Shift Light

[BiggestNizzy]

I have myself one of these (Cheers John)

http://www.omextechnology.co.uk/Revlimi ... ds/SLS.PDF

I plan on fitting it this week but I don't wan't to run cables through from the back do any of the electrical buffs know if there is a feed on the instrument binnacle I can run it from.

[robin]

Depends on whether you want to call them or not. By default the unit appears to be designed to connect to the LT side of the ignition coil, which is obviously in the back. They say that ignition systems with a tachometer output can be supported, but you need to call them. There is a tacho feed behind the dash (it's what makes the rev counter work). I don't have the wiring diagram to hand so cannot tell you what the colour code is for the relevant wire - I'm sure somebody else will know.

Cheers,
Robin

[Mikie711]

The tacho wire to look for is white/black. Not sure that will work, I run my data logger off that wire and it gives a crap signal. Never quite logs the engine speed correctly. Is that not a digital signal Robin, can't remember if it's switched to digital in the emerald or not.

[robin]

It is digital in the sense that it is not connected to the actual coils but is just a signal used for driving tachos, etc.; but it's an open-collector signal - i.e. it's driven low by a transistor but is pulled up high by a resistor. That's fine when there is no capacitance on the line, but the input capacitance of the stack plus whatever your shifto gadget has will slug the rise time of the signal; eventually, depending on your target switching voltage, you might find at certain RPM you no longer reliably reach that switching point before the start of the next pulse ... it's easy to fix this with a buffer circuit that decouples the MEMS/Emerald output from the shifto input, but then the circuit has to be built and has to have a home, etc., so may not be that reliable. Alternately you can add a second pullup resistor at the dash end of the connection; basically that will make it drive higher faster at the expense of burning more electrical power whilst being pulled down (irrelevant). I might try 1K or even 470R in parallel with the shifto input to +12v power supply (switched ignition input).

Before you fit that you should measure the ignition on, engine not turning voltage on the black/white wire - it should rise all the way up to 12v - but the stack may be clamping it at 5V. If this is the case then your parallel pullup will pump more current into the clamp circuit, possibly more than it's designed for (roughly (12-5)/470 = 7/470 = 15mA and around 0.1W - this is unlikely to be more than an automotive circuit will tolerate, but this is all at your own risk!).

A better fix would be to AC couple the shifto, but that assumes it will handle a balanced signal and switch on zero crossing - which might be what they did inside it already - maybe that's why they suggest you call them?

Cheers,
Robin

[BiggestNizzy]

[Mikie711]

Have to admit you lost me after "It is digital".

[mac]

Nizzy,

I've got the same thing fitted to the Caterham - if it helps your more than welcome to try and work out how it's wired. It's an EU2 car on MEMS so should be fairly similar although the wire colours may be different.


Mac

[robin]

Normal digital signals are so-called push-pull. That means that there are two switches - one connected to the +ve supply and one connected to the -ve supply. When the controlling circuit wants to drive the output high it turns on one switch; when it wants to drive low it turns on the other switch. If you look at the output signal on a scope it will have clean fast edges - it'll look square, even when you put a reasonable load on the circuit like the shifto or data logger.

In an open-collector (sometimes called open-drain) circuit, there is only one switch and it is connected to the -ve supply. The +ve supply is always connected to the output via a fairly high value resistor. So the controlling circuit just turns on or off one switch; if on the output is low; if off the output will return high, but much more slowly. If you were to look on this on a scope you would see a fast falling edge but a sloping rising edge - it will look like the charging part of this chart:

http://www.tpub.com/neets/book2/3d.htm

The problem is that the rising edge is slow and noisy - so for high speed signals you risk false triggering and missing edges. Whether or not any of this matters depends on the design of your input circuit.

Cheers,
Robin

[robin]

Mac, pretty sure the caterham uses white&black for the rev counter too - I did something with Lawrence on his hayabusa caterham and that was white&black.

Cheers,
Robin

[BiggestNizzy]

Mac, I might just do that.

I emailed them last night and await there reply, alas I am pulling 12+ hour days atm so phoning is a problem. I await there reply.

Robin, I think I get understand. my electronics is limited and rusty (higher tech studies but that was 15 years ago)

i.e. the signal isn't very good and adding something else in would degrade the quality

[BiggestNizzy]

I copy and pasted Robins reply to OMEX and got this back

Hi Graeme

No worries; it’s always best to check first if you have any doubts! What you friend has come up with is basically correct. Our unit triggers at around 7V, so if the Stack is only pulling up to 5V then we wouldn’t be able to read the signal, but generally it is only some Japanese tachos that do this, I think the Stack will be 12V. The input impedance of our unit is very high and so we don’t normally have any problems adding our unit into the system, but when we do, it is normally cured by fitting a 1k pull-up.

Basically, fit it temporarily at first, without cutting the wires down etc, and just try it. If it doesn’t work get back to us and we’ll find a solution for you!


Best regards


Andy Cornock
Omex Technology

[robin]

Yes - high input impedance is good sign - I doubt the stack is clamping the signal at 5v, but was just putting that forwards because I think you should test it before adding the pull-up - if it works without the pull-up no need to worry about the 5v.

Cheers,
Robin

[pete]

And you bought this why?

I've a multi meter )) (that's not much help is it?)

[mwmackenzie]

Cos he doesn't know when to change gear and likes fancy lights

[VXJON]

And you bought this why?

I've a multi meter )) (that's not much help is it?)

The same reason I bought it and one I know you will like





Its very cool