Let's Make Robots!

RC car

As some of you may already know, I'm building an RC car which will ignore user input and prevent a crash when necessary. It'll run on an arduino platform. I'm waiting for my parts ATM and doing whatever I can on the chassis I already had (making a motor mount, making a deck to put the electronics on, fitting in a battery...).

All pics can be found here.


UPDATE May 5th, 2011

I just finished a 5V regulator to power my Arduino UNO board. Of course I kinda forgot Arduino accepts a voltage of 6-12 V. Stupid me. Going to mount the motor the the chassis and maybe cut out the deck.


UPDATE May 7th, 2011

Got the motor mounted, and tested it... Boy this thing is LOUD and powerful!

My mailorder arrived, trying some things out with the arduino. Love that thing already :D I'm still waiting for my other mailorder with 20 transistors and 2mm pin sockets for my H-bridge and my 2nd xBee. Will keep you guys up to date!

PS: I know the servo doesn't work with PWM, made a mistake.


UPDATE May 12th, 2011

Cut out the new deck and it rocks! There's just enought space between the base and the deck to fit the battery between them. Also, I've mounted the servo and tested the steering. I'm still waiting for those transistors :( 

Update 2: mounted the Sharp IR sensors, pictures are still here.


UPDATE May 22nd, 2011

Got my transistors and soldered toghetter my H-bridge... Now I only need to solder a 5V regulator and som pin headers to make all the connections to my µC. I'll keep you guys updated.


UPDATE May 26th, 2011

I completed the PCB which holds the H-bridge and a 5V-regulator. There is a problem with the H-bridge though... I use the BD911 and BD912 transistors in my H-bridge and they are not repsonding as I expected them too. More info in the fourth vid.

Datasheets on the transistors can be found here. Can anyone confirm that I should feed 5V to the base to make these things switch on/off?


UPDATE July 19th, 2011

Since I couldn't get the H-bridge to work, I just tested the system with one transistor and a potentiometer (see pic). I found out that I will need a transistor to drive every transistor in the H-bridge. Too bad all radioshack-like shops in the neighbourhood are closed for vacation at the moment. Seems this project will take a lot longer than expected. I was happy to see the wheels finally spinning though :)


UPDATE March 20th, 2012

Alright, this has been some time! I kind of lost my eye on this project for a few months, but now I'm back on it! I've just finished soldering up the motorcontroller (pic below) and am now going to start figuring out how xBees work and communicate, so that will take a while to complete. At the moment, I'm kind of doing two projects at the same time, so it can be another while before I update this.

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First of, thanks a lot for the info!

My H-bridge is built up exactly as you described it (schematic). The problem is indeed the not being fully saturated part (as I said in the video, switch half on, bad with my words...). I originally used 5kOhm resistors for R1 and R2 but, when that didnt work, I went to 2.5 kOHm for both. Still nothing.

Do you know what value the resistors should have? is it even less? Or am I going completely the  wrong way with this and do I need a higher resistance value?

Again, thanks!

If your NPN transistors are turning on fully (saturated), and your PNPs are not, try a lower value for R1 and R2. I think you are safe with 1k, but I would really need to know how much current your motors are pulling.

Can you measure a few things for me?

  • What is the voltage drop from the collector to the emitter of your active NPN when the motor is supposed to be on?
  • What is the voltage drop from the emitter to the collector of your active PNP when the motor is supposed to be on?
  • What is the current going through your motor or into the collector of the active NPN when your motor is on?
  • What is the current going into the base of each transistor?

If the 1k value doesn't work for R1 and R2, and since you are already saturating the NPNs, I think you will need to use separate base resistors for each of the transistors. I have usually seen it done with each transistor having its own base resistor. This will let you play with different values for the resistors at the bases of the PNPs to try to drive them to saturate.

I'll do some measurements when I have the time (exams start in a few days). About the motor I can say the following:

  • Voltage: 6-8V
  • Unloaded current: 1.5A
  • Stall Current: 15-20 A (don't know for sure, don't want to fry my multimeter)

I'll try with even lower resistor values when I get the chance...


If your motor needs 6V to work well, you may need a higher voltage (~12V) instead of the 7.2V racing pack. You'll see when you take your measurements and determine how much voltage you lose across the transistors.

Good luck!

The following measurements were done without motor attached, with 2.5kOhm resistors and with a battery voltage of 7.61V

  • NPN which gets 5V: 0V
  • PNP which gets 5V: 4.67V
  • NPN which gets 0V (GND): 7.23V
  • PNP which gets 0V (GND): 0V
  • Voltage across motor connectors: 4.32V (positive side = 5V side)

I'm guessing the problem is in the PNP which gets 5V? :)

When I switch the 2 control pins I get the following:

  • NPN which gets 5V: 2.41V
  • PNP which gets 5V: 4.67V
  • NPN which gets 0V (GND): 7.21V
  • PNP which gets 0V (GND): 0.06V
  • Voltage across motor connectors: 4.36V (positive side = 5V side)


I think that the problem is indeed in the saturation part... When I find the time, I'll buy 4 potentiometers, so I can easily set the right resistor value for each transistor..

Does this sound about right? Thanks!

remembered reading something about hFE. I did a search and found a page that mentioned that using the current draw through the transistor along with voltage and hFE you can calculate the value of the base resistor. The page is http://www.rason.org/Projects/transwit/transwit.htm



I hope those values are NOT correct, otherwise I can not use my arduino to power these transistors: I can only supply 40 mA and according to that web page I would neet 60-400 mA... Seems quite a lot if you ask me.

instead of making a big mess with all these transistors, use a motordriver. I prefer the l298HN because it has everything you need to get it working, no need for extra flyback diodes and it can handle some current and voltage. STM has the IC for free sampling.

First of all, this motor draws 2.8A if it is just driving the wheels with no load. The L298H won't cut it! And motor drivers are expensive if you need to handle 20 Amps of stall current

Second, I'm a hobbyist, I like doing things by myself. I'm not going to take the easy way and buy a motordriver but I'll make this thing work and learn a lot of stuff I didn't know before ;)

Thanks anyways :)

Searched on about the topic and it seems to be right... Damn, now I'll have to rebuilde the whole thing to make room for a darlington pair :(