LEGO RC Car A.k.a Mark III

Hello all. Today I am going to share with you my long journey of building an RC Car. But first a little background. Ever since I was a kid i wanted to build my very own rc car. But it aint that simple. I realized that after some failed attempts. Four years ago i got a Lego technic as a gift. At first i built the truck model and played with it but i soon got bored due to its limited functionality. Hence i decided to make an rc car using the lego parts. Lego is easy to work with as it is a fairly sturdy platform where the pieces can fit with each other.

So almost three years ago i built the rc car and ever since then i have been playing with it and upgrading it along the way. The truth is that i am in love with hobby rc cars, the professional ones but they are too darn expensive so this is my poor mans solution to fuel the desire.

The form it is in now is very stable and i am happy with it but it took a lot of learning and breaking and repairing to get what i have today.

I will share with you the various stages of development and explain the final stage so that people like me who want to do something out of the box with their lego can give it a shot.

Well i used many things along way so here is a rough idea of things you will need:

1. Lego Technic pieces
2. Battery
3. Power Motor
4. Steering Motor
5. Motor Control System
6. Wireless System

The basic frame has been the same since i built it the first time. Party the reason is that I super glued it together so no way of taking it apart without damaging it. The only thing i would change now is the rear drive assembly. When i built it, i only had one large gear and one pinion gear so i went with that. But the large gear was almost as big as the tires so not a lot of ground clearance. If you have smaller gears, i would recommend building a gear box that you can change later on for various ratios.

I used four of the panel pieces for the main chassis. The steering system was built using two long lengths. The rear drive shaft is below the frame and the motor is above it. Front and rear bumpers provide protection to the wheels.

So at that point i had little idea about motor drivers and servos and stuff. So what i did was what i saw in cheap remote control cars. I used and elastic for the steering to self center and the lego motor to steer. I used a faster motor for the drive. The battery i used was rechargeable energizers in the lego battery pack. For the communication and control system, i used a circuit and remote from a broken rc car.

For a young maker looking to start out with his diy rc car, this is a cheap solution. Only expense was the batteries and that too was just a luxury. This was the first time the car ran and the results were promising. The steering was okay since it was only extreme position or center. Same with the drive. No speed control.

Soon i realized that the weight distribution was not optimal and i was getting good traction in the rear. Plus the cells were just not juicy enough. So i upgraded to more power from a lithium ion pack i made from cells from and old laptop battery. I placed them right over the rear. This made a huge difference. I added LED lights for looks and ran the car like that for months. It was really fun.

I broke it and i fixed it. Good thing about lego is that i had many spares so breaking something was no worries. Although one thing i learned soon was that if i used super glue and the part would break, it would be hard to extract the piece stuck inside for a replacement.

This worked for a long time until the tiny circuit gave up and finally stopped working. At that point i decided to upgrade.

Now my plan was to build something to add variable steering and motor control. To that end i built a custom dc motor controller using mosfets. For the controller i used an Aurora 9X that i luckily borrowed for that time. It was a very reliable controller but it was an airplane controller. The motor driver couldn't talk to the receiver directly so i used an entire arduino board and wrote a little code. For the steering i used a 9 gram servo. It worked directly with the receiver.

Now to the performance. Pro was that the motor driver was amazing. It provided great speed and full power. The servo was also responsive. Due to weight distribution, it had good traction and went fast. The trouble was that the motor driver kept blowing up. Literally with a bang, the mosfets would just burst. Despite many attempts the custom H bridge just wasnt reliable enough. So i decided to shift to something more reliable.

P.S. If any of you knows a good reliable H bridge design then please share it with me.

After fighting with the motor controller for a bit i decided to go with the L298 H bridge. Why did i not think of it sooner? Well i did and i even used it easily on but it causes a huge drop in output. Almost 3 to 4 volts which is very annoying. But now that i had no other option i decided to use it any way. So i bought a motor driver module and a arduino nano. I also used a 5 v regulator to power the servo and arduino. For the battery, i used 4s lipo 1600 mAh so that the motor gets the 12 volts it needs.

At first i used a small 9g servo but after going through 3 of those i switched to much higher power servo. It is super tough and wont get damaged even under high loads.

I also got rid of the big transmitter and shifted to a car transmitter. Now it really feels like an rc car.

Now i will explain the things i learned over the years about each part of the car that you should keep in mind if you are just starting out to build your own.

A car needs a bumper. Why do you need a bumper? To protect the wheels. I broke the wheels almost three times when i accidentally ran into stuff while driving. The wheel was the first thing to hit and it broke off. So if you plan on running a decent speed, you need bumpers. They will break if they hit at high speed but they are easy to replace.

I am not happy with my drive shaft because it has too many small pieces. What i would recommend is two sections only but with four supports. Dont use super glue on them but instead use screws to hold the sections.That way if something breaks, its easy to replace. Mounting the gear with the motor is always tricky. But what i did was a hole in the round lego shaft using a dremel. I constantly rotated the shaft so that the hole is centered. The i inserted a collet of the drilled portion and driller sideways into the end. I inserted a small metal pipe into that so that from the side it looks like a semicircle. Grind a notch in the drive shaft such that when you insert the drive shaft into the piece, you can lock it in place but pushing the pipe in the tiny hole. No need for any glue. For the other end, just make a sturdy support to keep the smaller gear spinning right.

I used a cheap motor so that i can over power it with no guilt. I just strapped it down using zip ties. Makes for easy maintenance. The motor has good power but you can buy a more expensive motor too if you want greater power. But i realized that the plastic gears would not be able to withstand any more force than this motor. They would strip away.

For the body the main lesson i learned was to make it more rigid. The panels were just too weak and they took a lot of beating so they broke. I had to fix that with a lot of epoxy and added fortification. Other than that the body has held up nicely. Also dont use super glue a lot. I did and it made many parts brittle causing them to crack. Use screws where you can and only glue where needed and as much needed. Not like me emptying bottles trying to make it indestructible.

The circuit is very simple. Just wire the motor controller input to arduino. The motor to the motor controller and battery to the motor controller and 5V regulator. The 5v regulator to the receiver and arduino. The receiver throttle out to the arduino and steering out to the servo.

I know i made it very confusing for you. And i wont simplify this for you. This is the part you have to go through yourself because you will learn. I wont spoon feed the schematics and diagrams. Use the old noggin and work it out :P

This circuit is very rugged. Even stalled current going through for long periods wont damage it. No blow ups or anything.

From cells to lithium ion to lipo. Why you ask? Because lipo is the most superior with the least weight. Not that weight is a large concern. But lipo wont sag under load and stuff. Although expensive but worth it. Dont buy when starting out. Lithium Ions are great for that. Lipo is for professionals only. And take care of lipos as to not discharge them. They will get damaged. And charging is also a pain.

I hope you enjoyed my stupid and crazy obsession with the tiny car. But its so much fun. To make it and to break it. I set up obstacle courses and try to pass them. Its always a blast. And it helps to understand differential and stuff. But mostly because of the fun.

Thanks for viewing and i am open to all questions ans suggestions. And guys please suggest a good H bridge.

And please vote for me if you think i deserve it. Thanks.