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Work Journal: R/C Fun with Bluetooth


Intro:
As a kid, I always wanted one of those R/C cars that I saw in TV commercials. Those were awesome. Around the beginning of summer, 2007, I wondered if there existed toys that I could control with bluetooth. A google search revealed that there doesn't seem to be many (if any) that you can commercially purchase. But I did find a do-it-yourself guide. That idea appealed to me immensely... even if I knew from the getgo that it wouldn't be a cheap project.

Components:

Here's a laundry list of components that I found necessary:

Putting it together

My first step in starting this project was reading about half of everything I could find on the subject. I recommend it for anything you do of this sort.... I approached this whole project with a "know before you buy" mentality.

I then proceeded to find a suitable toy car... I couldn't think of any friends of mine that had one I could use and I wasn't interested in garage sale hopping, so off I went to eBay. The NIKKO Alienator, sans remote controller was one of the first and in the end best (dirt cheap) options I saw. It was also lacking a battery (I'm sure the original, if this car is really two decades old was shot anyways) and electrical connectors for the 4 AA battery holder on the underside.

Upon returning from vacation to a large box with the car (larger car than I expected, but in hindsight, a good thing) I of course took it apart, and discerned that the only thing of real use on the car besides the chassis was the motor.

I started with salvaging the telephone wire and then wiring the serial adapter for the Mini SSC (4). The male 9 pin serial adapter was probably supposed to have wires soldered to it, but I went with crimping (with pliers) connectors on the end of wires. Didn't work too well.

Mounting the Mini SSC and it's 9V battery was quite straightforward. I simply did a bit of looking for where stuff would fit. The 9V battery is snuggly held between some ridges of plastic. It was just my luck that the mounting holes on the Mini SSC with two holes that alread existed for screws right next to the battery's spot. (5)

I didn't discover the compartment for four AA batteries on the car for around a week. The underside of the car has a 'door' that slides and pops into a locking position. Opening it gives you access to where the 7.2V battery goes. It also covers up a placeholder for the AA battery compartment. (9, 13) The compartment didn't have any terminals with which the batteries would have been electrically connected. I salvaged the necessary parts from two Hot Wheels chargers and two old lazer tag guns.

I bought two 7.2V NiMH batteries plus a charger on eBay at the same time I purchased four servos (spares are a great thing to have). (11, 27) Those two purchases were 40-50% of the project costs... and obviously I can use them for future projects. So I view that money as a reasonable investment.

Once the servos arrived, I started with mounting the steering servo. It didn't fit in the spot that contained the original, ancient, six wire servo, so I had to mount it elsewhere. I chose to use a wood block (19) to fit where the old servo had been, and mount the new servo to this block. I started with shaping a foam block to the shape then rough cutting the wood, and fitting it using a grinding wheel. (I lacked the ideal tools...) Once mounted, the wood started to crack due to drilling smaller holes for screws than I ought to have. (19) Once I better understood the mounting hardware that the servos came with, I replaced part of the wood block.

The linkage between the servo and the front wheel is mechanically a weak point, though it's supposed to be. The concept of a "servo saver" is that the mechanical linkage will fail between the wheels and servo before damage is done to the servo. Ideally, this linkage is easy to reset if broken. Alas, I wasn't fully able to use the servo saver that the car came with, but my current implementation works so far. The entire servo can actually be dislodged rather than take damage, and this happened a lot when driving over rough ground. I've since implemented a spring system that pulls the servo back into its slot, removing the need for manually reattaching the servo.

For speed control, I decided to use a potentiometer. By varying the resistance, with constant voltage, one varies the current that the motor can draw. Initially I salvaged a couple of pots from a recently replaced radio. They worked for a few seconds until some magic smoke escaped. I followed up with a 3 watt, 25 ohm (that's the resistance range, not maximum resistance) potentiometer (actually, a rheostat) from RadioShack.

I had to be cautious with the new potentiometer too - it builds up a fair bit of heat in the circuit I was going to implement it in. I disregarded this fact in my initial testing and ended up burning a small portion of the insides. (26) Thankfully, when using a potentiometer as a variable resistor, you only need two of the three terminals

I decided to modify the potentiometer. They still build up heat even if the motor isn't moving, and that's wasting energy. My solution is to turn part of the potentiometer into an on/off switch by destroying part of the resistive coil inside. This creates a section where current can flow with varying resistance, and then a section where, due to the break in the coil, current can't flow.

For the servo that would actuate this, I created a neat looking mount. (21) I cut the servo's profile into a piece of wood, making sure I had a snug fit. I lined up the servo's shaft with that of the potentiometer and then mounted the wood piece to a piece of plastic that the potentiometer was already mounted to. This assembly is attached to the car in a shaky makeshift manner, but it has yet to break or show any signs of wear... It just shakes a lot as I drive the car.

The third servo on the car is used to control the motor direction by reversing polarity of the power circuit. This is accomplished with an arm on the servo that moves to make the proper electrical contacts. (31) It's a rather touchy implementation. With better tools, I could do something more compact and less finicky. I've replaced the system entirely once with only a slightly better end result. Oh well. I'll probably post a simple circuit diagram in the future. If I wanted to pay the cost of the Mini SSCII again, I could buy a dedicated motor controller with built-in H-bridge.

Beyond that, everything else was a matter of making sure stuff was nicely attached and wouldn't go flying.


Getting the bluetooth working
Frankly, the first couple weeks of working with the bluetooth connection were a nightmare. The muggy summer weather combined with the constant silent asking of, "why isn't this working?," and praying the next attempted solution works, all contributed to this nightmare. The current setup is quite dependable and easy to troubleshoot.

To begin with, the actual function of the bluetooth in this project is to create a wireless version of a serial cable connecting a computer and the servo controller on the car. RS232 is the standard for the 9 pin serial connection. You can learn quite a bit about implementation of the RS232 standard for serial ports here. (I honestly didn't learn a ton about RS232... but the info on that site pointed me in the right direction upon reaching a rather large stumbling point.)

My laptop has built-in bluetooth. I pair this with the IOGear bluetooth serial adapter on the car.  This connection is best thought of as a simulated serial cable. I could implement this with my desktop, too. I recommend the very tiny, Bluetooth 3.0 (!) adapter by Cirago: Cirago Micro USB Bluetooth 3.0 Adapter Class 1.


The program
I've done three sets of programming for this car, the first in Visual Basic 6, the latter in Visual Basic 2005 .NET.

I started with Visual Basic 6 for a few reasons. With the DLL file provided by the Mini SSC makers, and the plethora of guides for and examples of code online, it looked to be the least painful method.

The Visual Basic 2005 version was started after I finished the VB6 version. This later version is absolutely better than the VB6 version. The car is controlled with the arrow keys, as is done in most car driving games on computers.

Code is available below the control GUI screenshots.

As I've worked through the coding and testing, my program and (more tangibly) the interface has changed quite a bit. Here a few choice screenshots of the GUI's development.


Third version.
Visual Basic .NET 2005 Code (8/8/2008):
Throw the VB stuff into a new project, and work out the kinks yourself... because I'm too lazy to do that.
Pictures of progress:
11
More info:
Here are some links that I found useful during the course of this project.

-6-07-12