Custom EOTech Reticle with an Arduino Inside

A TFB reader purchased a damaged EOTech very cheaply, then gutted the internals and replaced them with custom electronics. This video shows the result:

The possibilities are endless. Rangefinders, shot counters, timers etc. could be easily integrated and the data shown on the Heads Up Display (HUD).

It was built using a Arduino Pro Mini (about $7 on Amazon), a small variant of the popular Arduino micro controller board, which was squeezed into the battery compartment. The Arduino was wired to the EOTech’s switches and to a cheap I2C OLED display (about $5, search eBay for “i2c oled”). The OLED display replaces the laser module to provide the HUD/reticle. One neat advantage of using the OLED is that the HUD can extend “beyond” the users field of view, allowing the user to look through the scope at an angle and see additional information. The downside of the OLED is that it is not nearly as bright as the EOTech’s laser module and is faint in direct sunlight. A field ready version of this hacked EOTech would need a significantly brighter display.

I asked the reader if he was willing to share the custom code he wrote which runs on the Arduino, but he said it was very rough around the edges and not worth sharing with the world.

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • MrPhantom

    An integrated round counter would actually be a really neat feature on a sight, but I’m not sure that a proper laser-based optic would be able to handle displaying something like that.

    • allannon

      The two might not be mutually exclusive.

      A laser reticle and an OLED “peripheral vision display”; something as simple as a waterfall counter that (un)fills as rounds are fired. It wouldn’t be precise, but it’d be enough to tell you there’s a quarter mag left, without taking your eyes off the sight.

      • somebody

        How about an RFID type decice (no battery required) in the magazine base plate that would transmit data from a force sensor in the magazine base plate when pinged along with an ID number for the type of magazine (specifying manufacturer and model as all magazines won’t produce the same pressure range), the computer in the gun would then compare the ID number and the force sensor reading to look up the round count remaining against a table of different kinds of magazines (make it update able as new magazines come out).

        • MrPhantom

          Might be simpler, if a bit more expensive, to add an accelerometer to the sight that could be calibrated to the specific recoil impulse of the weapon and ammo. No chance of interference killing the system, lower power usage, and wouldn’t require any extra hardware. You’d need to dial in the capacity of your magazines though, which could be an issue if you had both standard and high capacity magazines, like a drum or casket mag.

          • Thomas Söderlund

            Oh, I didn’t see this comment before posting my own reply above 🙂 Yeah, we’re thinking the same thing. There are some really cool (and cheap!) accelerometers out there capable of handling decent amount of G’s and polling frequencies. Would love to see someone try it!

        • allannon

          Hrm…I wonder if it wouldn’t be easier to have a “magazine reader” unit strapped to/through the mag well that’s “networked” to the holosight?

          Theoretically, you might be able to have an array of “smart modules”, all feeding back to the Eohack, which is mostly just a display. Rangefinder unit, ballistic-correction unit (a couple accelerometers to get rate of angular change, track the target a bit, compare against a rangefinder and get a lead reticle), etc? If you did that, your mags might even be able to feed ballistic data back to the sight, if you had multiple round types.

          • somebody

            >Hrm…I wonder if it wouldn’t be easier to have a “magazine reader”
            unit strapped to/through the mag well that’s “networked” to the

            I just suggested RFID because it doesn’t have contacts that can get covered in dirt and can run off external power from radio signals, but getting the antenna for reading it as close to the magazine as possible would be a good idea so there wouldn’t be interference from other magazines (such as if you had 2 magazines taped together). As to additional features, it would be great to have it as modular as possible, you might not want the added weight of a laser range finder when you are shooting at close range but if you are shooting at longer range just throw it on the gun (possibly use those smart picatinny rails that allow attachments to send information to each other).

      • Thomas Söderlund

        Hey guys, original author of this hack here. This is a really cool idea. There’s really no need to trash the original reticle. Combining it with a (hopefully brighter) OLED would keep all the benefits of the original sight while adding extra info in the peripheral fov.

        As for counting rounds I had planned to use accelerometers to record force data of the mechanical components of the rifle going through regular operations (firing, mag insert, etc.) and build statistical models to identify these for the sight. So the sight would recognise what’s happening by the forces going through it. Unfortunately, other projects (and life) got in the way.

