The next generation of LEGO robotics: Mindstorms 51515 Robot Inventor [Review]

The year 2013 feels like eons ago. After all, a lot can happen in 7 years, and that’s how long it’s been since Mindstorms EV3 arrived on the scene. Now it’s beyond high-time for the long-awaited successor to LEGO’s premier robotics platform to hit the stage. Back in June when LEGO Mindstorms 51515 Robot Inventor was revealed, some people were ecstatic, but many were unimpressed with the features of the new system. In this review, we’ll take a deep dive to see if this set proves that you shouldn’t judge a bot by its cover. Robot Inventor contains 949 pieces and will be available beginning October 15 for US $359.99 | CAN $459.99 | UK £329.99.

The box and packaging

The colorful matte box is roughly the same size as its EV3 predecessor. But past that, it’s a very different experience. The EV3 came with an exterior sleeve that you cut to create a playmat, and inside the sleeve was a standard side-open box. In the case of Robot Inventor, there is no sleeve, and the box is two separate parts: bottom and lid.

An interesting feature of the new box is that both halves of the interior have a sorting aid. There are printed sections with 1:1 scale images of parts in order to help the builder organize everything. Of course, there would be way more brownie points if the sections actually had ridges to keep parts separate and prevent them from rolling around.

In any case, this is a set with a ton of small parts and it’s nice to have the box be useful as a container. Technically you could do the same with a side-open box, but this makes it so much easier to keep track of everything.

The contents

Unsurprisingly, there are no numbered bags in this set, as it’s designed to support a variety of builds. On the plus side, LEGO isn’t forcing the builder to start with a specific robot first. On the other hand, that box comes in real handy when you have to dump a gazillion (949) pieces to find what you need. Thank goodness the black pins come in their own bags!

Once all the parts are organized, it’s very satisfying, but some sections seem big for the number of parts they contain while others seem too small. Also, although it’s a great concept in theory, and certainly helpful to a degree, it’s hard to imagine many people (especially kids) utilizing the box in this way. I would guess that most would either just dump everything and build, or find another container/s to organize into.

Moving along, it’s immediately apparent just from looking at the box exterior that teal (dark turquoise) was going to be the primary pop of color in the set. It’s a great choice because it’s playful, fresh, and stands out from all the other LEGO robotics kits. The other excellent thing about it is that it also means the set brings several elements that have never been in this color before. Interestingly, a single teal element, the 2×4 L-shaped liftarm, did come in two previous sets, a full 20 years ago when teal was introduced the first time. In addition to the teal parts, we have a couple of other newer elements in new colors. And the 2×6 shooters with their projectiles, reminiscent of the EV3 touch sensor, are new for 2020 but have appeared in a few other sets already.

Instructions and stickers

A small page of stickers comes with the set, and sports a fun range of designs and matching colors. Whether you like stickers or not, it would be fair to say that if there had to be stickers, these are a nice choice. You can see and appreciate the mix as being an attempt at better gender neutrality. It’s also very fitting for a pre-teen audience. The best part is that the builder is encouraged to use them wherever they like instead of specific locations.

On the other hand, the set does not come with paper instructions. Instead, there is a small booklet titled “Quick Start Guide” that really isn’t a guide at all. It just tells the builder to download the app for instructions. The booklet is mainly intended to fill the legally required purpose of stating all the electronic component warnings. But fortunately, it does have one useful advantage, which is the inclusion of the parts inventory and a 1:1 scale Technic beam and axle identification page.

As previously mentioned, the builder is directed to download the app for further instruction. It may be due to working out the bugs before it hit the market, but our pre-release version was quite difficult to get downloaded on smaller devices, despite the fact that we were using it only shortly ahead of the product’s launch. iPads that don’t support iOS 13.0 cannot run the app, even though the SPIKE Prime app works fine. At this point in time, the app also cannot be used with a new HP Chromebook, even though the same device also supports SPIKE Prime without issue. And at first, the app wouldn’t run on either a Samsung Galaxy S7 Edge or an iPhone 11. But after multiple attempts separated over several days (and likely updates on the back end), the phones finally worked. On the flip side, the program downloaded instantly on a Dell laptop running Windows 10 and works like a charm.

