1. Now that Im looking at building three or four of these to measure oil and water temp, oil pressure and fuel, I can see a small problem with such a basic design.

The sensors used throughout the car are very basic; thermistors and pressure variable resistors, connected to the ground of the car. The physical guages that measure them have voltage regulators on them, but if you're not using those guages and trying to take the reading straight from the sensor wire, isn't the reading going to vary depending on engine speed? Vin is not constant - the power will be from 12.6V to 14.4V, depending on how the alternator is spinning.

Or am I overthinking this? I got my readings from a multimeter measuring resistance between sensor wire and car chassis (ground) and watched it drop from 1000 Ohms to 750 Ohms for oil temp as I started the engine, but 23 Ohms to MINUS 125 Ohms for the oil pressure between IGN-off and engine running. Am I doing something massively wrong?

2. The way I think I understand it, I may not even need to use a voltage divider--just use the +5vdc terminal on the Fusion Brain to provide a constant voltage. The resistor (sensor) has a known range like 0-90 or 240-33 ohms.

The resistance will change the input voltage between 0 and 5 vdc, with minimum resistance producing 5 volts and maximum resistance producing 0 volts, all accounted for in the formulas you devise in FCC.

I think...

3. I dont think that would work, because the ground pin of each Fusion Brain input is not the same as the ground of the car (which is what each sensor is grounded to.)

4. I just looked back at my notes. +5vdc from the FB goes to an appropriately sized resistor. The other end of the resistor goes to the sending unit AND FB signal input. You don't use the FB ground.

What you are measuring is, in effect, resistance to ground for the sending unit (sensor).

(Sorry for the crude diagram below).

FB +5vdc
|
|
>
< (this is a resistor, 330 ohms in my case)
>
<
|-------FB Singal input
|
>
< (this is the sending unit, 240 max ohms in my case)
>
<
|
|
___
__ (Engine block ground)
_

5. If that is it, then that is exceptionally simple. Can greenman or Toaster chime in to clarify?

I did resistors at high school, and yet this discussion makes me feel so retarded. I thought I was going to need something like this

Looking at that diagram though, it still doesn't make sense. The thermistor (oil temp) is grounded to the engine block. How do you get from there to the FB in order for the Vin to have any kind of reference point? Otherwise I might as well be holding the other end of the resistor in my hand and calling that "ground." See pic:

6. Originally Posted by Grrrmachine
Now that Im looking at building three or four of these to measure oil and water temp, oil pressure and fuel, I can see a small problem with such a basic design.

The sensors used throughout the car are very basic; thermistors and pressure variable resistors, connected to the ground of the car. The physical guages that measure them have voltage regulators on them, but if you're not using those guages and trying to take the reading straight from the sensor wire, isn't the reading going to vary depending on engine speed? Vin is not constant - the power will be from 12.6V to 14.4V, depending on how the alternator is spinning.

Or am I overthinking this? I got my readings from a multimeter measuring resistance between sensor wire and car chassis (ground) and watched it drop from 1000 Ohms to 750 Ohms for oil temp as I started the engine, but 23 Ohms to MINUS 125 Ohms for the oil pressure between IGN-off and engine running. Am I doing something massively wrong?
So power them from a regulated source....

7. Originally Posted by Grrrmachine
I dont think that would work, because the ground pin of each Fusion Brain input is not the same as the ground of the car (which is what each sensor is grounded to.)
Fusion Brain ground is tied to whatever you ground it to... so if you power it from your car, it shares the same ground. Power it from a wall socket, and it no longer does.

8. Originally Posted by Dan2008
I just looked back at my notes. +5vdc from the FB goes to an appropriately sized resistor. The other end of the resistor goes to the sending unit AND FB signal input. You don't use the FB ground.

What you are measuring is, in effect, resistance to ground for the sending unit (sensor).

(Sorry for the crude diagram below).

FB +5vdc
|
|
>
< (this is a resistor, 330 ohms in my case)
>
<
|-------FB Singal input
|
>
< (this is the sending unit, 240 max ohms in my case)
>
<
|
|
___
__ (Engine block ground)
_

I may have missed something, but I re-read and... I don't know... So I'll just ask anyway:

If you have switched from dividing 12V down by using 330 ohms and the sensor, to using 5V supplied by the brain, then why are you still using the 330ohm resistor. Isn't that going to limit your sensed voltage to the bootm half of the scale. It seems to me that not only would it do that, but it will also significantly reduce the sensitivity of the signal. My understanding, which I hope someone will correct if I'm wrong, is that for maximum sensitivity and the best range when using a setup like what's quoted here, a resistor should be chosen who's value is equal to the normal operating value of the sensing element.
(i.e. if you operate at 250F normally and resistance of the element is 2k2 at 250F, then the paired resistor should be 2K2. Or if you have a critical temp or press, that you want the indication to be more sensitive at, use the equivilant resistance for that value.)

This also gives you a mid-range operating voltage for that value. If you only operate in a small range of the sensor's capability, this can be especially helpful.

9. Originally Posted by greenman100
Fusion Brain ground is tied to whatever you ground it to... so if you power it from your car, it shares the same ground. Power it from a wall socket, and it no longer does.
That's what I needed to know - I didn't know if some internal rewiring of the brain meant that the sensor grounds might have a voltage difference to the actual Fusion's ground.

10. Originally Posted by h3rk
I may have missed something, but I re-read and... I don't know... So I'll just ask anyway:

If you have switched from dividing 12V down by using 330 ohms and the sensor, to using 5V supplied by the brain, then why are you still using the 330ohm resistor. Isn't that going to limit your sensed voltage to the bootm half of the scale. It seems to me that not only would it do that, but it will also significantly reduce the sensitivity of the signal. My understanding, which I hope someone will correct if I'm wrong, is that for maximum sensitivity and the best range when using a setup like what's quoted here, a resistor should be chosen who's value is equal to the normal operating value of the sensing element.
(i.e. if you operate at 250F normally and resistance of the element is 2k2 at 250F, then the paired resistor should be 2K2. Or if you have a critical temp or press, that you want the indication to be more sensitive at, use the equivilant resistance for that value.)

This also gives you a mid-range operating voltage for that value. If you only operate in a small range of the sensor's capability, this can be especially helpful.
I don't know??? I was just going by the solution greenman provided in an ealier reply.

I'm SO confused...

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