Home made rich/lean indicator

[Gassed'58]

Have a box of these at work

"Mixture Display Kit For Fuel Injected Cars

Refer: Silicon Chip November 1995 This very simple kit will allow you to monitor the fuel mixtures being run by your car. This type of sensor is also known as an E.G.O. (exhaust, gas, oxygen) monitor. You can use it as a tuning tool, to help in vehicle modification or simply to see the behaviour of the engine control module. Indication is via 10 LEDs to show mixtures rich, lean and normal. The circuit connects to the EGO sensor mounted in the exhaust manifold and the cars battery. PCB, LEDs and components supplied. "

if anyone would be interested?

Hi SootySport, the kit came this morning and looks great. Many thanks for supplying it.

For anyone else interested the kit is as it looks in the picture and is a preassembled kit made by " Jaycar Elecronics " and saves a lot of faffing around.

There is a large amount of info / data/ instructions included with it too !!! Hardly worth making yourself at this price. Just my thoughts.

Anyway, thanks again mate, guess what I'm doing today whilst it is dry

Gassed'58

[ThreeSheds]

Hopefully people won't be too upset with me resurrecting this...

I have started looking again into using an LM3914 as FF suggested above, but I can't work out how I can make it read 0-1v - it would appear to have a lower reading range of 0-1.25v at least all of the circuits I have found on the web are 1.25 or higher..

I did think of amplifying the input signal, but FF's post states that two pre-set pots can achieve the same effect. I have been trying to work this out for a few days now, but have drawn a blank - could anyone point me to a web resource where I can learn enough to work it out please? Then once I have the answer I will post it back here

Roger

[FridgeFreezer]

I've just re-skimmed the data sheet, the reference voltage is 1.25v which is usually applied straight to Pin 6 (Ref Hi) with Ref Low grounded (0v), and then you use a potential divider to drop input signals down to the range 0-1.25v.

However, with a 0-1v signal, you use a potential divider to scale Ref Hi down to ~1v, the circuit & calculations are in the data sheet (Voltage = 1.25 (1 + (R2 / R1)).

[ThreeSheds]

the circuit & calculations are in the data sheet (Voltage = 1.25 (1 + (R2 / R1)).

Aye mate - I saw that, but:

As you say: Voltage (in this case, if I understand correctly: the programmed voltage which will light all LEDS) = 1.25 (1 + (R2 / R1) so even if we make R2 zero so that R2/R1 is zero we still have:

V = 1.25 (1) which equals 1.25 - and that is where I get stuck...

Sorry to come back like this FF - I hate it when people keep loading a willing helper with more and more questions which is why I was asking for a learning site, but any further help would be appreciated...

The problem is (after hours of trying) I just can't find a way to program the LM3914 to light all ten LEDs with less than a 1.25 volt signal. The only answer I have come up with is to boost the input signal with an op-amp (I have an LM339 left over from my last go at this).

Roger

Edited:

Belay that! I think maybe I see it now so i'll have another think

[A Twig]

Let me get my Image Processing exam out of the way tomorrow and then I'll have a crack at a circuit for you...

[ThreeSheds]

Let me get my Image Processing exam out of the way tomorrow and then I'll have a crack at a circuit for you...

Thanks Twig - but hopefully I will be able to post up my layout before then Image processing eh? Sounds like a doddle to me... NOT!

Best of luck with that

After reading the FF comment again and trawling through the data sheet again I now think that all that matters is the voltage difference between R_HI and R_LO - if R_LO is ground and R_HI is 1.0v then you have a ten stage 0-1v meter - easy peasy. The problem was that I had not noticed any circuits with a resistor between

REF_OUT and R_HI, Now that I have seen that I should be ok.

Thanks guys - I'll let you know how I get on.

Roger

[FridgeFreezer]

I thought you just made a voltage divider twixt Ref Out & ground and picked Ref Hi off it. Raising Ref Low just raises the bottom of the graph above zero, which is used to expand the scale.

