adjustable voltage relay

Hi,

I have done a bit of a search for this, and found a couple of similar postings, but the answers were all too far over my level of experience.

So, first of all I have a basic understanding of Relays, Diodes, Resistors, etc, but terms like "op-amps" have left my head spinning :)

Essentially this is the situation that I have:

I have an Exhaust Gas Control Valve (EGCV) in my car (1994 TT Toyota Supra) that along with other valves controls the operation of the sequential turbo system. These valves are operated by pressure fed actuators that activate through powered solenoids that are controlled by the ECU. What I want to do is have the ability to cause the EGCV to close slightly, therefore creating a restriction in that particular part of the exhaust system.

We've tried using adjustable Hobbs switches but the hyisteris is too great (~2psi) and by the time the pressure drops back down enough to allow the valve to open back up a bit the valve has completely closed.

So what I was thinking of doing is putting a MAP sensor in the pressure line that feeds the actuator, and given that we know that the actuator opens & closes, and is partial at specific pressures, I could use the feed from the MAP sensor to trigger a relay that opens / closes another solenoid that allows air to bleed out of that pressure line (instead of closing the solenoid that feeds the actuator thereby causing it to snap shut).

The MAP sensor outputs at between 0-5v, with a specific voltage indicating a specific pressure / vacuum.

So what I need is a type of relay (preferably simple) that I can adjust the voltage point at which the relay switches on (e.g. 2.5v) and it must be able to cycle a few times a second.

Also, we'll be using an adjustable Hobbs type switch so that this circuit is not active until the manifold pressure reaches a certain amount.

If given a parts list, & wiring diagram I am pretty sure I'm competent enough to be able to put it all together, but if there is a simple off the shelf product that will do the same job for not a lot of money then that would be just as good.

Hopefully I've explained everything clearly enough.

Thanks.

Thanks for your reply.

I get the basics of what you're saying, and I had come across comparators in my searches, but I don't quite get that wiring diagram, it's a bit above my understanding.

Is the Op Amp the same as a comparator? (Or does the Op Amp describe the entire circuit?)

Any chance of getting a dumbed down version?

From what you're saying I was thinking it would be something as simple as:

12v Source -> Variable Resistor -> Comparator

                    MAP Sensor Signal -> Comparator -> Solenoid

Would a variable resistor work ok in this situation or would it not be accurate enough? (Or maybe I need to bring the voltage down to 5v first and then use a sensitive variable resistor to tune it in?)

Also, would I be better off using a solid state relay in this instance, and how would I build a hyisteris into it?

Sorry for what I am now realising is a greater lack of understanding of electronics then I thought I had, but I'm a fast learner, so this shouldn't be too painful :)

WOW, that's really good, and best of all I'm pretty sure I understand it all :)

Feeding some of the output back into the base input to get the hysteresis is so simply clever :).

I take it that I would just use resistor/s to drop it from the 2.5v to 0.5v (or whatever)? Is there a specific type that I should use for doing this rather then just the generic ones?

So, my last question: Where do I get a comparator of the appropriate type from? I've done a bit of a Google and they all seem to be suppliers for industry level stuff.

Thanks again.

LOL, that's alot cheaper then I thought it would be.

Seeing as how I'd have to have the reference voltage adjustable to tune in the limit a variable resistor would be best after the zener. I can also use this to provide power to the MAP sensor as it runs on 5v.

Thanks for all this, I think you've pretty much doubled my knowledge of Electronics :)

Ahh, hadn't realised that with the Zener, I might see if I can find a CU managed 5v line to tap into instead. The plan is to use a seperate MAP sensor so that I'm only referencing from the line going to the actuator, which is what we have to keep to within a 0.5psi range.

Funnily enough after going through all of this I was looking through the Jaycar (Aussie supplier of all things electronic) website and their kits, looking for something completley different, and came across this:

http://www.jaycar.com.au/productView.asp?ID=KC5377&CAtid=25&form=CAT&SUBCAtid=347

The REALLY annoying thing is that they were the first people I rang when I had this idea and asked if they had something like this and the idiot sales person said "Nope, we don't have anything that'll do that, in fact I don't think it's possible...."

So anyway, thanks for all your help, but it looks like this is what I need to do the job.

That does look like exactly what you were looking for, doesn't it?

no fuss, no muss.