC power supply to the Vin and same results.

The code is below and pretty simple (although I am sure there is a lot better way of doing it but this is my first project in C so go easy on me and the function for digits is from a website and not written by me).

If a picture of the project would help I can upload one.

Any ideas or thoughts? Thanks, Steve

#include "LedControl.h"
#include <stdio.h>
#include <math.h>

// Arduino Pin 7 to DIN, 6 to Clk, 5 to LOAD, no.of devices is 1
LedControl lc = LedControl(7, 6, 5, 1);
int sensorpin = 0;      // analog pin used to connect the sharp sensor
int val = 0;            // variable to store the values from sensor(initially zero)
int cnt = 1;

void setup()
{
    // Initialize the MAX7219 device
    lc.shutdown(0, false);   // Enable display
    lc.setIntensity(0, 10);  // Set brightness level (0 is min, 15 is max)
    lc.clearDisplay(0);     // Clear display register


}
void loop()
{


    val = analogRead(sensorpin);       // reads the value of the sharp sensor
    if (val >= 650)
    {
    
    //write values to LED
    if (cnt<=9)
    {
        lc.setDigit(0, 0, cnt, false); // Display 4 to Digit 1-9, " "
    }
    if (cnt >= 10 && cnt <= 99)
    {
        lc.setDigit(0, 0, getDigitFromNum(cnt,0), false); // Display 4 to Digit 1-9, " "
        lc.setDigit(0, 1, getDigitFromNum(cnt, 1), false); // Display 3 to Digit 1-9, " "
    }

    if (cnt >=100)
    {
        lc.setDigit(0, 0, getDigitFromNum(cnt, 0), false); // Display 4 to Digit 1-9, " "
        lc.setDigit(0, 1, getDigitFromNum(cnt, 1), false); // Display 3 to Digit 1-9, " "
        lc.setDigit(0, 2, getDigitFromNum(cnt, 2), false); // Display 2 to Digit 1-9, " "
    }

    delay(400);
    cnt++;
    
}
}

// Function: getDigitFromNum returns a digit at a given index of a integer.
// Goes from right to left with the starting index of 0.
int getDigitFromNum(int num, int digit) {
    num /= pow(10, digit);
    return num % 10;
}
// Function: getDigitFromDec returns a digit at a given index of a double.
// Goes from left to right with the starting index of 0.
int getDigitFromDec(double dec, int digit) {
    dec *= pow(10, digit);
    return (int)dec % 10;
}
// Function: getDigitFromNum returns the decimal values of a double.
double getDecimals(double dec) {
    return dec - (int)dec;
}