I have a classic Lotus with a fuel guage needle that bounces on rough roads. It's not the guage that is the problem, it is the sender bouncing in the fuel tank, which is a well known problem. I need a circuit that will average out the signal from the sender with a time constant of a couple of minutes. The long time constant will keep the guage from moving even around long bends.

This would be a simple smoothing circuit but for the following problem. I don't want to wait several minutes when I turn on the engine to see how much fuel I have. Any ideas for a circuit that will instantly fix the output at the input voltage as soon as it is turned on and thereafter give an accurate average the input?

 

Wouldn't it be easier to modify the sender instead? a segregated section(a bottle)of the tank, with a very small entry/exit orifice would maintain a level much better. The small opening would keep the level inside from rising or falling too quickly(dampened instantaneous response), but would let the level stay true to the larger tank fuel level over time.

 

Kermit2's idea is fine, but it may indicate that he/she hasn't worked on older cars. Often, servicing an old but working component can turn it into an old and non-working component.

I would imagine a reasonable solution would be to develop your low pass filter for the signal, but have a timer that only switches it into the circuit after, say, 30 seconds after the car starts.

 

I've been around a lot of automotive fuel sensors and this isn't anywhere near as easy as it sounds to take care of mechanically. Depending on the year of the car and the technology they used it could be anything between a large variable resistor formed from nichrome wire feeding a hot wire ammeter to a sensor that moves a pot which operates an electronic meter driver.

If it's the old style you're going to have to introduce some mechanical damping on the float similar to the way a shock absorber on a car works - by passing fluid through a tiny orifice.

If it's the newer electronic kind it could be as simple as adding capacitance across the sender unit or the meter itself.

 

What you are describing is an asymmetrical Low Pass Filter. Basically towards full it will have a lower impedance controlled by a forward biased diode and towards empty there is only the parallel high impedance signal path. A capacitor in parallel across the meter movement makes it low pass.



Depending on the load of the fuel gauge meter you might need to use an op amp to buffer the signal.


This is could depend a great deal on what the circuit for the fuel sensor and gauge actually is.


A capacitive sensor is different than a resistive sensor and then it might be inductive, thermal, pressure, mass sensing. Chances are it is simple but what kind of simple is the complication.


If the guage has a high current draw then a filter capacitor across it is not good enough. You would very high Q coils for the impedance or else they will make the scale of the gauge inaccurate by adding too much series resistance. You would need to have an Op Amp Buffer and put the filter across the Op Amp inputs. But you will also need to tune it so that it deflects the guage properly. You actually need two tuning elements. A reference trimmer and gian trimmer.


Basically start with a low tank and mark where the gauge indicates before adding your filter and then tune the the filter to that point with the reference trimmer.


Then fill the tank and adjust the gain trimmer so that it correctly indicates a full tank.

Beware that you migh now have the guage telling lies about how close you are to empty. It is best if you have the gain set high at the start and need to reduce it to prevent the guage from reading high at full. That way you will know that your guage won't tell you that you still have gas when you need to refill.

 

I just noticed that he called the car a "classic Lotus" so my guess is it's the old method fuel sensor & gauge setup - large variable resistor and hot wire ammeter for the gauge.

From Wiki:

In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current.

They used these as they were somewhat slow in response to begin with.