Need resistor circuit please

Hi ,I need some help in designing a circuit for my boats outboard motor .
What I have is a trim/tilt linear potentiometer ,very basic from 0-80 ohm .
0 ohm when the motor is fully down and 80 when fully tilted up .
Now the problem I have is the gauge I have is back to front ,as it requires 80 ohm
to be fully down and 0 ohm fully tilted up .
The gauge is sealed so I cannot get inside it
I hope there is an easy way to convert the signal .
Any help will much appreciated

Toxie said:

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I hope there is an easy way to convert the signal ...

Click to expand...

Actually, this is a very hard problem, requiring a regulated power supply, opamps, resistors, potentiometers, etc. What is your electronics building skill level?

Thanks for the reply .My building/ electronic skills are ok ...perhaps more rusty than any thing .Being a qualified TV/Radio serviceman ,but haven't serviced much for the past 12 years .
Look forward to seeing what you have in mind
Thanks again

Ok, then let's get started. Do you know what a Thevenin Equivalent is https://en.wikipedia.org/wiki/Th%C3%A9venin%27s_theorem?

I need a Thevenin Equiv for the gauge. I'm guessing that the sender connects to two terminals on the gauge. Is one of these terminals common to either the +12/14V power or Ground terminals on the gauge? (I hope it is, because that will make the job easier).

Power the gauge with 14.2V (I'm assuming the engine is running; alternator charging, so use an adjustable bench supply set to 14.2V).

With no sender connected, what does the gauge indicate? Zero or Full scale? Up or down?
Measure the open circuit voltage (using a DMM) at the sender terminals. Note the open-circuit voltage.

Now set the DMM to read DC current at a few 10s of mA. Connect the DMM leads across the sender terminals and measure what the gauge delivers into a short. Note that, too.

Now get one each 10, 22, 33, 47 and 75 Ohm 1W resistors, connect them in place of the sender while measuring the voltage across the sender terminals. Note the voltage, and the gauge needle position. Make a table of voltage/gauge reading vs resistance. (You could use a 100 Ohm pot wired as a rehostat to make these measurements. Use the DMM Ohmmeter to preset the pot to friendly resistance values, like 10, 20, 30, 40,..., 80 Ohms).

If neither sender terminal is common to either +supply or Ground, make the measurements outlined above anyway, but at each step, keep track of the voltage at the two sender terminals, and add them to the data you report.

If you can make these measurements, then we can come up with a "resistance inverter" circuit... (I told you it wasn't going to be easy).

Do you care if the new circuit gives you only 2 values? i.e. just 80ohms and 0ohms? It's very easy to give that.

Well... here's a circuit that'll give you 5 graduated positions depending on the input Pot. value.

Output of ~{82, 60, 40, 20, 0} for input resistance of ~{0 to 14, 14 to 28, 28 to 43, 43 to 59, 59 to 80}.

What I have is a trim/tilt linear potentiometer ,very basic from 0-80 ohm .
0 ohm when the motor is fully down and 80 when fully tilted up

Click to expand...

Is it really a pot (3-terminal device) or just a variable resistor (2-terminal)? If the former, then you already have both 0-80 and 80-0 available.

dougy83 said:

Well... here's a circuit that'll give you 5 graduated positions depending on the input Pot. value.

...

Click to expand...

Clever, assuming that the one end of the sender is grounded.

Why not just go whole hog? Use a MCU with an ADC to quantize the sender resistance to 256 or 1024 steps, and then use a R-2R DAC to create the variable resistance...

alec_t said:

Is it really a pot (3-terminal device) or just a variable resistor (2-terminal)? If the former, then you already have both 0-80 and 80-0 available.

Click to expand...

Even if the sender only has two terminals (or only one, if it uses chassis ground), it might be easier to change the sender than to create an elaborate circuit...