Looking for advice on a DIY car HUD

Projects General Guidance
1

Hi everyone,

I’m building a simple DIY HUD for my 2004 Toyota Corolla and would love feedback on my approach.

Goals:

  • Display speed on the windshield
  • Future additions: cabin temperature + fuel stats
  • keep cost low

Why not OBD2 or GPS?

  • The car only has OBD1 (no speed/revs).
  • GPS seems unreliable (lag, bad connectivity, higher cost).

Current Plan:

  • Tap into the Vehicle Speed Sensor (VSS) signal that runs from the transmission to the speedometer.
  • Use a solderless T-connector so I don’t cut wires.
  • The VSS outputs a 12V pulse. I’m scaling it down using a voltage divider (220Ω + 330Ω resistors), which should reduce 12V → ~4.8V for the Arduino Nano.
  • Display the data on a small OLED screen, then project it onto the windshield using a small reflective film (haven’t found the right kind yet—recommendations welcome).

Questions:

  1. Is the resistor divider safe/reliable, or should I be using a transistor/optocoupler for the VSS signal?
  2. Any recommendations for cheap but effective reflective film for HUDs (ideally from Amazon AU)?
  3. Any other pitfalls I should watch for with this setup?

Here’s the OLED I was planning to use: Amazon Link.
Here is the schematic from Tinker cad Tinkercad VSS Reader (only program school offered for schematics that I could simulate the Arduino on. I am open to free alternatives.) if it helps.

Thanks in advance for any advice!

2

Welcome to the forum

Using a display of that size, how large do you expect the speed display on the windshield to be ?

3

Not massive, enough to read just off to the side of the windscreen.

Do you think it’s too small? Should I be looking for a larger size?

4

It's less than an inch across

Is that big enough for you ?

5

It’s recommended. Count on electrical noise from the 12 volt car system.

35 years ago the low voltage side of the ignition coil was tapped for engine speed using an opto coupler. It worked fault free for 10 years. All based on a Z80, 64 bit aritmetics. For speed the gear in use was detected and tuned to give correct readings.

6

I thought with the reflective tape on the windscreen it would be big enough to read. What size would you recommend?

7

Thanks for the advice.
I’ve come from more of a coding background so I’m not overly familiar with circuits, if you could provide an example or something similar that would be much appreciated! And by noise could you be more specific as in >12v or just small random pulses not connected to the actual speed that need to be filtered.

8

I have no experience of HUDs, but wouldn't a reflection be smaller that the original ?

You mention projecting the display. How would that work ? Can you post a link to an example of what you are trying to do ?

9

Sorry, no such available. Look at datasheets for opto’s and application notes how to use them. An oscilloscope applied to the signal You want to use would be the best. A common noise is voltage spikes in the pulses. Random, short pulses, not likely.

Prepare for doing some tests and evaluate that. Designing circuits for guessed noise is a waste of time and efforts.

10

From my research a mirror will reflect it at the same size unless it’s a convex mirror.

As for an example this isn’t using an OLED display but it’s the same concept with the reflective film Reddit - The heart of the internet

11

Ok thank you for the reply, I will make sure to give it a look.

12

Yes, please do. The “common opinion” here is that cars of today are terrible noise emitters. I doubt it. I worked on a 1985 model car having no digital electronics, only an electronic ignition module that died after 100 000 kilometers when the car sat at the shop for service…