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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

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I too have always dreamt of remote control lego for 35yrs. I will have a look at your project.

H

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Ok, this is AWESOME! Great work! I would love to buy one of these from you. :grin: If you evder decide to do something like that put me at the top of the list...for two!

Mike

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Very neat indeed, almost full proportional control as well, the 9398/41999 really need this in my opinion.

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I'm really surprised that LEGO doesn't offer a good RC system. They're really cheap nowadays, and you know everybody wants one. I feel like they don't want to offer one even if they could for cheap, but I'm not sure why. Maybe they know it will result in buckets of mangled gears and axles. Maybe it would compete with Mindstorms. I read somewhere that the IR system was designed specifically for trains, and the features that make them good for trains make them bad for cars. That seems like an odd set of priorities.

After I put this receiver through the wringer to make sure it's reliable long-term, I'm going to build another so I can race cars against each other. It's almost as easy to make a dozen as it is to make one, so maybe I'll build a few extra sets at that point. I need to buy some L and XL motors to try with this.

I'm excited to figure out a way to use an 11.1v LiPo battery, which would be easy except for the low voltage cutoff circuit that needs to be utilzied, and I don't know anything about that yet. I also want to make a 4 channel one for use with a thumbstick RC transmitter, for robot battles. Wouldn't that be fun? LEGO battlebots. Woot!

Brian Z

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Can it handle buggy motors? I think one or two may work, but that's why I went with the beefier motor controller for my 4 buggy motors to run at once.

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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.

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Brian, this is really good but I doubt TLG could make it cheap enough - even if they could supply the transmitter at the same price level as HobbyKing, there's $15 of extra components, and labour, so receivers would end up costing at least $30-40 by the time they were in the shops - depending on numbers produced of course. So a system would be $60 or more, which is a lot more than the existing IR system.

Great for us AFOLs but probably not a big enough market at that price.

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They're a great investment! I've collected 4 so far and together they make a monster of a vehicle:

I think Lego would be wary of buying $90 hobby-grade motor controllers in bulk :tongue:

Edited by z3_2drive

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that's amazing dude! great job :)

@z3 impressive performances o.o

Haha thanks, I never really made a topic about it, I guess I should soon as I'm already starting my tank project :wink:

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@BrainZ: Amazing! You did an excellent job on building this circuit. Thank you for putting up instruction on how to build it. Definitely looks compact enough to work on most models. Keep up the excellent work! What gets me is How Fast the Servo can turn, That is a major step up from what the PF speed remote is able to do. It is a shame tho, that is why I use the toggle switch remote for steering. I want to include that It is possible that a full proportional servo steering is on the TLG's future list and a dedicated steering remote as well. That is why I building my RC circuit to be very compatible with the future PF devices like a full proportional servo and remote that could potentially be down the road.

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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?

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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.

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Hi Brian, a fantastic work!!! Have you managed to test with 11.1v? I would like to build such circuits and I think I should be able to. Something is not clear to me - if I want to use 4 L-motors (1 on each wheel) and lets say one or two servo motors (steering) on a model - will I have to have 3 receivers and 3 circuits or can I connect the 3 circuits to one receiver - 2 circuits on one channel and 1 circuit on the other? Will post the same question on your blog:

http://brianzawesomeblog.blogspot.com/2014/01/lego-24-ghz-power-functions-radio.html

On your blog you say:

"The HobbyKing receiver just plugs right into that socket I built out of 3 four pin headers side by side."

but I don't see how are they connected to each-other. Maybe I haven't red carefully enough :blush:

The 11.1v are very important to me since I find the L-motors need over 11v to perform according to my needs - building rock-crawlers and also planning to build a fast car.

Also which is the UK site you found that is selling the components - if I understood correctly that there is one?

Thank you very much!!

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I think I understand it now. If I use two L298N on top of each-other I should get 8A and should be able to supply 4 L-motors and 1 or 2 servos with one circuit - right? I have a DC/DC convertŠµr which can provide constant voltage on the input (lets say 11v) and the rest of the components should be able to handle this setup I believe. Then I can connect 4 power cables to the site of the bridge for the motors and 1 or 2 cables to the other site for the servo/s - is that correct?

Thanks a lot!!

Edited by ddimkin

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wow! that's cool man! i'd love to have something like that for using mocs outside...great job!

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