        • allannon

          Life may have gotten in the way, but you’re still further ahead with your project than I am…I don’t even own an Aimpoint. 😀

    • John

      I actually have a patent on a firearm round counter displayed on a holographic sight. At the time we were using an external LCD display to show the magazine capacity, bolt position (open, closed, hold), rounds per minute, recoil, etc, so putting the same info on a EoTech was the next logical step.

      Just looked up the patent, it explicitly says holographic sight for the display of this information.

      Kaaa-ching. Probably not, but it sure was a very fun patent to do.

      • Shadow

        If you’ve patented this without true intentions to implement it, you are a scourge on society that stifles innovation. THIS is why we can’t have nice things.

    • Chrome Dragon

      Replace the laser with a laser plus a DLP projector. Sweep the beam like the electron gun on an old-school television.

  • Paul Yeager

    Totally Bad ass and a great starting point.

  • John

    What would be an interesting concept would be to have a PEQ-15 style mounted rangefinder, and a changing BDC based on range along with distance readout

  • TV-PressPass

    Looks like something the guys at Applied Ballistics should be jumping on!

  • Trick

    Please keep the community updated about this situation. If he ever gets the code polished, I’m sure plenty of people would be willing to donate him some money to get access to a DIY

    • allannon

      If I know the Arduino/FOSS crew, several people will start fiddling with teh code, and several more people will start much with the basic hardware idea.

      This time next year, it’ll be Bluetooth-connected so you can check your phone without taking your eye off the sight. 😀

      • No One In Particular

        I just got my bleduino yesterday. I wonder if you could also use an IMU for lead prediction (at a given distance).

        • allannon

          I dunno. Elevation would be pretty easy (feedback from a rangefinder; not small, but just basic math).

          But predicting lead on a moving target? I don’t think Arduino-class hardware could do it, and I don’t know if standard wireless protocols are fast enough (latency-wise) to really do it well.

          • Chrome Dragon

            Predicting lead isn’t hard at all. You could pretty much solve that with a lookup table.

  • MattInTheCouv

    Protect this idea! even if you open source it for personal use this could be a huge cash cow for the guy. Assuming he’s the first, that is. Don’t let L3 make money off this without getting some yourself.

    • Alex

      EOTech already has the capability to change the HUD image.

      • MattInTheCouv

        Reticle change is vastly different than what this is

      • hami

        My understanding is that an EOTech’s reticule actually exists in three dimensional space (in the form of a holographic image) and is projected into the user’s line of sight. This custom sight appears to bypass the holographic aspect of the sight and instead projects a completely digital display for the user.

  • No One In Particular

    I’m tempted to call BS on this one but I’m not sure how an EOTech works. Does it project a laser through the hologram onto a semi mirrored glass?

    • allannon

      There’s a bit more to it than that, but basically.

    • flyingburgers

      Basically he’s using the EOTech as an optical combiner and threw away the holography part. You might as well use a open piece of glass for that. You can preserve the parallax-free part by using a technique called computer generated holography, but it requires much more computational power and pieces than an Arduino.

      • allannon

        In the video, it seems to remain parallax-free. The reticle, when the camera moves, seems to hover over one point.

        • Chrome Dragon

          Parallax-free can be achieved using the curvature of the screen, so not a real problem.

  • Rogier Velting

    Huh, never thought of something like that. I do still have an Arduino somewhere… now I need an old EOTech.

  • USMC03Vet

    That is hot.

  • James

    You know how EoTech now has those battery covers with the laser in it? Replace that with a small camera. Add a better processor, some computer vision algorithms (face detection?) and then pipe the whole thing back up to the display.

  • Patrick Mingle

    Wow this could be a real game changer if you could apply some useful tech to it

    • Alan Ellis

      I’m working on this same type of project, and I’m looking at adding Tilt sensor to show when rifle is level, plus show you distance you’re holding over for, etc. Plus Temp, Barometric, etc, sensors. Depends on how bored I get 😉

  • Danny Gonzalez

    get a kickstarter project going based on this!

  • Gambit Seven

    incase no one noticed, this is an eotech clone. glass is really close to the rear toward the buttons, the frame housing the glass is way thicker than all Eotech models, also you can see the emmitter.

    very cool nonetheless

    • allannon

      Yea, I think most of us are far more intrigued by the idea than the specifics of the sight. 😀

      • Guest

        kinda makes a difference… unless it’s been done on a real Eotech it doesn’t really matter. Fake ones use LED instead of the “holographic” technology like the ones used in fighter jets.

