DIY AF gauge

THAWA · Post by **THAWA** » Wed Mar 17, 2004 7:14 am

Anyone have instructions on how to do this? Like the one magnux made? Google only turns up something for a scirocco, so I dunno if all the voltages and stuff are the same. TIA

vrg3 · Post by **vrg3** » Wed Mar 17, 2004 4:31 pm

If it's meant to interface with a narrowband oxygen sensor, it's the same.

They're pretty much all based on LM3914 chips, so try Googling for "lm3914 oxygen sensor" or something.

THAWA · Post by **THAWA** » Wed Mar 17, 2004 5:16 pm

damn cool, that doesnt look too hard at all. The other day when you were talking about puting the leds in the shape of the sine wave did you mean this: http://www.ffp-motorsport.com/tuning/o2/ffplambda.gif or like this: http://www.wps.com/LPG/o2sensor.html about 2/3 of the way down it's in ascii. But yeah, the 20led doesnt look too hard, I'll prolly try that later.

vrg3 · Post by **vrg3** » Wed Mar 17, 2004 5:24 pm

I meant putting them in the shape of the first curve (but probably inverted horizontally, since we usually think of rich as a "higher" state than lean). That lets you see some idea of what the signal means.

Using 20 LEDs wouldn't be hard, but you won't really get much out of it. It'll certainly look cooler, and give you a better idea of the voltage coming from the sensor, but it's not all that useful to know the voltage to such high precision.

THAWA · Post by **THAWA** » Wed Mar 17, 2004 5:40 pm

ah okay, so like this? http://www.ffp-motorsport.com/tuning/o2/o2ffp-01.jpg

vrg3 · Post by **vrg3** » Wed Mar 17, 2004 5:41 pm

Yeah, something like that.

THAWA · Post by **THAWA** » Wed Mar 17, 2004 5:56 pm

one more question, as i plan to go buy most of this stuff today from radio shack. Does the value of the resistors matter that much? I dont think any two designs ahve the same ratings for resistors, so it wouldnt seem as they matter that much. Reason I ask is they dont have 2.3k resistors at radio shack only 2.2k so I'm wondering if the 100ohms makes a big difference?

Also since im sort of in the diy mood, what other gauges could be run off an lm3914?

vrg3 · Post by **vrg3** » Wed Mar 17, 2004 6:16 pm

Most of the designs online probably involve potentiometers or adjustable resistors for calibration... so a 2.2k vs. 2.3k would probably be okay. Most resistors you buy are +/- 10% anyway.

The LM3914 can be used to make a bargraph voltmeter for any purpose... The oxygen sensor is the only one on the car that lends itself easily to this, though; others require some interface circuitry. A tachometer or sequential shift light is one example -- you can use an LM2917 frequency-to-voltage converter.

You could hook one up to the MAF sensor signal to see airflow

(although the signal from the sensor is . Or to the TPS to watch throttle position. Properly configured, it could be used to measure alternator charge current or battery voltage. With some fancy circuitry it can be used to monitor injector duty cycle (like the little box boostjunkie has in his car). Barometric pressure, too.

Are you planning on getting an LM3914 from Radio Shack? I don't think they carry it.

THAWA · Post by **THAWA** » Wed Mar 17, 2004 6:24 pm

cool, sounds like with a lil work and some different circuittry i woudlnt need to buy any aftermarket gauges?

yeah i didnt see the lm3914 on their site so ill be looking elsewhere for that, they have everything else though. Thanks much homes, I'll let ya know how this turns out, and maybe even do a writeup specifit to our car about it

vrg3 · Post by **vrg3** » Wed Mar 17, 2004 6:27 pm

Well, if you only want 10 LEDs to indicate all the stuff going on in your car...

Needles are much better in my opinion. The only reason the LM3914 is okay for an "air/fuel ratio" gauge is that the signal is so vague to begin with.

I've been meaning to come up with a version of the circuit that would interface to our cars' factory dimmer... maybe I'll try again

Have fun.

ciper · Post by **ciper** » Wed Mar 17, 2004 8:32 pm

Dont waste the time with those. Such low resolution!!!

