Tech Article Title Author Date
S4/A4 Boost Gauge Don Pavlik 2000

So you want a boost gauge in your A4 or S4.  Sounds easy enough right?  Just get a gauge and put it somewhere.  Uhhh, not so simple.  In later cars you can't use a A-pillar mounting pod... airbag lives in there.  Do you really want a gauge slammed into your head with explosive force in the event the airbag deploys?  And, cosmetically there aren't many good places that you can glance at quickly.  After spending the cash to get an S4 simply screwing or gluing a gauge pod to the dash was not an option for me.  I had a mounting solution for the gauge in my A4 but the '00 console is different and won't fit.  My solution?  A LED bar graph at the bottom of the instrument cluster.

I found a document, AN1304, published by Motorola for a LED bar graph but the example used their MPX5100 series sensor which is only rated to 15 psi.  They also offer an MPX4250 which is good to 36.3 psi and designed for use as a MAP (Manifold Absolute Pressure) sensor for turbo engine boost control.  Based on the specs the 4250 was a perfect replacement for the 5100.  I made a quick stop at my favorite surplus store for most of the parts and a call to for the sensor.  I built it on breadboard first then assembled it once I was sure it was what I wanted.  

The gauge consists of 10 segments; 8 red and 2 amber LEDs.  Red and amber match the dash very well.  I calibrated the display to read full scale at 20 psi so each bar is 2 psi.  I'll get 1 amber at 18 psi and 2 amber at 20 psi. I used a ribbon cable to mount the LEDs remotely.  Turns out an MFM hard drive 20 pin signal cable (remember those?) is an ideal holder for the LEDs. Thanks to the location it's protected from most direct sunlight and very easy to glance at quickly.  I still have not decided if I need to add a light sensor to control LED intensity.  I'm going to use it for a while and see.

Info about the gauge. All documents are in PDF format.  You'll need Acrobat Reader from Adobe to view PDFs if not so equipped.

  • AN1304 Bar graph display local
  • MPX4250 Pressure sensor local
  • LM3914 bar display driver local
  • I purchased the hard to find sensor at Newark

I will not be building these for sale nor do I know of a company offering this or a similar device.

The completed gauge.  I tucked the electronics up under the dash.  Only the boost sensor signal line runs under the hood.  The ribbon cable obscures the LM3914 IC and a few other components.  The green and blue pots are for zero and full scale calibration.  The 3 posts next to the pots are the analog test point for the sensor, zero calibration test point and full scale calibration test point.
Underside of the circuit board.  Please, no snickering or scoffing at my amateur soldering and cheesy construction techniques.  I'm a car nut / computer geek... not an EE.
Excuse the poor picture--camera doesn't autofocus very well with all that red.  The gauge is 10 segments.  8 red and 2 amber LEDs.  Each segment is about 2 psi.  6 psi shown.
Just another shot. 9 segments lit (about 18 psi)  

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