        In other words, the technology is entirely different. If it did not matter, might as well take the innovations that happen in airsoft and say “oh, guess we can use that in real guns also”

      • Gambit Seven

        kinda makes a difference… unless it’s been done on a real Eotech it
        doesn’t really matter. Fake ones use LED instead of the “holographic”
        technology like the ones used in fighter jets.

        In other words, the technology is entirely different. If it the specifics did not matter, then we might as well make an argument about the using the innovations in airsoft gun and applying them to real guns. I think we all know how that would turn out.

        • Dan

          Well seeing as how he completely replaces the electronics and the display it doesn’t really matter. Based on what this guy did it could be put in any kind of casing the parts would fit in.

  • loopydupe

    The idea of extra information shown if you look into the glass at an angle is very fascinating to me; I hope that concept goes somewhere.

    • nadnerbus

      Yeah I actually really like that idea. Invisible and out of the way when you have a proper cheek weld and are aiming down sight, but right there and always available with a slight cant of the neck. Really fascinatingly simple yet so much potential upside. Keeps the main reticle clean that way. Combine that with a user programmable layout and we are really talking.

  • dan

    Is this the work of jumpifnotzero? I am only stereotyping because of his computer lingo name

    • JumpIf NotZero

      Ha, no 🙂

      I’m a pro. Like literally a professional EE working with the design of embedded micros. You’d see me using an Arduino when I was broke and methed out if my mind, at the end of a noose maybe. It’s all custom pcb, fpga, all my own designs from layout to firmware for me.

      I have been to the factory that EOTechs are made in Michigan though. The laser/led unit is about the size and sort of the shape of a kidney bean. To answer the question above, there is no room for both a screen and a factory unit.

      I’ve done some work with taking EOTech’s apart to look at how the “NV Modes” work. I think it’s stupid the “NV” model is a different cost considering the changes that are made to make that happen. I won’t discuss it here, as a courtesy to an A2 company. That said, Trijicon service blows, I’d post all sorts of hacks about them if I had them.

      This is a cool little hack, but asking for the source code is entirely pointless! It’s an Arduino with an I2C driver. You’re talking 20 minutes maybe to get rolling in C. “Rough around the edges” doesn’t surprise me. It’s taken me personally about 12 years to go from “rough” code to real deal proper embedded state or interior machines. I definitely give him respect for being the first I’ve seen to do it.

      • Shadow

        > I won’t discuss it here, as a courtesy to an A2 company.

        Apparently shorter duty cycles on the PWM are secret now.

        • JumpIf NotZero

          Um, lol,

          It wasn’t a technical discussion, more a marketing distinction.

  • Lance

    A round counter that be awesome. it be as close to Dela Force Black hawk Down is to real life LOL.

    • John

      If anything, it’d be like the round counters in Halo. I wonder if you could fit a little compass in there too…

      • Chrome Dragon

        It’ be trivial, honestly. Combine that with a wireless (bluetooth low energy?) data link to a PDA (maintaining a constant GPS fix), a gun-cam, and a laser rangefinder, and you can have a drone airstrike for a grenade launcher.

  • billyoblivion

    “he said it was very rough around the edges and not worth sharing with the world”

    Oh hell no.

    That’s what Open source is GOOD for. Put that shiznit on github and *someone* will fix it.

  • Benedict Tan

    If he wired it to display the status of the safety, rounds in magazine and CCIP he’d make a ton of cash 🙂

    • M40

      Interesting… maybe a small prox sensor in (or on) the magwell? Each time the sensor sees a drop (magazine out) and a rise (new mag inserted), it could reset the round count to zero. A round counter display on the HUD could be green in rounds 1-10, change to yellow in rounds 11-20, and begin flashing red for the final 10 rounds alerting the user that the magazine is running low.

      • M40

        PS – Eventually, this type of thing WILL be adopted by our military… it’s already in the works. Each soldier will have integrated comms, GPS, etc. That means that you could see an IFF of sorts, where if a soldier aims at someone down field, the gunsight will warn if he’s aiming at a friendly.