Get a decent volt meter and a voltage to air fuel ratio map.
DONE!

mikec · Post by **mikec** » Wed Mar 17, 2004 11:08 pm

vrg3 wrote:With some fancy circuitry it can be used to monitor injector duty cycle (like the little box boostjunkie has in his car).

Any idea what this fancy circuitry might consist of? Is it a counter or something similar?

ciper · Post by **ciper** » Wed Mar 17, 2004 11:20 pm

VOLT METER for air fuel ratio!

BAC5.2 · Post by **BAC5.2** » Wed Mar 17, 2004 11:20 pm

THAWA - Do a writeup if you do this!

vrg3 · Post by **vrg3** » Wed Mar 17, 2004 11:22 pm

ciper - I think we all agree that an "air/fuel ratio" gauge is just eye candy....

mikec - I don't know off the top of my head... Maybe we should ask boostjunkie to pop the case of his box open and take a look. :)

BAC5.2 · Post by **BAC5.2** » Wed Mar 17, 2004 11:31 pm

Seems like you could just use a capacitor. As current pulses through it, it unloads evenly and lights up the lights. As more current pulses through it, it starts to stream the power through it and lights up more lights.

Seems reasonable, but then you'd probably blow the capacitor pretty quickly, so I dunno.

ciper · Post by **ciper** » Wed Mar 17, 2004 11:36 pm

vrg3: In other words RICE?

BAC5.2 · Post by **BAC5.2** » Wed Mar 17, 2004 11:41 pm

I wouldn't call it rice. I'd say it's easier to read on the fly incase the car starts acting weird rather than pop the glove box and read a chart of voltages:a/f numbers.

It's easier to hide 10 LED's than it is to hide a display with letters (that you can't even read at night clearly). The Voltmeter is borderline dangerous IMO.

vrg3 · Post by **vrg3** » Wed Mar 17, 2004 11:54 pm

BAC5.2 - Well, the design would certainly have to include a capacitor or two. :) Maybe an integrator that resets on rising edges, and a sample-and-hold buffer that locks on falling edges.

ciper - I think it's a little harsh to call it rice, since it's still functional. Precision and accuracy are different things; even a precision voltmeter won't give you very accurate readings with the stock oxygen sensor. That's why ECUs treat the sensor as a just "rich/lean" indicator. If you used a 4-wire sensor and had a datasheet from that particular sensor's manufacturer, then maybe you'd have a meaningful advantage with the voltmeter.

ciper · Post by **ciper** » Thu Mar 18, 2004 12:04 am

BAC5.2: You dont need to look at the conversion chart once you get used to it, using your argument why not just create a two led unit showing rich and lean?

vrg3: The readings will be accurate enough to know which side of the hump its on and if its too far from the middle.

vrg3 · Post by **vrg3** » Thu Mar 18, 2004 12:13 am

ciper wrote:The readings will be accurate enough to know which side of the hump its on and if its too far from the middle.

As will also be the case with ten LEDs.

ciper · Post by **ciper** » Thu Mar 18, 2004 12:27 am

It takes just as much to learn the above which number is lean and below which is rich, so again why have the LED?

vrg3 · Post by **vrg3** » Thu Mar 18, 2004 12:30 am

It costs less than a voltmeter, can be mounted neatly somewhere on the dash, and can express the different conditions with different colors so it's easy to see without having to focus one's attention on it.

THAWA · Post by **THAWA** » Thu Mar 18, 2004 5:29 am

I went and bought most the stuff today after work. Got the resistors, didn't know what wattage to choose so i just bought .5w and .25w couldn't find the 10bar led thingy or the lm3914 but I got everything else. Looks like someone is going to be having fun this weekend assuming he can find those two things.

evolutionmovement · Post by **evolutionmovement** » Thu Mar 18, 2004 5:43 am

Just wondering, could you embed the LEDs in the dash pad above the gauges so you could see it through the wheel.

Steve

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DIY AF gauge

DIY AF gauge