4 replies to this topic

[Mockster]

Hi Everyone, I'm building my first Weather Station data logger using a Netduino Plus. So far I have a TMP36 temperature sensor and a MPX4115A air pressure sensor connected to the Netduino. When I feed the sensors 3.3 volts both sensors respond with expected and what seems to be accurate results. However, when I feed both sensors 5 volts from the Netduino the analog input that the pressure sensor is connected to displays unexpected results. For example the Debug.Print statement of the raw GPIO_PIN_A1 is 1023 which is what I would expect if the sensor is outputing 5 volts. My multimeter shows that the output from the pressure sensor is only 4.12 volts. The pressure sensor seems to work better with the lower input voltage, but it's data sheet states that the sensor input voltage should be 5 volts (4.85v min and 5.35v max). Also, I get the same result when switching to another analog input (GPIO_PIN_A2). The analog input that the temperature sensor is connected to does not change with the switch to 5 volts and so there is no issue. Interestingly, when I connect the same two sensors to an Arduino Duemilanove and feed them 5 volts from the board, there is no issue. The 5 volt output voltage from both microcontrollers is nearly identical - 5.05v Netduino and 5.02v Arduino so I don't think the problem is with the sensor input voltage. And the fact that my multimeter shows nearly identical output voltage from the pressure sensor with 5 volts input would seem to reinforce this (4.12v Netduino and 4.09v Arduino). Here are two examples of what I'm seeing when feeding pressure sensor 5 volts from each micro: Netduino - raw sensor reading from analog input pin is 1023, measured sensor output is 4.12 volts Arduino - raw sensor reading from analog input pin is 835, measured sensor output is 4.09 volts Any ideas why the Netduino is converting the 4.12 volts into 1023? Thanks in advance.

[CW2]

Any ideas why the Netduino is converting the 4.12 volts into 1023? Thanks in advance.

Netduino analog input voltage has to be in range 0 - 3.3V, resp. 0 - Aref (Aref is connected internally to 3.3V on Rev.B boards). Anything above 3.3V reads as 1023. But you should not connect 5V to analog inputs, they are not 5V tolerant (unlike when configured as digital I/O), you'd probably need to use a voltage divider for that.

[Mockster]

Netduino analog input voltage has to be in range 0 - 3.3V, resp. 0 - Aref (Aref is connected internally to 3.3V on Rev.B boards). Anything above 3.3V reads as 1023. But you should not connect 5V to analog inputs, they are not 5V tolerant (unlike when configured as digital I/O), you'd probably need to use a voltage divider for that.

That explains it! Thank you. I obviously did not know that. That makes using sensors that output higher than 3.3 volts slightly more difficult. As you said, I guess I need to scale the voltage down using a voltage divider.

Thanks again!

[scardinale]

What formula are you using to convert the Alalog output of the MPX4115A to a presure reading?

[Fahdil]

What formula are you using to convert the Alalog output of the MPX4115A to a presure reading?

 

it's explained on datasheet. or you can refer to sensitivity of the sensor to get millibar or pascal.