CarHUD System Overview

Before diving into the details of the project, I should discuss the system as a whole.  To begin, we need to ask ourselves a question.

How do we get information from the car?

All modern cars contain a diagnostic port with various implementations of the On Board Diagnostic (OBD) system.  There are many protocols used by different manufacturers to talk to various components of the cars electrical system, but they are all covered by the blanket of OBD.

With a quick Google search, you can find tons of hits for OBD, specifically version II.  You'll also find a lot of devices being sold on eBay that will talk OBD-II.  Most all of these devices use the ELM-327 and interface to USB or Bluetooth.

I picked a ELM-327 to USB device from a company called Interfuse and had it shipped from Wisconsin.

So where does this put us?  We have an OBD-II device and our Raspberry Pi.  How does one put these together to make a HUD system?

Phase 1: ELM-327 to USB based HUD

For simplicity sake, the first HUD system will use a commercial off the shelf (COT) ELM-327 device hooked to the Raspberry Pi.  Here is the system diagram:

The diagram shows a pretty simple looking setup, you have a power supply, the Raspberry Pi, your ELM-327 device and a projector system.  We'll get into the projector design later on in this blog as it's out of the scope of this post.

This setup isn't without its flaws.  The ELM-327 devices all pull 12 volts from the connector interfacing to the car.  This allows them to get proper signal voltage levels to talk to the cars computer and also power the device itself.  The USB device will pull some power over USB, but is not able to talk to the car without something providing 12 volts.

The dependence on needing 12 volts from the car battery is a concern because you cannot leave the device permanently connected.

This leads us to Phase 2.

Phase 2: STN 1110 based HUD

The STN 1110 chip uses the ELM-327 command set, but houses the ELM-327 code on a faster processor.  This can only mean good things as you can then talk to the microprocessor and the car faster.  I haven't found a pre-built device using this chip, so I intend to build my own.  Here is the system diagram for this phase.

As you can see, it's very similar to Phase 1, but it has it's own 12 volt line allowing me to keep it hooked up permanently and freeing up my ELM device for diagnostic use when necessary.  I won't go into the details of the Phase 2 design, we'll leave that for a later post.