I'm sure this is easy, but I'm too much of a n00b to see it. I want to interface a VDO oil pressure sender to an arduino-based in-car display. The sender grounds to the engine through the threads and presents a variable resistance ranging from 10 ohms@0PSI to 180 ohms@150PSI. So far, I've laid out the schematic below using a simple voltage divider. The problem is that making R1 big enough to keep current to a reasonable level reduces the total voltage range at the 'duino pin to around 1V (realistically, much less, since I'll never even get close to 150PSI).
If I've done my math right, R1=100ohms will give me an output of 2.62V to 3.68V and a current of 13 to 24mA. Is there a way to make the output cover more of the 0-5V range?
Schematic:
You could use an op amp as a voltage follower between your pressure sensor and the Arduino. This is essentially creating a buffer between the sensor and Arduino. The input impedance (resistance) to the op amp is very high. The output from the op amp can then be fed to a voltage divider and reduced to a usable level.
Are you sure your schematic is right? From my reading, closing switch S1 has no effect on the reading. Or is that just some kind of status lamp?
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The Quick Shield: breakout all 28 pins to quick-connect terminals
IIRC, that is typical for Detroit "Idiot Light" technology. (A separate system.)
I'm just surprised that the sensor/sender implements both modes.
You got it. The VDO sender replaces the factory switch and adds a real gauge to the system.
Richard, your suggestion was spot on. I knew it would be obvious once someone suggested it! If I use a 9V regulator, I can get a 0.6V - 5V range.
I do have series resistors on my inputs, they're just on another board. I'm trying to be paranoid about protection, given the nature of car electrical systems!
Thanks for the help!