Reading 500ohm resistive sensors

[Glenn]

I trying to build something and want to read a value from a inbuilt (as in, I can't change it) sensor that have a resistance from 0-500ohm. I am going to use PIC16F887 (with internal oscillator) and have very few pins free, basically just 8 (RE0/AN5) and 9 (RE1/AN7).

pot command does not support such low ohm values, and also, to be honest I have had alot of problems using pot in the past, so I guess using an ADC is better, especially since the free pins actually are analouge too.

So how would I do this ? creating a voltage divider I guess.. but then how do I calculate this ? if I simply build it of the sensor and another 500ohm resistor I going to have a swing of 0-2.5V, thats fine I guess, IF I could put a reference voltage of 2.5V to VREF ? ..but I can't since that pin is used..

If I take like well 10ohms, I get a swing between 0.01-5V, fine but then I get two other problems, it wouldnt be linear at all and the total resistance would in "worst case" be 10ohm, and that would draw 500mA (U/R=I), wich is way too much.

What would the best way to solve this be ? use an OPAMP ? ..would rather not if I can avoid it..


..As I said in the PBP-book thread earlier.. it would be really nice if there was a "cookbook" in how to interface different types of sensors from PBP with different solution for different types.. I would buy such a book without blinking, especially if I dont have to read five chapters of "how you install this really old software on this really old operating system" in the beginning of all books.

[Acetronics2]

Hi, Glenn

The obvious solution would be to use a Current generator and send 5 mA into your sensor ... to get a 0 - 2.5v voltage to be measured.

looks an OPA is to use for the current source ... to get a decent result.

Alain

[Charles Linquis]

Op amps will solve a lot of interfacing problems.



You can get linearity in one of two ways:

1. Use a constant current source. That could be made easily with a PNP transistor, two diodes and two resistors - (a section of an op amp can be used as well)

2. Use math or a lookup table in your software.