BrianZ

Hey all, I finished building my 2.4GHz circuit board for controlling Power Functions motors and servos with a hobby RC transmitter. It turned out as well as I hoped, giving full proportial speed control, all 7 servo positions on either side of 0 degrees, and the servo snaps back to center, just like you would expect it to when you let go of the steering wheel. You can drive two motors, or two servos, or one of each. It fits in a 4x6 stud area, 3 bricks tall. If I used a shorter voltage regulator I could have gotten it down to 2 bricks tall, so not much bigger than the IR receiver. I posted my bill of materials, the PCB etching art, and the microcontroller program on my blog so that anybody with basic electronics skills can build their own. There's also pictures of the process and a video of the final product. Here's the link: http://brianzawesome...ions-radio.html I have wanted to have radio controlled LEGO cars since I was a little kid, and now I can. I have also wanted to document a project on the internet for a while too, so I'm pretty pumped on both counts. Brian Z

I built a circuit that lets you use a hobby RC unit with Power Functions a while back. I updated the H-bridge to the one LEGO uses from my first try at it, but I have not done a writeup on it yet. I was having inconsistent results with my surface mount soldering techniques when I put this project on the back burner. I need to get it finalized and written up. Here is the writeup for the V1 of this project though: http://brianzawesomeblog.blogspot.com/2014/01/lego-24-ghz-power-functions-radio.html

I get my solder paste from Sparkfun, at $9 for more than I would ever use. A rework station might be nice though. My version 2 of my 2.4GHz Lego remote control receiver is giving me fits, as my first surface mount project. Too much paste, not enough, solder bridges, and whatnot. It's harder than I thought, but I think that I'll get better quickly. Brian Z

What are you trying to do with it? I built one of these, but it's probably not the simplest possible way. Pulse width modulation is your friend here, if you are going to design a circuit for it. Brian Z

H-S, The DRV8833 comes in two forms, the square one with pads on all 4 sides, and the one with pads on two sides. I used the one with pads on two sides, and I used solder paste and an electric skillet to solder it. I put a blob of solder on the thermal copper surface I made just below it, and it seemed to solder and dissipate heat just fine. I have had to re-work every single one of the ones I have made; apparently I am using too much solder paste. They all turned out fine with solder wick and an xacto knife in the end though. My point is that I have had to reheat every one of the DRV8833 chips, sometimes with the skillet and sometimes with my soldering iron, and they all survived. This is the first thing I ever soldered that was surface mount, so experience isn't critical, apparently. Pololu offers a DRV8833 board already soldered that I would like to try. It's small. I'm going to do a writeup on my updated system any day now. I'll post here when I have more information. Brian Z

That is super impressive. I'm using a laminator with toner transfer to make double sided boards, but those vias on your board fill me with envy. That's just something you can't do with home-fabbing, and It makes my board quite a bit bigger than yours, even though it looks like I have fewer components. I can't put a via under a chip, because mine are a soldered wire going through the board, and it makes a lump. Again, impressive work, and I'm excited for you. I know how good this part of a project like that feels. Keep up the good work, and post a video of the final project. Brian Z

As long as I'm flapping my yap about what I would like to see in Technic, it would be fun to see sets that are motorized walkers, and odd machinery that doesn't have a real-life counterpart. Like giant versions of Yoshihito Isogawa's simple machines, wheeled vehicles, and the other book, which I can't remember. Whimsical things. They probably wouldn't sell well though. It would be cool if they had a great general Technic techniques book, that taught a lot of principles about building with Technic, and sold all the parts with it so you could build the examples. Like what if you could buy Yoshihito Isogawa's books with all the parts included? Or better yet, Sariel's book The Unofficial LEGO Technic Builder's Guide. Man that would be sweet.

