[svenakela]

I'm renovating these pieces: http://bit.ly/10JfF9jIt's a throttle house for a VW Vanagon (T25 in Europe) WBX engine. The switch is used in both end positions, when the throttle is closed it activates the idle logic and near fully open the acceleration boost is activated. Unfortunately the micro switches can't take the abusive environment (neither could the originals) and even if I buy expensive switches they fail or are difficult to adjust.


So, I thought I might try a three wired proximity switch instead. The micro sw is simply short cutting two wires from the ECU. The proximity switch, triggering a solid state relay like this one http://bit.ly/11zpDvO, will do the trick. But...


For a while there was a guy who made a better solution and that is a pressure sensing trigger. The buses fitted with that device ran totally smooth so he was spot on. unfortunately these devices aren't made anymore. After some discussion with some guys I decided to skip the proxy switch and aim to make a pressure device instead. I first though it might be an easy task with a PIC, but it should be even easier (and solvable too) with two 555's (one 556) instead in a double Schmitt trigger design.
Problem is, I have no idea if I'm thinking correctly here and I really need help to figure this out.

The PDF is a spec on a pressure sensor at a decent price, the image is a sketch how my idea has been floating.
When at idle, the pressure will be near standard outside pressure.
When at full throttle, lowest pressure will be reached and the boost should be triggered.
I think it's a good idea to let both the low and high pressure threshold be adjustable a bit, so I added two trim pots in my mind.

From here, I'm lost. Can someone please help me with a working design?

[H500]

What you need is window comparator circuit. The one you drawn won't work. There are many circuit examples on the web.

Would hall effect sensors and a magnets be simpler and cheaper? Do a search on Ebay.

What voltage and currents are involved. You might not even need the expensive relay.

[[email protected]]

I think you are over complicating this. I would look at using a magnet and a couple reed switches or Hall effects. It will be a lot simpler and adjustable too

[holbieone]

I'd use a pot with an Arduino board

you can then program in your max and min position

[Al_The_Man]

I wouldn't think you need an atmospheric sensor? The hall sensor idea should work.
If you can get Allegro semi's there, you could look at something like the ATS617 prox. sensor.
Al.

[Al_The_Man]

IF you think the atmospheric pressure sensor will give a more accurate method of reading, most are 4-20ma output, this could be received with a RCV420 and input into a small pic with analogue input?
Al.

[underthetire]

You could always use a map sensor from any old car, and micro squirt FI.

[svenakela]

You guys totally ruled out yourselves!
A pressure sensor, a 556 and an opto isolator is less than a quarter of an Arduino (which still needs the pressure sensor), and let us not mention a MS FI (which I've built a few...).

The problem with a mechanical switch, that still is present with any type of mechanical triggered switch (Hall, inductive etc) is that they kick in at an assumed point measured from the angle of the throttle, which gives a signal that always will be approximated. The pressure sensing device - which never will be sitting on the shaking throttle as it will be connected through an existing hose - will give the correct trigger point. Always.

Ah well, I'll go on with my PIC circuit then. It is already working...

[svenakela]

IF you think the atmospheric pressure sensor will give a more accurate method of reading, most are 4-20ma output, this could be received with a RCV420 and input into a small pic with analogue input?
Al.

That's worth investigating. I couldn't find anything about current out from the sensor though. On the other hand I'll get one anyway so it's just a matter of measuring. The RCV420 should be much better than measuring the tiny voltage span the sensor has.

[Al_The_Man]

The sensor you showed appear to be 40mv? I wonder if this may could be a high impedance unit? if so the industrial standard 4-20ma may be more rugged in this environment.
It is also usually simple to rig up a 4-20ma receiver with an op amp.

[svenakela]

The MPX2200 is not written in stone, it can be replaced for sure. The MPX4250AP (which is used in Megasquirt) has a bigger voltage output span. Still, the spec isn't saying that there is current out.