Circuit design, 7-segment LED, vehicle speed sensor

Projects General Guidance
1

Hi, I'm a recently retired noobie, and this is my first Arduino project. I don't have any experience with electronics but I learn quickly. I have an older Yamaha motorcycle that has a difficult to read analogue speedometer. I'd like to augment (not replace) it with a 7-segment LED display. The bike uses a 3-wire hall effect 'gear tooth sensor' which is mounted to the transmission and feeds into the ECU, which feeds into the speedometer. The 3-wires are +5v, GND, and signal. I've done some development with a Uno_R3, an Adifruit 7-seg LED with I2C backpack, a used OEM sensor I bought off ebay, and a 3D printed "gear" to generate signals. This is all done on the bench top at the moment, I have not touched the bike yet. So far everything is working well; I can calculate RPM and display it on the LED display. Later I'll convert RPM into speed and display that instead.

The next step is to hook it up to the bike but I'm unsure how to design the circuit due to some complications. On the bike I can't hijack the gear tooth sensor and use it exclusively for my LED speedo because the ECU uses it for engine management and other stuff. I must 'sense' the output signal without affecting it. I also don't want to draw any power from the +5v/GND wires because they are supplied by the ECU and I don't want to risk damaging it. My speedo needs a separate power supply which might end up being an Adifruit buck converter rated 1.2A at 5V. My request is how do I design the circuit?

Adifruit buck converter: MPM3610 MPM3610 5V Buck Converter Breakout - 21V In 5V Out at 1.2A : ID 4739 : Adafruit Industries, Unique & fun DIY electronics and kits
Adifruit 7-seg LED: Adafruit 0.56 4-Digit 7-Segment Display w/I2C Backpack - Yellow : ID 879 : Adafruit Industries, Unique & fun DIY electronics and kits
This is my bench top design ...

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2

You have presented a good design. Remains the buck converter it looks like.
What's Your worry?

3

Will be a need to consider isolating noise from the ignition.

4

Only minor thoughts…

  • Use a smaller CPU, it’s easier to hide.
  • Plan your power supply and filtering.
  • Determine the sensor output with an oscilloscope (waveform and frequency)
  • Ruggedise your design against vibration, water and heat.
5

LED display will be difficult to see in daylight and I doubt it would be an allowable mod under any machinery specs.
I have yet to see any production analog gauge as " difficult to read" unless of course it has faded into obilivion.
In that instance it would be like for like replacement requirement.

6

lastchancename - Yes, later I'd like to use a Arduino Micro without headers hand soldered to a Adifruit buck converter. Planning the filtering is the tricky part for me. I don't have a sillyscope but I'm working based on the assumption it's square wave. Frequency will be proportional to engine speed. Yes it will be ruggedized when the time comes.

The schematic below shows what I want (please don't laugh) and I've tested it, but it doesn't work. Maybe ground is floating and the Uno can't make sense of the output signal. I did connect ECU GND to Uno ground but that also didn't work (random numbers on LED display). I will reiterate that I don't want to draw any power from the ECU because I don't want to risk damaging it. My problem is that it seems I am stuck with having 2 power sources; ECU+Geartooth Sensor and Uno+LED display.

How can I make this work?

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