Howdy! I love what you are doing. I recently built a 2.4GHz receiver for Power Functions. You can see my progress here. It has a simple housing that I made out of two plates and 6 2x2 panels. Well, the pictures on my blog are 2x3 panels, but I've gotten it to fit in 2x2 panels. You said earlier that you were having PCBs fabbed. Have you tried etching your own, and soldering with a hot electric skillet? I learned everything I know about etching and soldering SMD parts from working on this project. My new version is all surface mount, and is working awesome. I'm going to make a new blog post for the new version here in the next week or so. I hope you keep it up and find sweet glorious success! Brian Z

I've only been on these forums for a little while, but the interests people have in Technics surprises me. Beforehand I assumed that people bought Technic sets pretty much exclusively for the parts. Maybe they built the set, but then they tore it down and built their own models, learning and exploring about mechanical engineering as they did so. This is what I would do if my kids didn't protect their instruction-built models so fiercely. In this way, I would run the Technic line as a parts bin, selling sets with liftarms, gears, pins, or whatever. This doesn't seem to be what the users of this forum (and probably the public in general) would like though. It's good for LEGO that I don't work for LEGO. Still, I wish I could buy a set of liftarms in a particular color, or a set of wheels.

Use the Power Functions servo motor for the steering. It's pretty sweet.

I recently built a radio controlled car with the medium motor and a servo, and it was ok, but the L motor was way better when I finally got one. The XL is even more powerful though, but quite a bit bigger. When you say competitive, what do you mean? I built my own circuit to use my LEGO motors with a hobby grade 2.4GHz radio system, and now I'll never go back to the infrared controls. I posted instructions to build it yourself at my blog, here. My son and I have been building and racing small LEGO cars all over the house, and it's the most fun I've ever had with LEGOs.

That's really cool. I'm going to keep that in the back of my mind for future solutions.

Man that's sweeet. Please update with a video when finished. What battery do you use with your system?

Hey all, I'm working on the second phase of my 2.4GHz radio controlled Power Functions, which is to run them off of a 11.1v 3S LiPo battery. I have read posts on this indicating that it may not be so bad for the motors, but the IR receiver is probably not up for it. That's no big deal to me because my radio receiver can handle up to 30v. So I'm all full steam ahead, and I just now remembered that the servos have electronics in them that may be sensitive to those voltages. Has anybody run a Power Function servo at 12v yet? Did it work? Brian Z

Oh, Ok. I get it now. I was thinking controller=microcontroller in the housing of the thing that snaps to the motor, you were thinking controller=transmitter. Yeah, my transmitter has a return-to-center spring, but sometimes people will put a foam doughnut behind the wheel on the transmitter so that it does not return to center. What is your ideal wireless communication method between the transmitter and your model?

I'm really surprised at how poorly the speed remote works. It doesn't seem like it would have been tough to make it return to center. Are there not more Lego car and truck builders than train folks? I ended up building my own system, and it works way better, for the same price as the IR system.

Using a 555 is hardcore old school. There is no spring for return-to-center. The Lego servo uses a pulse width modulation signal on lines C1 and C2 (which is a high power signal, used to actually power the drive motors) to tell the servo where to go, and it uses the 0v and 9v lines to power the motor in the servo, via a control circuit. It's a wierd signal protocol because 0V on both C1 and C2 will not power a motor, but that's the signal to go to center for the servo. The output of your device must emulate that PWM protocol in order to control the motor. The device that hooks to the motor must have a vairable resistor that the microcontroller can read to tell it where it is, and use that information to calculate the difference between where the motor is and where it's supposed to be, and send an output signal accordingly. If you want to get into microcontrollers and you have no previous programming experience, let me suggest the Picaxe. It would make this project so, so easy. It is only a dollar or two more expensive than a 555 anyway. I will admit that the Picaxe has very little street cred though, certianly less than the 555.

I've got to think about that. A do-it-yourself solution would involve a microcontroller that would listen to the signal the receiver is sending, and also to the value of a potentiometer that was mounted around the shaft, and it would output a signal to a motor driver that would turn the motor in the direction needed so that the difference between the motor position and the receiver signal position is 0 degrees. Which is, of course, how servos work, but this microcontroller would need to interpret the pulse width modulation signal of the IR receiver or my radio controlled receiver. I think the parts on my circuit board (the Picaxe 14M2 microcontroller, the SN754410 motor driver, and a 5v regulator) are all you would need to make this happen.

