Friday, February 27, 2015

Bit Char-G - Doing research long after it matters. And making a 1/64 scale proportional car.

I want some r/c cars to drive around my computer room.  The room is about 10x10, and has a bunch of furniture and other stuff in it.  That means even 1/32 scale cars are a bit big, if I want to set up a course and do laps.

Several years ago, I was into these cars called Bit Char-G.  They were made by Tomy, and sometimes were sold on the MicroSizers name by Hobbico in the US.  I got most of mine from a little Japanese mall, and mail order.

Here's a Bit Char-G next to a AAA battery...

Their turning radius is about a foot, so they're great for rolling around on linoleum floors.

So lets talk about the cool stuff on these cars.  First, they have replaceable final drive gears.  They come in three color coded sets.

Now, this seems great, excepting they're not proportional.  They use a magnet and field coils to steer, and have what amounts to a pager motor to make them go.  Admittedly, they have enough power to really go, and be challenging to drive... so that's not the problem.

Now.. how do you give proportional steering to a car that's 55mm between body mounts.

Happily, there are servos that are "about the right size".  The one thing that's missing, is a decent micro speed controller.  "micro" speed controllers seem to start in the "few amp" range, and go up from there.  The smallest brushed one I can find, is a 3amp one for airplanes.

What we need here, is a 1 amp speed controller, with coast, brake, and reverse.

But.. do we really need 1 amp?  I pulled 1 amp out of the air, based on the run time of the car.  Which is "a few minutes" on 50ma.  I think we need to do some testing....

So testing we shall do.  From a fully charged 50ma NiCd cell...

No load: 33ma
Some load: 55ma (finger dragging on the wheels making them slow down)
Full stall: 99ma

I was expecting "amps" not under a tenth.  But.. I'll take that.

The usual answer for "I need a really tiny ESC" is to gut a larger r/c servo, glue the pot  on center, and call it a day.  I want a really good tiny ESC.  If this project pans out... I'll need to do some research to find that sort of thing.

I started digging through my gear to see what I have that might suit this build.  

How are we going to squeeze a battery, ESC, Servo, and RX into that?  Specifically this gear:

It looks like the receiver would fit upside down, without interfereing with most bodies.

And the servo looks like it will fit flat  if I carve out the bottom

But.. in the end, those weren't going to work tonight.  If i want a chassis like that, I'm going to need to build this up as a pan car.  (a completely sane, if time consuming option..)  That, was not in the cards tonight.

Speaking of tonight, I needed an ESC.. now.  Thankfully, a servo was willing to spill it's guts for me.

I'm decent at soldering, but lets say this was a challenge.  Much to my joy, it worked.

And then.. I got to busy to take pictures.  But, in the end, we have a Bit Char-G chassis, with a lipo battery, an ESC, and proportional steering.

This "may" be the worlds most expensive Bit Char-G.  It's definitely one of the only ones that's proportional.

I did drive it around the floor in my computer room a bit.  Next comes packaging it properly.  I may only be able to run this in the xB body, for receiver and battery room.  But that doesn't bother me to much.

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