Having reviewed both Boost and SPIKE Prime, there is something about this new release that feels more polished. The colors and format, while still playful, elevate the platform in maturity. More on this later.

Programmable components

Now for the meat of the set; and likely the reason you’re reading this review. If you’re a “moving parts” junkie like me, it’s all about the electronic components that make it come to life. Without getting into the overly nerdy specs, let’s take a little tour of the different elements that come in the box.

As mentioned before, the 5×5 pixel grid “Hub” comes in teal this time, in stark contrast with the SPIKE Prime yellow. But aside from the color, they are exactly the same. The device has a rechargeable, lithium-ion battery that composes most of the unit itself. The actual “brains” only make up about a third of the weight. But that’s not saying too much, because the whole thing is half the size and weight of a 6-AA-battery-laden EV3. There are connection points on all 6 sides, 6 universal motor/sensor ports, and a 6-axis internal gyro sensor and accelerometer. Charging is done via included USB cable, which can also be used to connect to a computer. Alternatively, connections can be made via Bluetooth.

The designers made the big decision to forgo a touch/pressure sensor in favor of an additional motor. Therefore, this set comes with 4 identical ones, in a new color combo of grey and white! While there are benefits of having a touch sensor, perhaps this is a wise choice. In a world where we prefer things to be voice or motion operated, buttons become less useful. And in a toy like this, the hub, color, and distance sensors can arguably accomplish the same needs, while adding another motor enhances playability options. Less input, more output.

Perhaps it should be noted that SPIKE Prime comes with two of these motors in addition to one larger one. I have found that the smaller motors are more versatile in terms of embedding them in a robot. They also seem to provide a decent amount of power for basic needs, although less efficient than its cousins. Another feature is that they have integrated rotation sensors with absolute positioning.

The color and distance sensors are the same as those debuted in SPIKE Prime. The former of which has updated, more accurate technology in comparison to its EV3/NXT predecessors, and can even recognize a tiny dot. It is capable of distinguishing 8 different colors in both darkness and bright light. It also emits light in addition to collecting data and is half the size of previous sensors. The latter mentioned above looks very similar to its old counterparts, although more squared off. In practice, it has a shorter range than EV3 (200cm vs 250cm) but is more accurate, especially with quick detection. It also has light rings around the “eyes” that are more programmable (compared to EV3’s preset “eye” patterns) and hardware integrations that allow 3rd party connections.

We’ll come back to further comparisons, but based on initial impressions, one of the primary benefits to the new components is that they are far easier to incorporate into builds than similar elements from the past, simply based on their shape and connection points. It’s important to note, though, that these components give a fixed-length cord, versus the interchangeable cords offered previously.

The builds and programming


I chose to start from left to right in the app, which had me beginning with “Charlie” the chubby little dancing bot with a treasure-chest belly. I must admit, I started this review with skepticism, but playing with Charlie made me smile. The build techniques are fairly simple, yet he’s got a few tricks, and that oversized jawline is just too cute.

One issue with Charlie (and all of the bots for that matter) is that the builder has to be very careful about motor and gear positions. All of the motors have an absolute 0° marker, which denotes the proper starting point. With the way the programming works, it’s vital that this is correct. The place where it’s easy to make mistakes are if the motors are at 0°, but the gears are slightly turned, or vise versa. In Charlie’s case, the head won’t work properly.

The most fun I had with Charlie was during an activity where the builder creates a drumset for him to play. What’s really neat about this activity is that it gets the builder thinking about timing. In this case, timing is about musical rhythm, but it also directly correlates to mechanical timing and rhythm. Music meets engineering.


“Tricky” is the smallest of the bunch. Aptly named for it’s “trick shots” with the included Mindstorms/Duplo ball. The base model isn’t terribly complicated or exciting, but this was my first adventure into the remote control options.

The application indicates that tablets and phones are better for RC, which is to be expected. Attempting the program from the computer results in smooth functions, but holding two buttons at once isn’t possible. I was surprised to discover, though, that my Galaxy S7 Edge connection with the robot provided jerky, difficult-to-control movements. Fortunately, I was able to snag my partner’s iPhone 11, which proved to be best overall. This probably has little to do with Android vs iOS and is more a matter of device age. 