      • Benedict Tan

        I think a more reliable sensor solution would be to modify the magazines themselves to track where the follower is. Something along the lines of a row of reflective IR indexes that will be blocked as the follower rises up the mag. On each firing the index is blocked by the follower and the sensor which looks down the magazine picks up a different signal return which corresponds to a round count. The problem with resetting the round count is that the assumption that you are starting with a full magazine of x rounds is… well… an assumption. Which defeats the purpose 😀

        • M40

          That’s creating some expensive magazines. (and you’re proposing a power source in each magazine… and some kind of wireless comms?).

          I didn’t say there was a round count-DOWN. I would actually have it counting UP. That way if you loaded a magazine to 25 rounds, it still counts up and still flashes as you get into the 20’s.

          Probably the simplest way to accomplish an infallible round count is to have the sensor in the proper location on the magwell, so that it sees the density drop from rounds to follower. Depending on sensor placement, you could have it start alerting you with 5 rounds left, 10 rounds left, etc (it’s just seeing the follower reaching a certain point). No magazine mods required.

          • Benedict Tan

            No need for power in the mags, just a change of follower and an insert. I figure if you are going to go with such a venture it would make sense to be accurate to the round. If the system knows the number of rounds in the magazine then you can also differentiate between a stoppage and running out of ammo.

    • Alan Ellis

      Radetec ammo counters do this, and technically could be modified to work with this display – I’ve had plenty of chats with the manufacturer about it in the past (can even hook to google glass ;)). I am already working on it using the same setup this guy did his display.

  • gunslinger

    que the “why not both” OLED and laser?

    • Chrome Dragon

      Because a tricolor (quadcolor, with an IR diode for NV mode?) laser DLP system simulating a virtual retinal display would be far more interesting.

  • Michael R. Zupcak

    I see we have a LOT of video game players commenting

    All kidding aside, this probably will happen at some point

  • ozzallos .

    DO WANT.

  • 1leggeddog

    He’s onto something…

  • guest

    1) Make this as rugged as the real Eotech
    2) mod the laser battery cap to be a laser rangefinder battery cap
    3) add drop compensation reticule controlled by the rangefinder
    5) Profit!

    • Chrome Dragon

      Sounds a little like the Burris Eliminator, honestly.

  • Geoffry K

    How about a better video instead of 2/3 black bars?
    Learn how to use your iPhone, people!

    • Thomas Söderlund

      I agree! I apologize for the aspect ratio. But the damn thing is just so comfy holding upright. I propose they tilt the camera 90° instead! 🙂

  • Zachary marrs

    It needs to display my action points, my weapon condition, my health, and ammo count.

    The next logical step will be vats

  • Michael MacConn

    Why aren’t we funding this?!

  • jeff

    if you watch on youtube it says this is a knock off eotech REFLEX SIGHT.

    i love the idea but the title is misleading

  • Rick

    okay, I have one of those that doesnt see much use after I bought a Meprolight M21. It’s been riding one of my .22s. I have a larger Arduino Uno that I could use for development before switching to the tiny version to fit inside….ooh, multicaliber BDC calibrated to each of my rifles individually, no zero change needed going from my 22 upper to 556 upper to planned .300BLK upper, just select the option….rangefinder to adjust drop and inclinometer to handle those uphill/downhill shots?

    Creator should make sure to take credit for this, even “rough code” when released demonstrates “prior art” in a way that’s impossible to ignore later.

    File for patent STAT! just getting the process started locks this in!

  • Tothe

    Pass it along to Defense Distributed for open-source Creative-Commons distribution and development!

  • Ben Pottinger

    This could be done with a custom pcb stuffed to the brim with interesting bits. Think compass, gyros (the fancy kind or sandwich kind, your call), temp, altimeter, hygrometer, even GPS or other radios. Just look at the teensy 3.1 for an idea of how cheaply you can stuff a decent amount of CPU power into a tiny space (it’s a ARM Cortex M4 at 72mhz and can run arduino code). A better choice. As for the screen, I’ve seen a few other options that might work. A dot matrix style VFD panel would be very bright but who knows how reliable it would be in such a rough environment. A simple LED matrix. It’s small enough you should still be able to find some led displays of reasonable density that are still just LED and not LCD. A 128×96 or something might work.

  • Ryan Arendt

    Anyone want to make some money changing my recital? PM me on Facebook and we’ll talk.