Arduino UNO Ohmmeter 


Using a Wheatstone Bridge configuration. We will calculate the 
resistance of an unknown resistor using Kirchoff's Laws.

Part (1) Voltage Divider
Part (2) Calculating for Unknown Resistance
Part (3) Finished Product with Program

This example use the following:
(1)  Arduino UNO
(1)  External Battery Pack with (3) 1.5 Volt AA Batteries
(3)  10k Resistors
(1+) Test Resistors
(1)  Software program capable serial port and ANSI emulation.

[NOTE] All electrical circuits must be handled with caution. 
       Also I'm just an amateur trying to learn theory not 
       a certified electrician. So if you make this circuit
       and are not certified. You are also an amateur trying
       to learn theory. Thus if you hurt yourself or your 
       property from this post by proceeding to read or 
       carry out actions to create the circuit that makes 
       you in agreement that you're fully responsible for
       your actions. 


Part 1 [Voltage Divider]

Using a voltage divider R1 and R3. [Note R1 and R3 must be the same]
We can get the voltage of a external battery.

Read A0 Pin * 2 = external battery voltage (Reference between R1 and R3)

Example (1):

A0 reads 2.25 volts

2.25 * 2 = 4.5 external battery voltage.

With this arrangement of a voltage divider even if the external battery
falls to 4.4 volts we can still get a good reading from our circuit.

Example (2): 

A0 reads 2.2 volts

2.2 * 2 = 4.4 external battery voltage.


Part 2 [Calculating for Unknown Resistance]

Using the Voltage Divider in a series circuit we discovered the external
battery voltage from the left bridge. Now we will create right bridge. 
The right bridge it will consist of R2 a known resistance. From the
right bridge and Kirchhoff's law of resistance we will determine the 
resistance of an unknown resistor at R4 or Rx using this formula:

A1 = voltage read from A1 pin 

(Rx)ohms = 10k * A1 / (A0 * 2) - A1

Example (3)
     Rx = 100 ohm (we don't know that yet but we are finding for)
     A0 = 2.25 V    (2250 mV)
     A1 = 44.55 mV  (44.55 mV)
     A0 * 2 = 4.5   (4500 mV)
     A1 * 10000 =   (445500) 
     Rx = 445500 / 4500

     Rx = 99 ohms

Part 3 [Finished Product with Program]

I know this project has some weakiness yet to be discovered by myself
or more likely others. However for the most part it does show how 
voltage and resistance measurements can take place using a Wheatstone 
Bridge. Allowing for other sensors to operate like a strain gauge,
light sensitive diode, temperature sensor and much more. 

For the software used mostly Hyperterm and VT100J emulation.

However using Putty Translation (CP866 manually type it in)
with font based OEM and ANSI.

Anyone willing be to offer some *constructive* criticism to make the
circuit more accurate is welcome.

Attached is the sketch for the microprocessor on the uno board:
Here is the raw code:

// start copy here
#define A0IN 0
#define A1IN 1

int analogamount = 0;  // used to convert voltage left bridge
int analogamount1 = 0; // used to convert voltage right bridge

float voltage =  0; // left bridge voltage
float voltage1 = 0; // right bridge voltate
float voltage2 = 0; // used for voltage correction 

float answer = 0;
int cv = 0;       // counter variable used screen drawing

void setup() {
   analogReference(EXTERNAL);   // use AREF for reference voltage
   Serial.begin(9600);
   set_screen();
}

void loop() {
  
  analogamount = analogRead(A0IN);
  analogamount1 =analogRead(A1IN);
   
  voltage=analogamount * (4500 / 1024.00); 
  
  voltage1=analogamount1 * (4500 / 1024.00); 
  
  voltage2 = 0;

set_xy(23,1); 
save_cursor_pos();

if(voltage == voltage1) { 
  set_xy(10,25);
  repeat_char(10,0x20); 
  set_xy(10,47);
  repeat_char(10,0x20); 
  set_xy(14,49);
  repeat_char(10,0x20);