Getting Tricky to score a goal isn’t as easy as it seems! But the “Chain Reaction” activity was much easier and far more satisfying. The best part is the large number of parts that are left over upon completion of this build. It inspires you to add on to the contraption.


One might consider “Blast” as the main build because it resembles the primary humanoid builds for the EV3 and NXT sets. But other than standing in the center, it doesn’t command the show. That said, it does take more of the parts than the other robots and is pretty dang cool. Perhaps the most enjoyable part of building Blast is that you build on a custom jack, adding and testing each segment at a time.

Similar challenges as those faced with Charlie come into play with Blast. The builder must be cautious to get the gears and motors properly aligned. The unique differential gear comes into play in the torso, allowing dynamic movement of the limbs and head.

Another hurdle faced (and one that spans across all bots) is the good ol’ Technic frustration of trying to align and click in several pins at a time. This is something you often see in the most complex models under that theme, therefore it’s a little surprising to see in a 10+ build. It’s even more surprising when several of the connections are with pins that float and move around as you try to connect them. The images below are a couple of the more simple, yet more easily visible examples of this.

Like all epic robots, and following its namesake, Blast gets a pair of flick shooters to step things up a notch. With a few tries, getting him to hit targets is a breeze – and incredibly satisfying.


If you’re going to have a diverse collection of robots, you need to have some kind of RC vehicle. “M.V.P” (Modular Vehicle Platform) is your bot for the job. It has a unique body design and is a ton of fun to zoom around the room. The sensors also give it features that only a smart car can have, like distance sensing.

The most fun sub-build, hands down, is the crane. You could spend hours just trying to perfect the art of hooking an object, and moving it back and forth to different locations. This is one build where having a sensitive controller is extremely helpful.

One issue that does come up is the transition from base buggy to the crane. The instructions do not indicate that you take the top off and recycle the pieces. The first page of the crane add-on instructions calls for parts that are still part of the base model. It also doesn’t indicate that the base model needs to be flattened to convert into the crane. It was at this point when I finally realized that red pins indicate sections that change or come off and interchange with others. This is not explicitly stated anywhere.

As a nitpicky aside, there are a handful of locations spread throughout the builds where the instructions are confusing in terms of where connections go. In the image below, the bottom arrow disappears and you have to look at the picture a couple times to figure out what you’re doing. It’s also tough to get your fingers in there. This is one of the simpler examples, and a couple will make your eyes cross. But this is likely a side effect of more compact electronics making for more condensed robots. Therefore, I can’t complain.


I don’t know what it is about robots that walk on four legs, but I find them fascinating. I was very excited to give this bot a try. And for the most part, “Gelo” didn’t disappoint. But it’s also the most finicky of the bunch, requiring extra care to make sure all of the pieces are on just right, and that the cords don’t get caught up in the motors.

The other odd thing about Gelo is the impractical need to chase it around, controlling it from the hub. This is definitely one where you want to create a program that utilizes the remote control features.

Fortunately, in addition to the stock controller screens, you can customize the controller to fit your needs. This is useful for all sorts of applications!

App/Program Platform

I’ll say this multiple times, but the look and feel of the app are sleek, modern, and refreshing. The builds have introductory video snapshots to get you pumped for the build. Cheekily, they cut off just before the good part.

I have mixed feelings about one aspect of the build instructions for this set, which is the lack of initial “building” one does in the app. After completion of the physical models, the builder is given a finished program. Once the original program is demonstrated, the builder is encouraged to alter its block components to customize the bot. It’s a little surprising, as other platforms have the builder learning how to use the program through increasing from basic to more complicated steps. In this case, for custom builds you must feel your way around after playing with the stock programs. On one hand, it potentially encourages the builder to learn (or “invent” as the namesake goes) on their own. On the other hand, this could backfire for those who get easily frustrated without step-by-step instruction. At least the programs give helpful tip bubbles to steer you in the right direction.


Now let’s get back to the nitty-gritty regarding Robot Inventor versus the rest of the LEGO robotics family. First to bat is the programming software. We won’t go too in-depth, but rather give you an overview of what to expect. As one would guess, NXT programming is smarter and easier than RCX, and EV3 is slightly smarter and easier than NXT. BOOST is not the best comparison, as it is intended for a younger audience. SPIKE Prime moves into the territory of similarity with NXT and EV3, but its Scratch-based programming is easier to use as well. It makes the introduction into more advanced programming smoother.

