Reversing a signal, ohm "damper?" n00b help

It's likely there's a "yes" answer to both of your questions. To dampen the voltage swings, the universal first choice is to see if an RC low pass filter will work. This is a combination of a resistor and capacitor. You probably want a time constant of on the order of 5 to 10 seconds or so. That's equal to the product of the resistance in ohms and capacitance in farads. Thus, you pick one and calculate the other. Example: use a 1 uF capacitor. Suppose we want a 10 s time constant. Then the resistance calculates to 10 Mohm (million ohms). A 10 uF cap would let you use a 1 Mohm resistor. Get a voltage rating on the cap of 25 to 30 volts minimum and note the polarity if it's an electrolytic capacitor.

For the inversion part, this is tailor-made for an op amp. You'll have to make some measurements of the voltages needed to have the gauge read empty, half, and full, along with the voltage outputs of the sender when it is reading empty, half, and full. That's going to take a bit of work on your part, but once it's done, it shouldn't be hard to get an appropriate circuit (it will probably be an 8 pin op amp chip and a few resistors). With a couple of pots, it could also be adjustable.

 

OK, we'll be gentle. We all come out of the womb with zero knowledge (well, except politicians, they start out negative and have to study for 80 years to reach zero), so just ask when something is confusing.

Let's work on one thing at a time. The RC filter is probably the easiest, so let's start with that. Unfortunately, if you do a web search for "rc filter", you'll get a bunch of stuff that looks a bit intimidating, although most of it is at the freshman physics level or beginning EE level. Here's a picture of the circuit. The main thing to know is that the time constant of the circuit is equal to the product of the resistance in ohms multiplied by the capacitance in farad. (Note: "farad" is the technically correct SI nomenclature, but you'll see almost everyone use the plural and say e.g. microfarads in conversation.)

So, pick the time constant you want and choose the values of the resistor and capacitor. Long time constants (here, "long" is longer than around a second) can require high resistances with small capacitors, so the longer you want the time constant, the higher the capacitor's value needs to be. If you want 60 seconds, then choose a 100 uF capacitor. The the resistor is R = 60/(100e-6) = 600 kohm. One of the disadvantages of such a long time constant is that the meter won't read correctly for at least 2 to 3 time constants, so I'd recommend using a 500 kohm pot and you could then tune the response time to your liking.

For the next step, inverting the signal and perhaps amplifying it, you're going to need to make the measurements I indicated (this can be done with a $5 Harbor Freight digital multimeter). Basically, the approach will be to use an inverting amplifier made with an op amp. Here's a description, but a lot of it will be engineer-speak, so you can ignore that stuff -- just try to understand what the circuit does. I'd probably pick an op amp like the LM324, as there are four of them in one package, so this gives you three more op amps in case you want to do something else down the road. BTW, since you live in WA (I'll be up there next week visiting my sister), you could just send me a SASE and I'll send you a couple of these chips, as I have a number of them. If you're interested, PM me and I'll send you the parts.

The answer is yes to your remaining question. I'll be happy to try to help you through this (time may get a bit scarce next week because of some commitments), but fortunately there are quite a few knowledgeable and helpful folks here who will pull our bacon out of the fire if necessary.

 

You could consider that to be a ground connection. It's just that a voltage is referenced to another conductor, so it's really a reference point.

The time constant needed will be driven by usability. You don't want to see the fuel sloshing around in the tank, so you make the time constant much longer than the sloshes.

One suggestion is to see if you can just reverse the inputs to the gauge. Often, analog gauges have two isolated inputs and you can reverse the direction of the needle's response by reversing the leads. If there's only e.g. one lead connection and the body of the meter is connected to ground, then this won't work (unless you can electrically isolate the body of the gauge too). If this does work, then it's likely you can solve your problem with the RC filter alone (i.e., no op amp needed).