Being *very* basic about it:

Ref Out

250 Ohm resistor

->To Ref Hi

1KOhm resistor

Ground

So Ref Hi = 1k/1.25k * Ref = .8 * 1.25 = 1v

Be aware the "Ref Out" line also sets the LED current, so choice of resistors gets a bit more complicated, but the ratio is the important thing.

[mad_pete]

Are those cool 10 LED baragraphs still about ? I can't find one with the 3 colours.

[Max_torque]

I don't know how much you guys know about how a narrow band lambda sensor works, but unfortunately they are really next to useless for accurately determing the current AFR of your engine. They are designed to have a large voltage swing as the AFR traverses the stoichiometric zone, and this enables std engine management to know when the exhaust afr has crossed lambda 1. It does this because to maximise catalyst conversion efficiency the exhaust AFR must be oscillated either side (in a very small window, typically 0.97 to 1.04 lambda) of lambda 1 continuously (the rich-lean conditions allow the different active components in the 3 way catalyst to convert the pollutants, i.e. lean to oxidise the THC's and rich to minimise the NOx etc)

Because of the sensors characteristic, it is highly innaccurate at telling you anything other than you are lean, or rich, HOW lean or rich (which is what you probably want to know) is so far up or down the voltage curve as to be virtually worthless (also sensor ageing massively effects the actual output voltage values either side of lambda 1)

It could be useful if all you want to know is that your engine stays rich when you accelerate hard, but that rich could be as lean as 0.99 lambda, rather than the 0.85 (or less) required to stop your engine going bang.

The real answer is of course to use a wide band sensor, but these take much more electronics to drive as they compare current flow across a reference cell as a comparitor to estimate exhaust gas oxygen content.

Typical narrow band curve attached, see how small the voltage swings are between say 0.8 and 1.0 lambda etc

Please don't take this as a negative comment on your diy systems, i'm all for people doing stuff like this themselves, but i wouldn't want you to take the output values as gospel and cream cracker your engines!!

(edited to add: EMS systems that have narrow band sensors do NOT use these sensors as feedback devices at high loads, when target fuelling is not lambda 1, they go "open loop" at this point. This is why most newer EMS's use wide band sensors for continuous closed loop control (well almost continuous, disabled during deccels and large load transients etc)

[Quagmire]

I'd like to try to put one together to give me a starting point on tweaking carb engines.

If i can zero in on the point where the AFR crosses over, and then go back and forth over it in smaller steps, i can theoretically find roughly stoich and then richen a bit?

The hardest thing then is how much is "a bit"

[mad_pete]

I've got a wide band sensor :-)

[ThreeSheds]

I don't know how much you guys know about how a narrow band lambda sensor works

<..>

Thanks for the detailed info and I agree that it's a good idea to point this out to people, but I do know about how the voltage flips (my first thoughts were just a two colour LED flipping from red (rich) to green (lean) and since my reason for this whole thing is to help tune my very basic LPG I thought this would be enough (Up until now the mixture has been done by the old tweak and listen method so I reckon even a narrow band lambda has to be better than that..) - then FF came along with his tempting LM3914's and TEN LEDs and I was hooked! Who cares if the middle four never light, it's the bling that counts

Actually the reason I resurrected this thread it is that I need an electronics project to build for a course I am doing, and I just happen to have the engine out so fitting the lambdas will be a breeze The project needs to be 'interesting' without being so hard that I can't make it work. I have a Maplins kit robot in case I can't get this designed in time for the weekend but that would be the easy way out.

Cheers & thanks for the help chaps - I will keep you informed. Fingers crossed I don't end up doing the robot!

Roger

[A Twig]

That bit of the data sheet in the attached file is what you need to get your input voltage range down to 1V. Don't need to worry too much about Iadj. Essentially your RefOut needs to be 1V larger than your Ref Adj.

Good luck with it all, if you need any help...

[ThreeSheds]

<..>

Essentially your RefOut needs to be 1V larger than your Ref Adj.

<..>

Excellent - I can do that

Thanks

[ThreeSheds]

Been a while, but:

:D :D

Running open loop (just as an indicator) on LPG.

[A Twig]

Good job! Always satisfying when little projects like that come off! Shouldn't be too hard to make it closed loop now.