The Robot Inventor obviously has the same brain as SPIKE, but we’re given a whole new app to fit under the Mindstorms umbrella. (Remember, SPIKE Prime is technically LEGO Education’s domain.) I have to say that playing with this app is pleasantly surprising. The marriage of Scratch-based SPIKE characteristics with the slightly more mature Mindstorms theme actually works quite well. The drag-and-drop blocks are easier to understand than NXT or EV3, yet it doesn’t feel as dumbed-down as expected. (See closeup screenshots of this and past platforms in the gallery at the bottom of this article.)


Next up are the brains, otherwise known as hubs/intelligent bricks. As previously mentioned, the latest hub is much lighter and smaller than the older components. One major positive to this is that it’s easier to build around. On the other hand, perhaps the biggest negative is that the 5×5 light grid matrix is a step away from the seemingly more sophisticated screens of the EV3 and NXT. This has been one of the biggest frustrations for fans because it feels a bit dumbed-down. Even if the programming is capable of more complicated readings (for applications like FIRST LEGO League), you’d have to keep it connected to a smart device to see those readouts.

Additionally, you can’t program from the hub, again relying on a connected smart device. And the EV3 has an SD card slot for additional memory. That said, there’s something to be said for clean simplicity, and even after my hesitation, I found that I wasn’t missing out on too much for most applications. Plus, the internal gyro sensor is a huge plus, while EV3 relies on a separate component.

Motors and sensors

We could talk all day about the intricacies of LEGO motors, but today is not that day. Instead, let’s focus on the major points: size/shape and basic power. One of the biggest complaints of EV3 and NXT motors is their awkward shape. Even though the shape can make for interesting build designs, most fans (including myself) find them cumbersome and restricted. The new motors solve that problem in most applications. Unfortunately, the downside of these is that, while perfectly capable of most tasks with proper gearing, they are one of the least powerful and efficient of all LEGO motors. This includes Power Functions, Powered Up, and even some old school motors. A highly detailed motor comparison by PhilippePhilo” Herbain can be found here.

We don’t need to go into excessive detail on the color and distance sensors, as the most important aspects are noted above. The new versions can fit in more locations and have more connection points than the others. I believe leaving behind the old attachment points is a major step toward increasing creative options. Also, the new sensors and motors have grooves for the cables to travel through, again, making them less bulky and easier to implement in tight spaces. Previous elements are a real pain in that regard.

The photos below really highlight the major difference in size and shape. The shift to straight black and white allows for versatility in the color choices of builds, and they just generally look sleeker. (Please note that the EV3 “head” comes in two options, an infrared sensor for the standard set and an ultrasonic sensor for the LEGO Education set. The one pictured is the infrared, but the ultrasonic looks more similar to the others with screened holes instead of a flat surface. Also, the first NXT version came with the grey and orange light sensor in the image to the right, while the NXT 2.0 came with the updated color sensor directly in front of it.)

Cable connections

Finally, let’s talk about cables. This is a huge point of frustration amongst those who have already spent a fortune on past products. All the new elements, including those found in BOOST, Powered Up, SPIKE Prime, and this set, have a new cable. The benefits are that the connection is smoother and doesn’t get caught on things like the telephone-style cables of the past. The cords are also flatter, making them much easier to fit through tight spaces. The major issues are that these cables are a fixed length, reducing creative flexibility, and have an easier potential to become dislodged when tugged upon. Even more frustrating is that, up until this point, virtually all LEGO electronics have had backward compatibility via adapter cables. This is not so with the new products, eliminating the possibility to use them interchangeably.

One other thing to note is that the EV3 comes with a little remote, and Robot Inventor does not. Instead, you use your smart device. The Bluetooth connection has the potential to be more reliable than the infrared remote, but handheld remotes with physical buttons often have sharper, more precise reactions. Alternatively, both devices have the potential to connect to a 3rd party remote, like a Playstation controller, but the Robot Inventor is arguable easier to pair.

