IS my design is causing sensor to read wrong?

[donperry]

Ok. My project is going fine thanks to resources like members on this site. I have an issue that I'm trying to solve.

My project reads the Car's Air flow meter voltage (0-5V). For some reason, when i connect my device it causes the sensor to read less volts that it actually is seeing. So my car leans out a bit (it is responsible for air/fuel ratio)

I'm wondering what could cause this? Here is a diagram.


I tapped into the sensor wire, in parallel. Not sure why it's causing a 0.3V drop..

 

[MikeMl]

...I tapped into the sensor wire, in parallel. Not sure why it's causing a 0.3V drop..

Read post #18 in this recent thread. I suspect that is the root cause of your problem.

If you are running the Arduino on 5V, then the analog input to it should not be loading the MAP sensor. That would not be true if you are running the Arduino on 3.3V.

Follow the link in post #16, too.

 

[crutschow]

Remove the 10kΩ resistor R0. It's not needed.
That may be loading the sensor output.

 

[donperry]

And it was! Or so it seems so far! I still needed a resistor there for pulldown though, cause without one it was reading 2.2V when the car's logger was seeing 0.7. I dropped a 150K in there it reads 1.03 when the car is at 1.3

So I did a test and the problem i was having has disappeared. I just need to find proper resitor to put there now. I currently replaced it with a 150K

MikeMl, thanks for your input also.

 

[Diver300]

I still needed a resistor there for pulldown though, cause without one it was reading 2.2V when the car's logger was seeing 0.7. I dropped a 150K in there it reads 1.03 when the car is at 1.3

If you are reading an analogue voltage, you should aim for the impedance of the input to be as high as possible. The 150 Ω in series will not make much difference so it you are changing the reading with the resistor that you are adding, then you are affecting the voltage on the connection on the car.

With your circuit disconnected you should use a multimeter with an input resistance of 10 MΩ or more and measure the voltage and compare with what the logger says. If the readings are different, find out why rather than just using a resistor to make them the same.

 

[donperry]

If you are reading an analogue voltage, you should aim for the impedance of the input to be as high as possible. The 150 Ω in series will not make much difference so it you are changing the reading with the resistor that you are adding, then you are affecting the voltage on the connection on the car.

With your circuit disconnected you should use a multimeter with an input resistance of 10 MΩ or more and measure the voltage and compare with what the logger says. If the readings are different, find out why rather than just using a resistor to make them the same.

thats what i realized. Good thing the impedance of the mcu itself is pretty high without the 150Ω so it could even be removed. I can't rem how much Ω I got, but it was high from the input to the mcu.

The 10K Ω pulldown was really loading the sensor too much, which i then replaced with a 150KΩ. Had no idea 10K would be so strong in that location!

 

[KeepItSimpleStupid]

Reminder:

The car sensors are usually ratiometric to +5 or should I say the +5 that they are powered with.

Your reference needs to "Track" the sensor power and not be an absolute +5 volts.

 

[donperry]

OK thanks. There is a vref input on the mcu i could make use of it.