BMW K100 Gear Position Converter

ATmega328P-based signal converter that translates the BMW K100's 3-wire V+ gear position output to 6 discrete GND-referenced outputs -- one per gear plus neutral -- for use with aftermarket speedometers and displays.

Overview

The BMW K100 outputs gear position as a combination of voltage signals across 3 wires. Most aftermarket speedometers expect a simpler 6-wire input with one dedicated wire per gear. This converter reads the 3-wire signal and drives the appropriate output pin for the current gear.

This was one of my first PCB design projects, built during my senior year of high school. I also wanted to build a breadboard Arduino from scratch -- this was a good opportunity to do both at once. The ATmega328P was flashed in an Arduino Uno's DIP socket then transferred onto the custom PCB.

Fair warning -- this is massively overengineered. The same logic could have been implemented with a handful of basic logic gates and no microcontroller at all. But it was a great learning project.

Signal Logic

The K100 gear signal is read via analog inputs with a threshold of 500 (out of 1023).

Pin 1	Pin 2	Pin N	Gear
LOW	LOW	LOW	Neutral
LOW	LOW	HIGH	1st
HIGH	LOW	LOW	2nd
HIGH	LOW	HIGH	3rd
LOW	HIGH	LOW	4th
LOW	HIGH	HIGH	5th
HIGH	HIGH	HIGH	No signal

Why Not Logic Gates?

This could have been built with just a few basic logic gates and no microcontroller. The 3-wire input gives 8 possible states (2^3), of which 6 are valid gear positions. Each output is simply a boolean combination of the 3 inputs.

The equivalent gate logic for each gear:

Gear	Logic Expression
Neutral	NOT(Pin1) AND NOT(Pin2) AND NOT(PinN)
1st	NOT(Pin1) AND NOT(Pin2) AND PinN
2nd	Pin1 AND NOT(Pin2) AND NOT(PinN)
3rd	Pin1 AND NOT(Pin2) AND PinN
4th	NOT(Pin1) AND Pin2 AND NOT(PinN)
5th	NOT(Pin1) AND Pin2 AND PinN

Each output would need one 3-input AND gate and up to three NOT gates (inverters). A single 74HC04 (hex inverter) and two 74HC11 (triple 3-input AND) chips would cover the entire circuit -- total cost under $1. Instead, an ATmega328P was used. No regrets.

Hardware

Component	Details
Microcontroller	ATmega328P (custom PCB, breadboard Arduino style)
Programmer	Arduino Uno (chip flashed in DIP socket then transferred to custom PCB)
Input	3-wire V+ gear position signal from BMW K100
Output	6 discrete digital pins (one per gear + neutral)

PCB

One of my first PCB designs -- a minimal ATmega328P circuit with only the components needed for this application. Designed and built during senior year of high school.

See pcb/ for design files and schematics/ for the schematic.

Pin Definitions

Inputs

Pin	Function
3	Gear signal wire 1
4	Gear signal wire 2
5	Neutral signal wire

Outputs

Pin	Function
8	Neutral indicator
9	1st gear indicator
10	2nd gear indicator
11	3rd gear indicator
12	4th gear indicator
13	5th gear indicator

Files

File	Description
`src/bmw-k100-gear-selector.ino`	Main firmware -- 6-wire output version
`src/bmw-k100-gear-selector-7seg-test.ino`	Development version -- drives a 7-segment display for bench testing

Flashing

The ATmega328P was flashed directly in an Arduino Uno's DIP socket using the standard Arduino IDE, then removed and soldered onto the custom PCB.

To reflash:

Remove the ATmega328P from the custom PCB
Place it in an Arduino Uno's DIP socket
Open src/bmw-k100-gear-selector.ino in Arduino IDE
Select Arduino Uno as the board
Select your COM port
Upload
Remove the chip and reinstall on the custom PCB