I Put Throttle-Driven Eyebrows on My Car. Now It Looks Mad When I Floor It

It’s mean, it’s fun, and it sure as hell isn’t street legal, but we’ve got the low-down on the build.

byLewin Day| PUBLISHED Oct 11, 2022 1:00 PM
I Put Throttle-Driven Eyebrows on My Car. Now It Looks Mad When I Floor It
Lewin Day
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Often, we think of cars as having faces. And despite what the movie Cars might have you believe, the headlights are the eyes. Cars like the Jeep Wrangler that have round headlights add to the effect. You might have seen “angry eye” kits that make these vehicles look mad with plastic slants—but it seems like a shame to limit a car to one emotion. Luckily, I found a way to give a car eyebrows that actually move, and tested it for you on my Mercedes.

A man called Peter on Twitter had the idea that Jeeps should have eye shutters that move relative to throttle position. The more you put the gas pedal down, the more aggro the auto gets. That sounded so funny I simply had to build it myself.

I didn’t have any components on hand, nor did I have a Jeep Wrangler. However, I did have a Mercedes E-Class with suitably round headlights, and I figured that was close enough to get started with. I dropped a few hundred dollars on parts and got down to work.

The resulting build was an utter expression of my creativity and joy. It may not be street legal, or even in good taste, but I promise you, it's a heck of a lot of fun.

Theory of Operation

With plenty of experience in electronics, it didn’t take me long to throw a concept together. All I needed was a microcontroller to read the car’s throttle position, and then use this to drive the angle of a pair of servo motors. The servo motors would be attached at the top of the headlights, and would move the eyebrows accordingly. 

It didn't take long to get a proof-of-concept up and running on the bench.

My early proof-of-concept build. The red circuit is the microcontroller board that controls the system. It reads the "throttle position," simulated here with the knob on the right. Based on that, it directs the servos to rotate the eyebrows on this charming cartoon character's face. Lewin Day

Desiring a quick and easy build, I lent on the components that I was most familiar with. I grabbed some basic servos, an Arduino Uno microcontroller, and some supporting components, and started throwing all the electronics together. Unlike Peter’s design, I wouldn’t go with the subtle rotating shutters. Instead, I’d go for proper angry cartoon eyebrows.

The first thing I had to figure out was how to read the throttle position. One way would be to grab messages from the CAN bus that relate to throttle position. This would work on a wide variety of modern cars, though it’s a little complicated on my 1998 E-Class. Instead, I went for a simpler method, and decided to read the variable voltage directly off the throttle position sensor under the hood.

The Arduino would then take this voltage, and as it varied, tell the servos to move the eyebrows. I set the system up so that the eyebrows were slightly relaxed at idle, and would then quickly move into a more aggressive angle as the throttle was applied. 

With the help of some shower curtain rail to act as mounting plates and eyebrows, l had a basic system up and running relatively quickly. Leaning into the throttle would make the car look angry, just as I hoped.

There were some problems, though. A car sounds angry based on RPM. However, my system would only look angry when the throttle was applied. If you leaned into the accelerator pedal, the car would look angry until you hit redline and had to back off. I quickly realized the system would probably work better with the angriness mapped to RPM instead of throttle position.

The other problem was that the metal eyebrows I’d created were simply not very visible from a distance, especially at night. I realized I could instead build bespoke eyebrows that lit up, and glowed a hot, furious red as the anger (and RPM) rose. This took a lot of extra work to make eyebrows that glowed consistently and looked the part. I cobbled together LED strips, translucent rulers, and foam, and leaned on my experience with zipties and adhesives to make something that would hold up and look the part.

These changes necessitated a switch to a more powerful microcontroller that could run the LEDs and servos all at the same time without skipping or jerking around. I had little time to optimize, so I selected the ESP32, which runs at a mighty 240 MHz and blows the Arduino Uno out of the water. 

I also had to find a way to scrape an RPM signal from the Mercedes, which I eventually found by hacking into the wiring for the crankshaft position sensor. I was well aware I might end up bricking my ECU or car in the process, but when you’re working fast, sometimes you just have to focus on getting the job done. 

The Results

After hustling in every spare minute I had, I managed to get the system up and running. Obviously, driving around with big angry red eyebrows on your car is a bit risky, so I drove down to a secluded dirt road before I fired up the Angry Eyes up for the first time. With the camera rolling, I leaned into the throttle, and made the humble V6 rev like a beast. The car’s face now showed it meant business, with eyes aglow and eyebrows glaring with all the menace red LEDs can deliver.

With the angry eyes ablaze, other motorists would see I meant business every time I surged past during an overtaking maneuver. Whether blasting down a dirt road kicking up dust, or revving at a stop, the eyes added a drama and an attitude that had been missing in my life until now. 

They also made me wish I had a bigger engine under the hood of the Merc, if I’m honest. I wanted something that more adequately matched the car’s expression with a suitably guttral exhaust note. 

Challenges

The build came together on a short timeline. There were plenty of hassles with tapping into the car’s electronics, and the hot underbonnet temperatures and spinning engine bits weren’t friendly either. The development process also claimed the lives of several LEDs, two microcontrollers, and a servo.

Despite all these challenges, the final result worked, and worked surprisingly well. If I wasn’t worried about police impounding my car, I could run these all the time. If I did, I’d probably add a function to let the eyebrows be indicators, too. The LEDs can light up any color of the rainbow, and animate too, so there's plenty of room for silly fun to be had.

The way I designed the system, it would actually be very easy to port to other cars, too. It could readily hook up to just about any car with a simple resistive throttle position sensor, or any car with an inductive crankshaft position sensor. Alternatively, it’s easy to reprogram it to work with other signals, and you could readily mod it to read the CAN bus instead. For interested parties, I’ll be eventually posting the build files online, but realistically, if you’ve read this article, you’ve got the basic info you need to recreate the project.

Should you do this to your own car? Absolutely, in my eyes. These would be an awesome mod for any Jeep Wrangler or Mazda Miata, and would get you all the laughs at your local Cars and Coffee. They’re even more likely to make you a star on the drift circuit or at your local burnout pad. If you've got a 240SX in Formula D and can flirt your way through scrutineering, let's talk about rocking these on your popups.

With that said, I don’t recommend using these on the street. Wiggling lights on the front of your car can be distracting for other motorists, especially when they’re red. If I had something this ridiculous permanently installed, I’d set them up to act as simple non-movable daytime running lights unless I specifically enabled them for fun. That’s about the only way you could get away with something this silly without getting a stern talking-to from the local constabulary.

Overall though, I had great fun putting this build together. I’d like to tip my hat to Peter for the idea, and I hope you all enjoyed the show. Do let me know if you build your own - I desperately want to see these in a drift video!

Got a tip? Let the author know: lewin@thedrive.com