} else {

set_xy(10,25);
voltage = round(voltage);
voltage = voltage * .001;
Serial.print(voltage);
if( voltage < 1) { Serial.print("mV"); } else { Serial.print("V ");}

set_xy(10,47);
voltage1 = voltage1 * .001;
Serial.print(voltage1);
if( voltage1 < 1) { Serial.print("mV"); } else { Serial.print("V ");}

set_xy(14,49);
voltage2 = voltage * 2;
answer =   10000 * voltage1 / (voltage2 - voltage1);
answer = round(answer);
Serial.print( (int) answer+1);  
Serial.print(" Ohms     ");  
}

restore_cursor_pos();
delay(3000);
}

void save_cursor_pos(){
   Serial.print("\x1B" "[s"); //  save cursor position     
}

void restore_cursor_pos(){
   Serial.print("\x1B" "[u"); //  restore cursor position     
}

void repeat_char(int r,char c){
  int rx;
  for(rx = 0; rx != r; rx++){
    Serial.print(c);
  }
}

void set_xy(int x, int y){
  Serial.print("\x1B[");
  Serial.print(x);
  Serial.print(";");  
  Serial.print(y);
  Serial.print("H");  
}

void set_screen(){
   Serial.print("\x1B" "[2J"); 
   set_xy(1,2);
   Serial.print("Ohmmeter");
   set_xy(3,2);
   Serial.print("\xDA");
   repeat_char(20,0xc4);
   Serial.print("\xC2");
   repeat_char(20,0xc4);
   Serial.print("\xBF");
   set_xy(4,2);
   for(cv = 0; cv != 2; cv++){
      Serial.print("\xB3");
      repeat_char(20,0x20);
      Serial.print("\xB3");
      repeat_char(20,0x20);
      Serial.println("\xB3");
      Serial.print(" ");
   }      
   set_xy(6,2);
   for(cv = 0; cv != 3; cv++){
      Serial.print("\xB3");
      repeat_char(20,0x20);
      Serial.print("\xB2");
      repeat_char(20,0x20);
      Serial.println("\xB2");
      Serial.print(" ");
   }   
   set_xy(7,25);
   Serial.print("R1 (10K)");
   set_xy(9,1);
   Serial.print("\xC4\xC1\xC4");
   repeat_char(19,0x20);
   Serial.print("\xB3");
   repeat_char(20,0x20);
   Serial.print("\xB3");
   set_xy(10,1);
   Serial.print("\x1B[10;1H" " \xC4 ");
   repeat_char(19,0x20);
   Serial.print("\xB3");
   repeat_char(20,0x20);
   Serial.print("\xB3");
   set_xy(11,1);
   Serial.print("\xC4\xC4\xC4");
   repeat_char(19,0x20);
   Serial.print("\xB3");
   repeat_char(20,0x20);
   Serial.print("\xB3");
   set_xy(12,2);
   Serial.print("\xC2 ");
   repeat_char(19,0x20);
   Serial.print("\xB3");
   repeat_char(20,0x20);
   Serial.print("\xB3");
   set_xy(13,2);
   for(cv = 0; cv != 3; cv++){
      Serial.print("\xB3");
      repeat_char(20,0x20);
      Serial.print("\xB2");
      repeat_char(20,0x20);
      Serial.println("\xB2");
      Serial.print(" ");
   }
   set_xy(16,2);
   for(cv = 0; cv != 3; cv++){
      Serial.print("\xB3");
      repeat_char(20,0x20);
      Serial.print("\xB3");
      repeat_char(20,0x20);
      Serial.println("\xB3");
      Serial.print(" ");
   }
   set_xy(18,2);
   Serial.print("\xC0");
   repeat_char(20,0xc4);
   Serial.print("\xC1");
   repeat_char(20,0xc4);
   Serial.print("\xD9");
   set_xy(7,46);
   Serial.print("R2 (10K)");
   set_xy(14,25);
   Serial.print("R3 (10K)");
   set_xy(14,46);
   Serial.print("Rx");   
}
// end copy here