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Air/Fuel Ratio guage Circuit

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Gotrek

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As the subject says I'm looking into building a circuit to display the air fuel ratio of a car, I was thinking of using 10-20 LEDs to indicate from lean to rich using various colored LED's. I found various plans online that use the LM3914 driver. I was wondering if anyone had any experience building this type of circuit (which in essence is just a voltmeter really) it will read voltage from the O2 sensor.

I was wondering if there was a way to only have on LED light up at a time instead of having them light up progressively. I'm not sure how to do this all the plans I've found light them all up as the voltage goes up.
 

fat-tony

Member
I've got answers :) First thing I'd like to point out is that you're probably looking for the LM3914, not the 9314...

http://www.electro-tech-online.com/custompdfs/2003/12/LM3914.pdf

But the second answer is better. If you check out page 7 of that, they describe how to put it into bar or dot mode, including a mode using dot display with over 20 LEDs.

All of these are set using pin 9 (the "mode" pin). For normal dot, you just leave the "mode" pin open. For normal bargraph, you short the "mode" pin directly to pin 3 (V+). The huge graph is more complex, and you can read all about it if you want.
 

Gotrek

New Member
You're correct about the chip I edited my other post thanks, and Thanks for the document thats exactly what I want
 

eemage21

New Member
gauges

I put a gauge pcb together for a friend. You will achieve the best results using curved segmented LEDs.
 

Gotrek

New Member
Yes that probably what we were going to do, possibly also using a larger LED to indicate Stoich, lean, and rich
 

Gotrek

New Member
I found a simple diagram I'm going to follow But since I'm not an electronics guru could someone tell me what kind of part T1 and T2 represent? They are valued at 5k ohm but are they resistors? Thermistors? I don't know anyone have a clue? (I'm sure you all do)

[/img]
 

fat-tony

Member
I'd suspect that T1 and T2 are just variable resistors. They set up the biasing for your reference voltage.
 

Gotrek

New Member
After looking up pictures of varible resisotrs I believe you are correct, thanks a bunch.

Like the silver boxes and the thing with the yellow screw type thing on tope right? Not the circled objects (nice tech talk ehh?)

 

Gotrek

New Member
I'm on Digikey ordering the stuff, can someon tell me What power dissipation value I want?

I need top adjusting, 5k ohm variable resistors but I'm not sure what wattage rating I need
 

Roff

Well-Known Member
That circuit will only monitor voltages of 1.25v or less. If you were to use that circuit as drawn, you should connect a 620 ohm resistor in series with T2, which is the LED brightness control. This will limit LED current to about 20ma each, which still may be too much current for the LM3914 if all LEDs are on and your LED supply voltage is more than about 8 or 9 volts.

What is the range of the voltage you are wanting to monitor? What is your supply voltage? How much current do you want to run through each LED? Answer these questions and we (I) can draw a complete schematic for you. I know that you said you wanted dot mode as opposed to bar mode. This will save power.
 

Eclipsed

New Member
Just so you know, that pic isn't even close to being correct.Outside of stoich there is no way of reading actual(or even ballpark) A/F from a normal oxygen sensor.They are effected too heavily from exhaust temperature.Over time they can help you get a "feel" of how your engine is operating, but don't think they are even close to correct A/F.I've seen more than one engine damaged from people thinking o2 voltage is some indication of A/F.
 

Gotrek

New Member
Ahh ok, the A, B, C on the graph represent the different curves based on different temperatures for the Lamda sensor, I guess that explains it
 

Gotrek

New Member
Sorry to bump up this old post but in the picture of T1 and T2 what are the pin assignements? I'm not sure how to wire it up.
 

njcfm

New Member
a a/f gauge off of a stock 02 sensor is pretty much worthless..

it won't really give you any USEFUL information other than the 02 sensor is actually working.
 

Gotrek

New Member
Well it's in a Truck that didn't have an o2 sensor to begin with, it's the help fine tune the Carburator and it's more for fun then anything, currently it's hooked up to a multimeter and as you can imagine it's a little hard to read since the numbers always bounce of course and the LCD display on the multimetter isn't quick enough to keep up so it's a blur.

it's just for fun and to have something to do.

BTW I figured out my pin out fromt he diagram.

Thanks.
 

Gotrek

New Member
Sorry to bump a way old post.

I built the circuit and it worked fine for about a month, now a strange problem has come up, the LED's don't light properly and some flickers and beahve strangely.

I narrowed it down the the Voltage regulator I added (12 volts to 5Volts)

Bassicly when the unit behaves strangely all I do is short the ground and output pins on the regulator and the unit will work fine again until the 12V power source is cut off (unit is inplugged or truck is turned off) Is the regulator bad? anything else that comes to mind on what might be happening?
 

Sebi

Active Member
Maybe the regulator oscillate.Have You capacitors (100nF) across the input and GND and output and GND? (Nearby the pins as possible)
 

Gotrek

New Member
Any other ideas? Basicly I'll probably restart from scratch, just wierd that it worked leaving my bench and for about 1 day in the truck and now it's bombed.

BTW shorting the ground and output pin on the regulator no longer causes the unit to work.

I've anaged to pull the circuit board from the case and tested all the connections and they are fine. I guess the problem is either with the voltage regulator or the display driver. I will rebuild the circuit without the voltage regulator and see what happens.
 
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