Let's Make Robots!

RC car

Ok, so I'm working on a plan to bring my little brother's little Niko RC car (you know, these models that suddenly break down for no reason at all after three charges) back to life. The plan is:

  1. Yank out old electronics (fun job, done ^^)
  2. Test steering servo and current draw of drive motor
  3. Build new electronics

Step 2 will be done by tonight but I have a few hickups on step three.

Basically, I'm going to refit the remote with two mini-joysticks and an xbee module. Now here's the thing: I've read that the xbee has onboard ADC's and I absolutely love this (no need for a microcontroller in the remote) but I can't seem to find a lot of info on how this whole system transmits it's data. With "transmitting it's data" I mean: what comes out at the receiving end and how can I read the ADC's output with my ARDUINO in the car (first time I'll be using arduino :D). Is there anyone here with any experience in this department?

Also, I've built a design for a simple H-bridge and wanted to know if it looks any good. So if you could comment on that, that would be great!

Thanks a lot guys (and girls)!

H-bridge.png56.92 KB

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I can't help with the xbees, but here are some thoughts on the h-bridges and electronics.

Your schematic doesn't show current limiting resistors on the base inputs to the transistors. These are recommended to protect you processor's outputs in case the circuit tries to draw too much current. What transistors are you using? Will they be able to handle the current of your motors? Same goes for your diodes; make sure they are rate high enough to handle your max motor current. Bigger (more current handling) components won't hurt, but they can save you.

You can measure the stall current of your motors by preventing them from turning while measuring the current at full voltage.

One last thought, have you looked at the original electronics to see what you can use? Chances are the original h-bridge in there is still just fine. At the very least, you may be able to re-use some components, or gain an understanding about what size transistors you need.



The transistors will probably be TIP31 or alike (NPN), depends on the current draw (which I'll measure within the next hour or so). You're right, I forgot about those resistors! Extra question about them: is there a specific rule about them (what value should they be)? If they are just for current limiting they just need be high values, or am I mistaking there?

The original electronics are: one PCB which has a lot of glue-like stuff on it, pretty nasty stuff... not usable, I'm afraid :/

Thanks for the reply!

OK, I've found the time to measure the current draw of the 'new' motor (that belongs to the 'new' chassis I found)... It draws about 1.5 A constant without a load. Stall current = 15 A. Now, I guess that I should foresee transistors that are capable of handling 3A. Or should it be more because of the stall current?

I was thinking that I could just install a fuse to prevent the transistors from smoking when stallig, but then again, I'm going to use PWM on this thing so it can draw lots of power when driving slow (correct me if I'm wrong please)

Does any of this makes sense?


A 1k transistor at the base of each transistor should be fine.

I forgot to mention before, be very careful that you never turn on both transistors on one side of the h-bridge at the same time (Q1 & Q2 together or Q3 & Q4 together). This will shoot current straight through from power to ground and probably smoke your h-bridge.

Some people like to use a combination of npn and pnp transistors for their h-bridges. This way, it takes a combination of a high and a low (referring to pin1 and pin2 your schematic) to turn on the h-bridge. When your microprocessor first wakes up, gets in a weird state, or you just have a programming error, it is possible that all output pins will accidently get set high. This puts your current h-bridge in a "smoke me" configuration.

I've used all npn h-bridge designs in the past. They work fine, but if you are building from scratch anyway, that is something to consider. Just replace Q2 and Q4 with matching pnp versions of your npn transistors.

Did the motor have a closed attached gearbox, or an open frame of gears? You may be able to find an appropriate sized motor for cheap and replace it.

Or... parts!

YEAH! I forgot about a very old RC car platform I had lying around in my bedroom... Going to use that, has a way more powerfull (or is it powerfuller? nah, that can't be right) motor :D Great!

Thanks for your tremendous help! That only leaves the xbees unanswered...

I've recently started using the L298 Dual H-Bridge available for only a few dollars.  This thing has current limiting (up to 2 amps per channel if i recall correctly) and TTL Compatible inputs.  Someone also made up a PCB design available from http://www.dprg.org/projects/1998-04a/ .  I've used 2 of these to run car window crank motors (with the two sides paired to get double the capacity).  This way if you draw a nominal 3A and if it goes up to 4A, no problem.  It just won't be able to draw more than 4A -- but no smoke!.

This motor draws 15A when stalled, at start up it should be able to draw that amount of current so that it can accelerate quickly. So 4A is definitely not enough

Thanks anyway :)

Just because a motor draws 15A during stall, doesn't mean it should or will be able to handle that much current -- not for long.  If the car hits a wall and stalls, you don't want the motor(s) drawing 15A continuously, so the current limiting of the L298 is a good thing.  Unfortunately, the electronics to allow a short - high stall current during initial acceleration are much more complicated than a simple H-Bridge driven by some sort of PWM signal. :(   Would be nice if it was that easy.

Yes, startup is similar to a stall and normally doesn't happen all that much, but in an RC style car there tends to be a lot of switching between forward and backward, repeated stalls.

Beware the blue smoke that makes electronics go!  Your motors are the most difficult thing to replace as you've designed everything else from scratch and can easily fix it.  If you go with 15 A capable transistors, you'll have to keep an eye on the temperature of the motors until you get a feel for how much abuse they can take.