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Old 08-01-2003, 12:06 AM   #1
Chav
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Default Any EE's out there? Need help with building a wideband -> narrow band

I built a voltage divider and tried it in the car only to realize that the 0-5v signal from a wideband is 0 = richest ------- 5 = leanest which is the opposite of a narrow band 0 = leanest --------- 1 = richest. The link plus has user defined tragets, but it assumes lower is leaner and higher is richer. So I built an inverting op-amp circuit with an lm741.

I used a voltage divider to send +7v to V+ and -7v to V-. The virtual ground goes to the non-inverting input of the op-amp. I also have the gain setup to be -.2 . The only thing I have forgotten how to do is to offset the output voltage by +1v. Now the output goes in the right direction (lower leaner -> higher richer), but it goes from -1v to 0v.


-Chav
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Old 08-01-2003, 01:21 PM   #2
vile
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throw out a drawing of what you have and what u need to do

i work better w/pics
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Old 08-01-2003, 02:12 PM   #3
uf300zx
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I agree, pictures of your schematic would be useful.

Im guessing you have a negative feedback; 1 resistor going from inverting terminal to output, and another resistor going from inverting terminal to ground?

Then have the input go to the positive terminal?

Kinda of reminds me a Schmitt trigger problem, I'd try adding a 1V source instead of the ground on that 2nd resistor.

I may be approaching this entirely wrong though, gimmie some schematics to look at.

Ryan S
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Old 08-02-2003, 04:17 PM   #4
vrg3
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So you're just trying to scale the 5-volt signal down to a 1-volt signal? You are aware that you won't end up with the same signal you'd get from a narrow band sensor, right? Narrow band sensors are nonlinear.

But, anyway, let's say you're trying that anyway...

You won't be able to do this using a virtual ground, since your input source almost certainly shares its ground reference with the vehicle.

Scrap the 741 and get an LM324. Radio Shack should have one. Use +12v as its positive rail and ground as its negative rail. An LM124 or LM224 would be preferable, as they are pin-for-pin replacements for the LM324 but can handle much wider temperature ranges.

So here's what I came up with off the top of my head:

The 324 has four op amps in it and you'll use two.

First, set one up as a voltage follower to duplicate the input voltage at high impedance (You could leave this stage out if the wideband controller is able to supply enough current).

Then, you pump this into a differential amplifier. That way you can subtract the input voltage from a reference voltage.

For the reference, you need a reliable voltage source -- you can probably get a +5v source either from the ECU or from the wideband controller. A divider network can cut this down to 5/6 volt (0.833 volts). Feed that 5/6 volt refernce into the noninverting input of the amplifier, and feed the inverting input a weighted average of the input and output voltages (input*1/6 + output*5/6). It will try to equalize the two inputs, so:

input*1/6+output*5/6=5/6
input+output*5=5
output=1-input/5

This is kind of what I'm thinking of (pardon the MS Paint artwork):

http://www.graphics.cornell.edu/~v/pics/5vto1v.gif

You probably would want to use potentiometers instead of pairs of resistors in order to get proper calibration, but that's the basic idea...
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Old 08-02-2003, 07:25 PM   #5
Chav
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Thank you very much vrg3. I'm using a link plus so the output can be linear. Also the wideband I'm using has a sensor ground reference which I was going to connect to the virtual ground.

I'm going to go with your circuit though. Thanks again.

-Chav
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