Boxer, are you suggesting a module that clicks on to the front of a PF motor, which sends position feedback to a controller? You could add servo functionality to any of the motors like that. The more I work with the Lego PF servo through my radio control circuit, the more I like it for steering though. It's the Lego IR transmitters that hold it back.

I agree about the non-Lego servos. I'm trying to strike a balance between using only Lego parts and parts that are not Lego. The way I'm striking that balance is to pretend that I work for Lego (because who wouldn't like to work for Lego?) and I'm in charge of developing a system for Power Functions that makes building radio controlled cars fun. I don't think my bosses at Lego would be cool with me mounting a Futaba servo in my car when we already have a servo developed. On the other hand, we don't build batteries, and I think customers would like a high-powered rechargable LiPo battery pack option, so I'm working on that. Before I started on this project I considered that my imaginary employer was Futaba instead, and what kind of Lego compatible housings could I build for our servos, receivers, battery packs, and motors? Basically a Futaba Power Functions system (which would be really cool, hint hint Futaba) that worked seamlessly with Lego. In real life I design food, dairy, and pharmaceutical processing equipment, so I have a lot of 3D modeling experience, and I thought about going that way. Research shows that it is difficult to produce 3D printed parts that work well with Lego though, due to the tight tolerances required, so I'm taking the Lego employment route. You'll know I've gone officially crazy though, when I start listing my imaginary employers on my resume.

I updated my blog with a video of my first road test after finally getting in my L and XL motors. Man, those motors are a big step up from the M motor! Other updates and insights I have had since I first completed this project: 1) 7 segments in the servo are fine for all practical steering purposes. I can't tell that it's just 7, and steers as well as a normal radio controlled car. 2) I was able to cut the top of the heat sink on my voltage regulator off at the hole, which shortened everything up enough that I could fit the circuitry into a 2 brick tall area. 3) The next big step on this project must be to run 11.1v 3S LiPo batteries. These motors seem like they would do really well at that voltage, and the batteries are a lot smaller and lighter for the same capacity (compared to alkalines), LiPo can put out a lot more current, and they're rechargeable.

I just looked at your videos, and the forum topics they linked to. Oh man. I have seen the dark path of my future.

How are you transmitting radio signals out of the IR transmitter?

I will say that now that I have had a chance to build and run a car with this, the 7 segmented servo is not that bad. I don't think that I would have noticed that it wasn't fully proportional. The only benifit I can think of with a normal servo is that you would be able to better center the servo with the trim if you needed to. Right now I'm using a rack gear mounted to an axle, and I can slide the rack gear left and right a tad to adjust my center steering. I only have M motors, which are difficult to get much speed with, but I did notice that the car is slightly slower through this circut than hooking the drive motor up straight to the battery pack. There is apparently a small voltage drop across the SN754410. I could get around this by building my own H-Bridge out of Mosfet transistors, like Boxerlego did, but the SN75441 is just so easy. I have not thought to test the IR receiver against hooking straight to a battery pack, but I'll bet there is a small voltage drop across the motor driver in the IR receiver too. How does the M motor compare to the L in building small cars, as far as speed and power goes? I know the L makes about twice the power at the same speed. How does this transfer to real-world car performance?

I don't think there is any way it cuuld handle the buggy motors. They have a stall current draw of 3.2A, so you would have to stack up 3 of the SN754410 chips, which is possible, but unwieldy. There are more powerful H-bridge chips out there, for still less than a few bucks though. $3 gets you the L298N chip, which will do 2A per motor. On the other hand, two stacked up SN754410 chips would do 2A, and I'll bet that motor would tear something up before it stalled. I'll do some L and XL testing, and cross the buggy motor bridge when we get to it. I need to get my hands on one of those.