as mentioned in the subject I need to read a sensors resistance but with 0.5v. as rev voltage
I use a simple circuit for reading the resistance, means a R1 of 50 kOhm where the unknown resistance R2 is the sensor itself.
My target is also to provide a switched rev voltage, means 5v provided by a digital pin, where in the first step I reduce the voltage to 0.5v using a simple voltage divider (in advance to obtain the needed rev voltage.
It works flawlessly IF I let the Arduino only being powered via USB only. BUT if I attach a 5v via power supply to the 5V VCC of the Arduino the voltage readings of A5 are significantly lower ... even if I don't use the D13 as 5v voltage source but the 5V pin where here the 5v power supply is attached to. I guess here the approach via the volate divider in the first step is responsable.
So how can I make it work to read the sensors resistance using D13 as ref voltage source and with its reduced voltage to 0.5 as voltage ref for reading the resistance of the sensor?
Suggestion, supply power from an external voltage of maybe 7.5V or more into the Vin pin and use the arduino 5V as your voltage for the reference. Plugging in and unplugging the USB should not change your readings. By the way D13 is a LED pin in most Arduinos, so that will have some interesting effects over time.
If your sensor is a lambda sensor , then it produces a voltage , according to the Nernst equation .
With this sensor you should not “power “ it as you have .
Any heater connection needs powering
Thanks to all for your replies, .. it took me a lot of time to get through the new behavior of this forum as IMHO its more complex and comes with less user friendliness.
Thats what I did beside the USB power, ---> applying an external 5V Voltage, and exactly this what it makes the Arduino reading result odd.
In my test circuit above, in this state the sensor itself wasn't even connected. I used a resistor instead to mime the sensors resistance.
