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 Current Sensor 30 Amp \$8.95 Arduino Pro \$19.95 Seeeduino Mega \$43.00
 This Arduino based current, voltage, and power sensor/meter tutorial was created for hacktronics by Steve Spence. For my off-grid Ham Radio and Solar projects, I needed a way to measure volts, amps, watts, amp hours and watt hours. There's a couple of commercial products that can do this, but not with the flexibility I wanted. I designed a Arduino micro-controller based solution that is very extensible. Right now it monitors the above values of attached gear, and I'm thinking about adding web monitoring and a sd card for data collection. Well, let's get started.Here is the circuit for sensing the battery voltage: The Arduino can accept up to 5v on a analog input. Our battery voltage can range as high as 17vdc in certain charge cycles, so we designed a voltage divider that would provide 5v at 17v battery voltage, and less at various lower voltages. See http://en.wikipedia.org/wiki/Voltage_divider for more information on Voltage Dividers.The code to read that value is as follows:  batVal = analogRead(batMonPin);    // read the voltage on the divider  pinVoltage = batVal * 0.00488;       //  Calculate the voltage on the A/D pin                                    //  A reading of 1 for the A/D = 0.0048mV                                    //  if we multiply the A/D reading by 0.00488 then                                    //  we get the voltage on the pin.   batteryVoltage = pinVoltage * ratio;    //  Use the ratio calculated for the voltage divider                                          //  to calculate the battery voltageI have 4 possible nominal battery bank voltages. Each battery bank has a higher possible charge voltage during certain charge cycles. I've called this max, and will prevent a voltage of over 5v being sent to the Arduino pin during all charge cycles including equalize.Solving for R1R1 = ((R2*Vin)/Vout)-R2with a R2 of 5k ohms, I get the following values of R1 for 4 battery voltages:nominal max R1 R2 Ratio12           17    12  5    2.4024           34    12  2    6.0036           51    12  1.3 9.2348           68    12  0.9 13.33All values are in k ohms.Solve for Vout to make sure Vout never exceeds 5vVout = (R2/(R1+R2))*VinMore details at http://arduinotronics.blogspot.com/2012/04/voltage-monitor.htmlAll parts were obtained from Hacktronics.com.Additional math notes:I measured the voltage at A4 in respect to gnd, and with a 12.46 Vin, I got a 3.52 Vout. I also reported the actual ADC output of the analogRead by printing avgVal to the LCD, and got 778 out of a max of 1023 (0-1024).I then calculated the multiplier for the ADC,ADC Multiplier = 12.46 / 778 * (R1/ R2)Sense that current!ACS715 Current Sensor Board:  The next step is to track the current flowing, or produced by a source. We are using a ACS715 Hall Effect sensor to track the current being passed. // read the analog in value:   sensorValue = analogRead(analogInPin);              // convert to milli amps   outputValue = (((long)sensorValue * 5000 / 1024) - 500 ) * 1000 / 133; amps = (float) outputValue / 1000;  Math alert!!! To calculate watt (volts * amps), amp hours (amps * hours), and watt hours (watts * hours) requires tracking the time component, and performing a bit of math: float watts = amps * batteryVoltage;   sample = sample + 1;  msec = millis();   time = (float) msec / 1000.0;   totalCharge = totalCharge + amps;   averageAmps = totalCharge / sample;   ampSeconds = averageAmps*time; ampHours = ampSeconds/3600;   wattHours = batteryVoltage * ampHours;  Serial output: We can now output the results of the calculations to the serial port using the following code:  Serial.print("Volts = " );                        Serial.print(batteryVoltage);       Serial.print("\t Current (amps) = ");       Serial.print(amps);   Serial.print("\t Power (Watts) = ");    Serial.print(watts);  Serial.print("\t Time (hours) = ");  Serial.print(time/3600);   Serial.print("\t Amp Hours (ah) = ");  Serial.print(ampHours);  Serial.print("\t Watt Hours (wh) = ");  Serial.println(wattHours); LCD Display:  Keeping a computer connected all the time is inconvenient, so I added a lcd display to the project.  