Conclusions and recommendations

I came into this review fully prepared to express major frustrations. As someone who is passionate about the possibility to make interactive builds, I spend a ton of time playing with LEGO electronics. I also thoroughly enjoy incorporating a wide variety of them in my builds, which is why the limiting factors of incompatibility frustrate me. (And in case you’re wondering, conversations with the designers yield a pretty firm “no” on potential future development.) That said, it’s understandable why LEGO chose to go this route. As a company, they want to be innovative and forward-moving. The outdated aspects of previous products minimize that goal. Additionally, it’s both expensive to find/create compatible solutions, as well as potentially loses sales when previous products don’t become obsolete. It’s a bitter pill to take from a company that prides itself on its formerly unwavering “system of play” but I can’t be too upset.

Why can’t I be too upset? Because this set is incredibly fun to play with! The stock robots will make you smile and laugh, and demonstrate a decent variety of potential builds. There are many, many hours of building and programming options, both stock and custom, and despite limitations, possibilities are virtually endless. Also, I am shocked by the number and versatility of the pieces. After building all the base models and several of their add-on activities, I was always left with tons of leftovers. This was another area where I was fully prepared to be let down, but instead was very pleasantly surprised. (The following picture is an example of what’s left after building Charlie, for example.)

If you want to talk about the numbers, 51515 has a price-per-piece of $0.38, while 31313 (EV3) has a price-per-piece of $0.58. Overall, the new set has a price point only $10 higher. In comparison, SPIKE Prime is abysmal, with a $0.62 price-per-piece and more limited play features. The Robot Inventor’s $360 price tag may be off-putting, but the sheer number of activities makes this a worthwhile consideration. If it comes down to a decision between the two, just don’t buy EV3 new. You can get it used in good condition in the secondary market for half the price. If you must buy new, go for the Robot Inventor. It checks lots of boxes in terms of ease of use, parts variety, and playability, and should hold up in terms of technology for at least a few years. Seeing as LEGO is making the old connections obsolete, you’re less likely to limit yourself from using future products.

So the set may not be perfect, and may not be everything LEGO Mindstorms fans hoped it would be. But it is fun, and users new to the platform are sure to enjoy it, while old hands can find some silver linings along with some new things to experience.

LEGO Mindstorms 51515 Robot Inventor will be available from the LEGO Shop Online for US $359.99 | CAN $459.99 | UK £329.99 beginning October 15. It may also be available via third-party sellers on Amazon and eBay.

The LEGO Group provides The Brothers Brick with an early copy of this set for review. Providing TBB with products guarantees neither coverage nor a positive review.

9 comments on “The next generation of LEGO robotics: Mindstorms 51515 Robot Inventor [Review]

  1. Charles

    Curious, how long are the wires? I build some big models, so the new set not having seperate wires of different lengths is a downside.

  2. Al Vodi

    That was a great self serving article. It spends a lot of time on counting the number of pieces in the set and how much “fun” it was to play with. This is supposed to be an ducational toy. Let’s see fewer reviews like this.

  3. Jester

    @Al Vodi, Did you even read the article? Bre breaks down the the set from box to build (as TBB does with every LEGO set they review). She even gets into the nitty gritty and goes into how the motors work, shows you the code, and compares it all to previous LEGO robotics sets over the past twenty-two years. What else do you want? Also, I’m not sure what a “ductional toy” is. If you mean an “educational toy,” what were you expecting? Floggings, timed math tests, and lines on a chalkboard? Let’s see fewer comments like this.

  4. Bre Burns Post author

    @Charles Great question! Sorry I forgot to mention it. I also build big models, which is part of the frustration. This seems to be a common concern among most builders. The cables (including the connector bit on the end) are roughly 10in (25.4cm) long.

  5. Mark Wallace

    Thanks for the extensive review. I don’t see any compelling reason to replace my EV3s. Yes, there are some pro’s, but also some con’s. LEGO should have offered compatible cables, so that EV3 motors and sensors could be used with the new robot. Also, it will be very interesting to see what LEGO Education does with Robot Inventor. So far, it’s not on their website.

  6. Brian Osserman

    Hi, thanks for the review! Is it right that the six I/O ports is down from eight on the EV3? If so, how big an issue is that? I’ve never used the latter, but from what I can tell from using Boost and vex IQ, four motors is kind of the minimum for a lot of things one would want to do, and it seems like having only two open ports left would limit options for sensors.

