Hello,
as part of my still ongoing CarDuino project, I would like to measure certain thermistor voltages (oil and coolant temperature) on my car, so I can later use that data with the actual CarDuino to calculate those temperatures.
For this purpose, I've built a "rough and ready" voltage meter on a breadboard which I want to connect to my car to get that data. I've got a laptop with OBD software which will show me the actual temperatures, so I will just have to draw a graph with my measured voltages and the corresponding temperatures shown by the OBD software.
Note: My car has OBD I, so I have to measure most gauges out myself and can't simply tap into the on-board electronics via OBDII to get those temperature values. The car's OBD I protocol is Rover/MG specific, so even if I was able to read out the hex values from the OBD, I wouldn't know what they meant, because very little about Rover's OBD I protocol is known.
Anyway, here's the schematic. Note that for the time being, I've hooked this circuit up to a rudimentary 12V switchboard with which I can control two channels with adjustable voltages.
And here's my sketch so far:
#define input1 A3
#define input2 A1
#define led 9
#define refVoltage A0
// defining input variables
int input1Value;
int input2Value;
int aRefValue;
// defining voltage variables
float input1Voltage;
float input2Voltage;
float aRefVoltage;
// Turning analog inputs into voltages
float makeVoltage(int inputValue) {
// calculation formula to turn analog value 0-1023 into voltage
// based on using 22K / 10K voltage dividers
float doMakeVoltage = (float) ((160 * inputValue / 1023) / 10);
return doMakeVoltage;
}
void setup() {
// With external reference,
// no readings on serial at all.
// Therefore, commented out at the moment.
// analogReference(EXTERNAL);
Serial.begin(115200);
pinMode(input1, INPUT);
pinMode(input2, INPUT);
pinMode(led, OUTPUT);
pinMode(refVoltage, INPUT);
}
void loop() {
// Reading analog input and calculating values
// input channel 1
input1Value = analogRead(input1);
input1Voltage = makeVoltage(input1Value);
// input channel 2
input2Value = analogRead(input2);
input2Voltage = makeVoltage(input2Value);
// ... and the car's current operating voltage
aRefValue = analogRead(refVoltage);
aRefVoltage = makeVoltage(refVoltage);
// Blink status LED
digitalWrite(led, HIGH);
delay(100);
digitalWrite(led, LOW);
// Printing out the readings
Serial.println("Input channel\t\tvalue\tvolts\n");
Serial.print("Input 1:\t\t");
Serial.print(input1Value);
Serial.print("\t");
Serial.println(input1Voltage);
Serial.print("Input 2:\t\t");
Serial.print(input2Value);
Serial.print("\t");
Serial.println(input2Voltage);
Serial.print("ext. voltage:\t\t\t");
Serial.println(aRefVoltage);
Serial.print("\n\n\n");
// wait 2 seconds before reading inputs again
delay(2000);
}
The problem is that right now, it's not exactly giving me what I want. I get weird readings on serial, and I also can't show the car's current operating voltage.
What I want the sketch to do is show me the value of the analog reading (0-1023) as well as the calculated voltage that those readings represent.
I'm not sure yet if it's best to measure the voltages against the Arduino's 5 volts, or against the car's current operating voltage using analogReference(EXTERNAL). What I'm really after is the changing resistance of the thermistors, but that can only be measured for my purposes as a change of voltage, can't it... but then the problem is that at a given resistance, I will get different voltages between when the engine is off, and when the engine is on with the alternator running.
Any help is appreciated.
- carguy
EDIT: The first schematic I posted had an error in it with the voltage dividers. I've updated that now.
Which would explain why the external voltage is displayed as 16 volts. In reality, according to my multimeter, I've only got 11.8V on my switchboard.