lcd.setCursor(0,0);  lcd.print(batteryVoltage);  lcd.print(" V ");  lcd.print(amps);  lcd.print(" A ");   lcd.setCursor(0,1);  lcd.print(watts);  lcd.print(" W ");  lcd.print(time/3600);  lcd.print(" H ");   lcd.setCursor(0,2);  lcd.print(ampHours);  lcd.print(" Ah ");  lcd.print(wattHours);  lcd.print(" Wh ");   All the code: The code, schematics, and photo's along with discussion is available at this URL: #include /* This sketch describes how to connect a ACS715 Current Sense Carrier (http://www.hacktronics.com/Sensors/Current-Sensor-30-to-30-Amp/flypage.tpl.html) to the Arduino, and read current flowing through the sensor. */ LiquidCrystal lcd(7, 8, 9, 10, 11, 12); /* Vcc on carrier board to Arduino +5v GND on carrier board to Arduino GND OUT on carrier board to Arduino A0 Insert the power lugs into the loads positive lead circuit, arrow on carrier board points to load, other lug connects to power supply positive Voltage Divider 11.66 from + to A4 4.62k from A4 to Gnd Ratio 2.5238 */ int batMonPin = A4;    // input pin for the voltage divider int batVal = 0;       // variable for the A/D value float pinVoltage = 0; // variable to hold the calculated voltage float batteryVoltage = 0; int analogInPin = A0;  // Analog input pin that the carrier board OUT is connected to int sensorValue = 0;        // value read from the carrier board int outputValue = 0;        // output in milliamps unsigned long msec = 0; float time = 0.0; int sample = 0; float totalCharge = 0.0; float averageAmps = 0.0; float ampSeconds = 0.0; float ampHours = 0.0; float wattHours = 0.0; float amps = 0.0; int R1 = 11660; // Resistance of R1 in ohms int R2 = 4620; // Resistance of R2 in ohms float ratio = 0;  // Calculated from R1 / R2 void setup() {   // initialize serial communications at 9600 bps:   Serial.begin(9600);   lcd.begin(20, 4); } void loop() {   int sampleBVal = 0; int avgBVal = 0;  int sampleAmpVal = 0; int avgSAV = 0;   for (int x = 0; x < 10; x++){ // run through loop 10x   // read the analog in value:   sensorValue = analogRead(analogInPin);     sampleAmpVal = sampleAmpVal + sensorValue; // add samples together   batVal = analogRead(batMonPin);    // read the voltage on the divider   sampleBVal = sampleBVal + batVal; // add samples together     delay (10); // let ADC settle before next sample } avgSAV = sampleAmpVal / 10;   // convert to milli amps   outputValue = (((long)avgSAV * 5000 / 1024) - 500 ) * 1000 / 133;    /* sensor outputs about 100 at rest. Analog read produces a value of 0-1023, equating to 0v to 5v. "((long)sensorValue * 5000 / 1024)" is the voltage on the sensor's output in millivolts. There's a 500mv offset to subtract. The unit produces 133mv per amp of current, so divide by 0.133 to convert mv to ma           */ avgBVal = sampleBVal / 10; //divide by 10 (number of samples) to get a steady reading   pinVoltage = avgBVal * 0.00610;       //  Calculate the voltage on the A/D pin                                 /*  A reading of 1 for the A/D = 0.0048mV                                     if we multiply the A/D reading by 0.00488 then                                     we get the voltage on the pin.                                                                         NOTE! .00488 is ideal. I had to adjust                                     to .00610 to match fluke meter.                                                                         Also, depending on wiring and                                     where voltage is being read, under                                     heavy loads voltage displayed can be                                     well under voltage at supply. monitor                                     at load or supply and decide. */   ratio = (float)R1 / (float)R2;   batteryVoltage = pinVoltage * ratio;    //  Use the ratio calculated for the voltage divider                                           //  to calculate the battery voltage                                                                                         amps = (float) outputValue / 1000;   float watts = amps * batteryVoltage;       Serial.print("Volts = " );                         Serial.print(batteryVoltage);        Serial.print("\t Current (amps) = ");        Serial.print(amps);    Serial.print("\t Power (Watts) = ");     Serial.print(watts);           sample = sample + 1;    msec = millis();    time = (float) msec / 1000.0;    totalCharge = totalCharge + amps;    averageAmps = totalCharge / sample;  ampSeconds = averageAmps*time;    ampHours = ampSeconds/3600;    wattHours = batteryVoltage * ampHours;    Serial.print("\t Time (hours) = ");   Serial.print(time/3600);     Serial.print("\t Amp Hours (ah) = ");   Serial.print(ampHours);   Serial.print("\t Watt Hours (wh) = ");   Serial.println(wattHours);      lcd.setCursor(0,0);     lcd.print(batteryVoltage);     lcd.print(" V ");     lcd.print(amps);     lcd.print(" A ");     lcd.setCursor(0,1);   lcd.print(watts);   lcd.print(" W ");   lcd.print(time/3600);   lcd.print(" H ");     lcd.setCursor(0,2);   lcd.print(ampHours);   lcd.print(" Ah ");   lcd.print(wattHours);   lcd.print(" Wh ");     lcd.setCursor(0,3);   lcd.print(ratio, 5);   lcd.print("   ");   lcd.print(avgBVal);     // wait 10 milliseconds before the next loop   // for the analog-to-digital converter to settle   // after the last reading:   delay(10);                     }Happy hacking. 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