This project has gone through a few different phases, but I think I've finally found the beginnings of a decent solution. The problem I have is that I just swapped a modern transmission into an older car. The modern transmission uses an electronic speed sensor, but the old gauge is mechanically driven. They make a converter for this kind of thing, but at over $300 and with questionable reviews I decided to try to make something myself for the fun of it. Also, the guage is a ribbon style horizontal setup as opposed to the more common round variety, so I haven't be able to find any electronic aftermarket ones to simply replace it and don't feel like mounting another one somewhere else. I took some microprocessor classes in college a few years ago, so I have an SX28 and BS2 on hand, but my preference would be to use the SX since the BS2 is currently set up for cruise control on a different car.
The simple solution would be to lock the magnetic couple on the speedometer and just use a stepper to position the needle appropriately, but doing this would render my odometer inoperable which I would rather not. To keep the odometer functional, I worked on trying to direct couple a motor to the speedometer where the cable used to attach. This worked okay, but finding a small enough motor that would work at very low rpm (less than 2000) without a gearbox was difficult. I managed to scavenge one out of a printer that worked okay, but if the coupling between it and the gauge wasn't near perfect, there wasn't enough startup torque for the guage to function at low speeds. The vehicle environment is also such that I doubt I could build a substantial enough mount to keep the motor in this near perfect alignement while driving that would fit behind the dash. I did prototype a circuit for this setup that consisted of the SX reading the input pulses from the speed sensor which then controlled a digital potentiometer that fed a 555 timer that I set up as a rudimentary PWM motor driver.
However, I've come across a speedometer from another vehicle that maintains an analog odometer while using only an electronic input. It appears the needle for the guage is controlled by a stepper and the odometer has a separate motor to run it. The new plan is to use the guts of this gauge behind my current speedo faceplate. I believe this new gauge functions with a square wave input that varies frequency based on speed. This makes the circuitry on my side a little easier as now all I need to do is read the speed sensor input and output a varying frequency as opposed to controlling a motor, but I'm unsure if the SX is capable of performing both tasks simultaneously. I need to read in pulses from a hall sensor, do some math based on tires and gear ratios, and output a corresponding square wave of appropriate frequency. The trick is that as much as possible I'd like to have a constant output (granted the frequency of the output will vary on an update cycle). My very basic approach would be to poll a sensor for a set period, then do my tire/gear ratio math, and update the frequency, but I'm not sure how to keep an output going during the sensor input and math operations. I bypassed this on my original design by offloading the output signal job to the 555, but I got to wondering if it would be possible to keep all the operations on a single chip to reduce circuit complexity. Also, the digital potentiometer I have is limited in the number of steps available, so if I was only using the SX I could potentially have more precise control of the output signal than what I can achieve with the RC circuit that drives the 555.