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post #1 of 7 (permalink) Old Jan 9th, 03, 05:32 PM Thread Starter
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Paul
 
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Location: Queen Creek AZ
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I am trying to build a set of custom gauges. I have two air/fuel senders mounted in my headers. The output from them is 0-1 volt so I just built a little circuit to convert the voltage to an air/fuel reading. That works fine, but here is my question.....I want to build an oil pressure and temp guage. Both of these senders are just variable resistors...something like 33-233 ohms. I'm wondering if anyone knows how to make a guage to measure the resistance and display something meaningful. I figure that I have to run a 5 volt regulator to smooth the voltage and then measure the current flowing???? I'm not sure if I'm on the right track or not.



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post #2 of 7 (permalink) Old Jan 9th, 03, 06:10 PM
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I pride myself on my electronics skills, it's how I earn a living, but with all the different gauges on the market I don't think I'd waste my time building my own. If it was the only thing I had to do, then maybe. It can be done with an array of LED's, resistors, and zener diodes. You could even make a digital display. I just don't have the time to figure it all out for free. If you have the time go to the library and improve your skills.

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post #3 of 7 (permalink) Old Jan 10th, 03, 04:48 AM Thread Starter
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Paul
 
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Thanks....I just like tinkering with electronics I think it's kind of fun. I suppose when you don't do it for a living it can still be fun to do in your spare time!. I work on software all day at work so I hate spending any time on it when I'm at home. I've got a couple of test circuits setup on the bench at home....I'll just keep messing with then until I get it right.

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post #4 of 7 (permalink) Old Jan 10th, 03, 07:29 PM
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I'm not sure what you're planning to use for guages - that will make a big difference in your circuitry. Most all meter movements are 50 micro-amp full scale and about 2000 ohms coil resistance. On the other hand, some meters have resitive circuitry already inside and have a full scale rating in the volts range. The later probably will not work very well since their internal impeadance is often too low for the sensor.

You'll have to figure out how to combine discreate resistance values to give you the range of meter movement you need. As you can imagine it depends highly on the sensor you use. For example, an oil pressure guage will vary from near 0 ohms full scale to about 250 ohms with no pressure. A temperature sensor will vary from 0 ohms at full scale (~ 300 degrees) to about 2000 ohms at room temperature. At 100 degrees, they are about 1000 ohms. The problem with temperature sensors is that they are non-linear. As a result, you'll notice the meter faces are non-linear as well.

As I said, you'll have to build a resistive circuit to give you the display range you need. You should avoid letting more than about 60 micro amps of current flow through the meter at any time lest it fry. And you must know your sensor's characteristics. A simple little resitive network can be ugly when you get down the the details.

-dnult

[This message has been edited by dnult (edited 01-10-2003).]
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post #5 of 7 (permalink) Old Jan 11th, 03, 06:38 AM Thread Starter
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Paul
 
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Thanks!!

My oil sender and temp senders both are between 33-240 ohms. I have a socket board setup using a potentiometer and an led bargraph. After I get the range right, I'm going to convert the analog signal to digital and run it through microcontroller and display the temp/pressure on a 3 digit LED.

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post #6 of 7 (permalink) Old Jan 11th, 03, 07:53 AM
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The A/D and microcontroller should simplify things quite a bit. As for sensor linearity, I put my sensor in a pan of water connected to an ohm meter. I stuck a resonably well calibrated thermometer in the pan and kicked on the heat. I measured R / temp at several points and plotted it on a graph. If you find the same non-linearity problem I did, you might be able to use a lookup table to linearize the display. Then again, you may find that in the area of interest (100-300*F) that it is linear enough. Consider using a current source to feed the sensor and send the voltage signal to your A/D. This is more linear that using a dropping resistor connected to 12 volts. Drop me an email if you want more details or schematics. BTW, I know how you feel. I've been known to rack my brains building things I could buy cheaply. However, it's fun to do it yourself.

-dnult

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post #7 of 7 (permalink) Old Jan 11th, 03, 01:42 PM Thread Starter
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Paul
 
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I talked to a couple of gauge manufacturers and they said that they would sell me the bare circuit without the pod housing and I could just unsolder the LED and use a 16 pin ribbon to mount the little LED anywhere that I wanted.....but what fun would that be?!?!

Ultimately I want to have a few circuits for various temps and pressures in one box and a small thin remote display that I can put somewhere like on top of the dash. With the small LED's the display panel wouldn't be very tall(maybe 3/4 inch), but could be six inches or so wide and display many things at once.

I think that I'll do what you did and plot the different readings and then convert it to a lookup table on the microcontroller.....

I'll email you when I get a little further along with the design.

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