Lilypad + Temperature Sensor

Other Hardware Sensors
1

Hello!

I have a Arduino LilyPad ATmega328V with a temperature sensor MCP9700. I have a correct read of temperature when the LilyPad is connected by USB FTDI of fixed 3.3V, because when I read the value of analog pin, I do this: lectura = lectura * (3.3/1024.0); But the problem occurs when I supply the LilyPad with a LilyPad Power Supply (AAA battery) (LilyPad Power Supply - DEV-08466 - SparkFun Electronics), because I think it does not supply a fixed 5V. It has fluctuation, and probably supply less than 5V because of the resistence of wire and other factors.

So I want to know the real power supply that the LilyPad is receiving, in order to do correctly this: lectura = lectura * (powersupply/1024.0);

I have read something about internal reference of 1.1V, but I dont know how to use it.

I am an amateur in Arduino, so I will very grateful of all helps. Thanks!

2

The wire thread designed for use with fabric and lilypads has a high resistance I believe, so yes you might get poor voltage regulation this way. Using analogReference(EXTERNAL) and feeding 3.3V to the AREF pin could be a solution? Can't remember if the lilypad brings out AREF to one of the pads though.

3

I thought use the internal reference of 1.1V in order to do a rule of three like that:

#define BANDGAPREF 14
int refReading = analogRead(BANDGAPREF);
float supplyvoltage = (1.1 * 1024) / refReading;

But the result is a power supply of ~15V, and this is impossible!

What am I doing wrong?

4

I have could decrease the fluctuation. Now, with the battery supply, I am getting values between 56-63. But these should be around 22-23 degrees. With the FTDI USB, that supply 3.3V, I do: read = read * (3.3/1024.0); and I get the correct 22-23. The battery supply provides 5V. In adittion, I am using the internal reference with the rule of thrre I show in the last post. It seems to be that the ADC is not using Vcc as the reference, why?
Thanks!

5

I show the code I am using (I use a bluetooth mate gold in order to transmit the value to an Android Aplication, so I only transmit a value of byte because the degrees measured wont be higher than 255, and bluetooth only write a byte).

//LilyPad + Temperature Sensor

int tsensorPin = 2; // Temperature sensor is connected to analog pin 2.
int tempValue; // Variable to store the value coming from de temperature sensor.

#define BANDGAPREF 2 // Special indicator that we want to measure the bandgap.

float lectura=0;
float supplyvoltage;
int refReading;
float temp;

void setup()
{
Serial.begin(115200);
}

void loop()
{

//Read the temperature.
lectura = analogRead(tsensorPin);

//Get voltage reading from the secret internal 1.1V reference
refReading = analogRead(BANDGAPREF);

//Now calculate our power supply voltage from the known 1.1V reference.
supplyvoltage = (1.1 * 1024.0) / refReading;

lectura = lectura * (supplyvoltage/1024.0);
temp = (lectura - 0.5) * 100;
tempValue = (byte)temp;

Serial.write(tempValue);

//Delay
delay(1000);
}