Gas gauge calibration unit

Bill,

Not trying to put you down, just trying to explain my reasoning.

The little 50 cent 8 pin micro is fairly simple, no xtal and other baggage. Now days I use them instead of op-amps. I typically do pressure sender conversion for offset and gain, for less than $1 in parts, no precision components or pots. The circuit is low power and is powered by the 5V sensor supply. The conversion fools PCM to do control operations for special performance applications, involving common rail diesel fuel pressure regulation.

In this circuit there are added parts, a current source to feed sender, IVR, buttons, LED, and MOSFET to drive gauge. It will cost a few dollars, but similar in concept.

This is just an exercise, to see if it works. It is not a plan to make money, or sell them.
If it works I will program chips for $2, to save people the expense of a chip programmer dongle and compiler. I do not plan to build circuits.

The idea of using series and parallel resistors to me seems more difficult to implement. Adding a resistor in series or parallel changes both ends of scale. It can be elusive for some, when only working on a single condition at a time. One would need to characterize the gauge, and sender, then work the numbers. Likely the sender will be diluted by the introduction of resistors, so the span will be decreased. To fix that would require gain increase. If the IVR voltage is increased, the in-tank sender sees more current, that could pose danger. The OEM system already has a current of ~167mA at full tank.

The concept of the calibrator is simple, it merely re-scales sender values of Empty to Full to gauge values of Empty and Full. The sender endpoints, and PWM endpoints are stored in non-volatile memory inside micro in the calibration process, in-car by pressing buttons. Since the sender resistance is taken out of the gauge loop, the PWM can over drive a gauge, if that is required, without raising IVR voltage. In the OEM system the 10 Ohm of sender Full, reduces gauge drive to ~67%. Mathematically the gain is adjustable, that is simple and convenient.