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I would like some assistance in designing a digital LED display vacuum/boost gauge for a turbo application. Answered

Hello!   I'm new here.

I would like some assistance in designing a digital LED display vacuum/boost gauge with both a linear LED display as well as two seven-segment display modules.
I plan to use a standard automotive MAP sensor to provide the -30Hg to  +30psi signal.   What I envision is having the two seven-segment displays sit next to a row of 28 green LEDs for vacuum to -28Hg, 7 yellow LEDs for pressure from +1psi to +7psi, and 8 red LEDs for pressure from 8psi to +15psi that light up incrementally as the pressure increases or vacuum increases from a center point of zero (clear LED always on). 

The problem is that I have absolutely NO idea of where to start other than the layout of the LEDs.  So any help would be appreciated.

Skullmaster20  (Chris)

Edit ----- 
This is a drawing of what I'm trying to create, and a picture of what I'm going to replace.


Count up the number of lamps you have to drive  - 37. Then work out how to drive 'em. I'd suggest something like this
MAX 7221, which can drive 56 LEDs with one chip.

Then you need a processor. Probably the easiest way is to use an Arduino, or derivative.

Then you wire them up and write a simple driver program.



Thanks Steve!

That looks like it would probably do the job!

Its been about 20 years or so since I built IC circuits.  I've forgotten pretty much everything that I ever knew about them except that it was fun when whatever I was building finally worked!

Let me know if you need any design help. The 7221 is pretty straight forward.

This is a non-processor solution to your problem.  I've got nothing against processors just think this might be simpler, or not :)

You could try using LM3914 ICs for the led bargraph.  The LM3914 was designed for linear voltage displays.  The chip can drive 10 leds in either bar or dot mode, and they can be cascaded to form larger displays.  They can run from 3V to 15V supply, so they are ideal for cars.

What's the voltage output of your MAP sensor ?  You may need to amplify it to a suitable voltage for the LM3914.  Also the sensor output may not be linear.

If your MAP sensor is linear then you just need to cascade several chips together and use the voltage reference of the top chip to set your maximum voltage and the reference of the bottom chip to set your minimum voltage.  I'm assuming that the sensor output ranges from something above 0V to less than12V (eg 1.25V to 10V).  The 0 position on the chip would be disconnected and a led just wired permanently on would replace it.

There are other versions , the LM3915 and LM3916.  The LM3915 is logarithmic with 3db steps between each led, the LM3916 is for VU meters.  These are a bit harder to cascade.

For the digital part, a voltmeter circuit like this one might work.  Though you might need to fiddle with the voltage references and use voltage amplifiers or dividers to get the correct numbers displayed.

Sounds like a very good posibility!  I'll look into it.  Thanks for your comment, it it greatly appreciated.


yo man

8 years ago

way cool can i use the same set up on a trubo diesel? Its a 1993 gmc truck

I don't see any reason that it wouldn't work!  It's just monitoring vacuum and pressure as a stand-alone system.  It should work just fine.

.  If the car has OBD, it may be better to tap into the car's brain (should be fairly easy to so with an Arduino, as mentioned by steveastrouk) and see what it thinks MAP is. Why buy another sensor?
.  With a little programming, you could display just about any variable in the car's brain.

there's too much risk on it, you have to know very well how yout ECU works and how you can get on it without damaging it. Every car is a world on that, and not any car allows you to enter in some features on it. The better way is to keep the ECU alone

The car is a 1980 Pontiac Turbo Trans Am.  It was the last year that GM built vehicles without ODB computers in them.

Although it does have a computer of sorts to manage the ignition timing based on feedback from a knock sensor, it does not know what the manifold pressure is.

The original setup has three lights - the first is "Normal" and comes on when the ignition is on, the second is "Medium" which is activated by a pressure switch at +1 to +3 psi, and the third is "High" which is also switch activated at around +7psi.

.  I have a '98 Grand Prix GTP (3.8L V-6 supercharged) that has a built-in boost gauge. No numbers on the gauge, but it works. Unfortunately, it gets its data from the computer, so one from a junkyard wouldn't work for you. :(

There is ZERO risk from reading it. Plenty of people make OBD readers, its almost trivially easy to access at least some of the internal data. WRITING to it is a lot harder to do, and is often protected by the makers anyway.

i agree with frollard, will be the easiest way. Maybe you can take the signal of the air flow sensor to make it, too. there are two kind, one with a trapdoor that moves a potentiometer, and other is a hot wire that with the amount of air, it colds and to mantain it hot, the car needs resistance, and is that resistance that makes the measurement. tooking the values, you could make the gauge.

Yes.  I have considered purchasing gauges very similar to this one.  The problem is that the gauges will not fit in the space that I have to work with.   That is why I need a linear gauge that is thin and long instead of a round one.

A solution is to use a ready-built gauge, and desolder the leds from the circuit;  Then all you have to do is mount the leds on acrylic (etc) how you want them and extend the wires to the original circuit.  That way all the hard way work is done, you just have to customize how the output looks.