    Also, while I’m asking questions, what do you mean by having to chase Gelo around and controlling it from the hub? How is that implemented?

  7. Bre Burns Post author

    @Brian Osserman You have excellent questions! You are correct that there are 6 ports in the new hub. This is something I intended to mention and forgot, so I appreciate you bringing it up. It truly depends on what you’re building. For this set’s robots, they have quite a range of possible functions (depending on how you code them) even with fewer ports. Not all of them use all four motors. So for someone getting into robotics, there are plenty of things you can do with what is given. As someone who works with LEGO robotics often, I have been able to do many of the things I wish to do with a single set, but then again, I often combine EV3/NXT sets together or with Power Functions for my larger builds. I have used multiple color sensors at once, for example, and you can’t do that here unless you buy more sensors, but the same can be said for EV3. As mentioned regarding FLL (FIRST LEGO League), you’re certainly more limited with this set versus EV3. What are you hoping to build?

    In regard to Gelo, I should’ve been more clear. You can program it to be used with a controller, but the foundation code has you pushing the center button and then only being able to stop it from running away by picking it up and hitting the button again. You can make it go slower or faster by pushing the left and right arrows, but that’s not easy to do while it’s moving. It’s not exactly the most practical bot without a remote control. It’s very odd, because this one arguably needs it the most, but is the only one that requires you manipulate the hub itself versus play/pause from the app.

    Hopefully this answers your questions. Please feel free to continue asking them!

  8. Brian Osserman

    Thanks for the clarification on Gelo. That does sound clunky; hopefully it’s pretty straightforward to add the RC support.

    For the number of ports, I get that it doesn’t impact out-of-the-box functionality, but I was thinking more in terms of expandability/compatibility with other existing pieces. Arguably compatibility is wasted if there are no free ports after using what’s in the set. I didn’t have anything too specific in mind to build, but with Boost have felt very constrained by the three motors. Basically, if you have full mobility that takes at least two of them, so you can’t add a full claw device (with up/down and open/close functionality). Or if you’re doing a humanoid you wouldn’t be able to have two arms moving independently, or any independent arm/head motion (of course, I’ve felt even more constrained by the Boost app, but that’s a separate conversation). But anyhow, I was thinking that for a claw robot or a humanoid you’d probably want (at least) four motors most of the time, which would rule out using a total of more than two sensors of any variety. I have very little sensor experience so I don’t have a clear picture of how much of a constraint that is, but that’s what motivated the question.

  9. Bre Burns Post author

    @Brian, I totally get what you’re saying. You’re right, Boost is quite limited. This is certainly a step above that, by far. One thing that I have to give LEGO credit for is that basically everything with the same cable connector, including Boost, SPIKE, and Powered Up, are all compatible with this set. Therefore, if you’re someone who has Boost, for example, you can program the use of those parts in with this new platform. Even the hub, perhaps, which would join up with Bluetooth. I believe there are some catch points with this, although I’ve heard they are working to improve compatibility in that area, at least. (I’ll be experimenting with my own sets after writing this!) But with all that said, I do see your point about wasted compatibility/expandability. You’re absolutely right; of course fewer ports would reduce your options in any case.

    When it comes to asking if you can still do the things you want to do, I would say it depends on how innovative you are. I know some people (magical LEGO wizards) who build very complex machines using only a single motor or two. With some fancy gearing, creative coding, and a remote, you can do quite a lot. There are many possibilities with only two external sensors, and we shouldn’t forget the internal gyro-sensor, which EV3 does not have. Another thing to keep in mind is that EV3 separates the motor and sensor ports. For example while there are applications where you can flip one, for all intents and purposes EV3 has 4 input and 4 output ports, while the 6 Robot Inventor ports can work either way. Basically, you can choose to run 6 motors/sensors at a time, while you can technically only use 4 of each on the EV3.

    I know that probably isn’t a huge help in your attempt to determine the extent of feasibility with specific applications. Thinking off the top of my head, without too much crafty mechanism work, I can see it being easily possible to build a humanoid with a claw and some sort of detection sensor/s for locating an object to pick up.

    Thanks for prompting me to experiment and get the gears turning in my head. :)

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