1976 BMW 2002: Digital Gauges

Modernizing the mechanical gauges in a calssic car

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Ever since I bought this car I've been working on modernizing it, one step at a time. I'm currently working on upgrading the mechanical components of the gauge cluster for digital accuracy. I want to retain the original look as much as possible, and reuse as many parts as I can, but everything behind the scenes should be up to the standards of a 21st century car.

The primary visible difference when I'm done will be the replacement of the odometer decade counters with small 7-segment LED displays. I tried to design a way to keep the decade counters, but it got too complicated. I think the 7-segment displays will be a nice semi-modern touch without seeming too gimmicky.

So far this has basically just been a good excuse to learn about microcontrollers, and PCB design. I expect the end result to look pretty good, too.

The whole circuit will be based on an Atmega328 microcontroller, because they're cheap and seem like a good starting point for learning about microcontrollers.

The odometer digits are driven by two MAX7221 display drivers. The speedometer gauge needle will be driven by a Switek x27 stepper motor, which will be controlled by an L293D motor driver IC. I plan to measure dstance traveled by mounting a diametrically magnetized ring magnet to the spindle that used to drive the mechanical gauge, and recording its motion using a US3881 Hall effect latch IC.

  • 1 × Atmega328p Microprocessors, Microcontrollers, DSPs / ARM, RISC-Based Microcontrollers
  • 1 × Switek x27 stepper motor
  • 2 × MAX7221 7-segment display driver
  • 1 × L293D Motor Driver IC
  • 9 × small 7-segment displays (8 red, 1 orange for tenths of a mile digit)

View all 9 components

  • PCB arrived, first tests

    Jonathan Perry-Houts03/12/2015 at 04:16 0 comments

    My PCBs arrived yesterday from seeedstudio. First PCB I've ever designed, and it looks pretty good, I think. Soldering SMD components for the first time was a bit weird. The solder joints don't look super uniform, but they're not as awful as I sort of expected they'd be.

    The board seems to work (the same as the breadboard setup I had, so I would hope it works). And it looks good doing it! A nice advantage of having a physical prototype that's in the final formfactor is that I can see what the whole thing looks like assembled. It's cool to finally see what the little displays look like behind the gauge faceplate. The pictures don't do it justice, it looks cool.

    One thing I realized recently is that I'm probably going to want to drive the stepper motor with a smoother PWM sine wave signal, which means I need to switch some stuff in the circuit. So this first iteration is already outdated. I'm going to play with this design as-is for a while and see what I can do, then think about my next iteration once I have a much more solid idea of what works and what doesn't.

  • Breadboard prototype done, moving on to PCB

    Jonathan Perry-Houts02/19/2015 at 05:10 0 comments

    I completed a simple prototype of the speedometer/odometer circuit on a breadboard. Prototype PCBs are in production now, from SeeedStuido. The PCBs came to ~$35 for 10 boards, and the components are almost $100 / board. Most of that cost is LED displays, and the MAX7221 ICs.

    I haven't written any code yet, except to test that I am able to drive the stepper motor and display drivers, via arduino libraries. Since this project is primarily about learning, I plan to write the firmware from scratch with the AVR-GCC toolchain.

    Of course I'll post everything on github when I have a chance. For now if you're interested in contributing contact me directly.

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zakqwy wrote 02/19/2015 at 15:29 point

Love the 2002--looking forward to seeing how this project changes the aesthetics of the classic instrument cluster. Phase II: S14 swap? ;-)

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Jonathan Perry-Houts wrote 02/20/2015 at 22:21 point

Haha, that might be phase 50 or so. I did already complete an EFI
conversion using megasquirt. Thinking of moving to FreeEMS eventually.
S14 would definitely be a nice goal on the *when I have several thousand
dollars burning a hole in my pocket* list!

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