Let's Make Robots!

Micro Wall Racer

Race around walls (hopefully :-)
IRF7105.pdf265.52 KB
MAX1811.pdf167.49 KB
MPSA14.pdf43.7 KB

I wanted to do a wall racer ever since I joined this site but I also wanted to do something a bit different. I'm now attempting a micro wall racer using a tiny RC car that was on special after Xmas. First thing I had to do was strip it out including the 1.2V NiMh battery.


This gives me a nice clean chassis to work with. Now for the hard part, fitting in not only a Picaxe 14M and sensors but a "H" bridge as well. I tested the motor and was suprised to find it drew about 200mA from the 3.7V battery. Admittedly it was originally running of about 1V so I shouldn't have been suprised. I did try another motor I had that was a similar size, it only drew 60mA but since this is a race car, I'll keep it as a last resort.


 As you can see the battery fits neatly in the top of the cover. Picaxe processors can run at voltages as low as 3V but you must disable the brownout detection circuit otherwise it will shut itself down below about 4.5V. The 14M is a loose fit in the chassis with some room to spare above it. I do have an 8 pin chip in mind that should make a suitable motor driver without additional external components but I haven't got one to test yet. 

Sensors are a bit of a worry, I could try and fit two of my new IR sensors that I tested recently on this battery. I got up to 1 meter range and drew an average of less than 10mA. I'll keep these as a last resort as the 5mm LED style phototransistors are a bit bulky. I'm going to attempt instead to make micro sonar detectors.


These are the inner workings of your standard ultra sonic sensors with the bulky covers removed. They are a 6mm square piece of piezo electric material with a tiny aluminium cone glued to them.The original casing did have these mounted on what looked like a resonance chamber so I'm not expecting to get the same range as I used to but if I can get a meter range then that should be heaps. They are delicate so I'm expecting that I'll have to re-glue the cones back on if the car crashes. 


Update 28-2-2009


After 2 months parts are starting to arrive. Futurlec have been veerryyy slow lately. I've decided to use SMD parts where possible due to the small space.




I found a Nch/Pch FET combo that is perfect for making a minature "H" bridge.

They have a max current rating of over 1A and have a lower gate threshold voltage of only 1V. The outputs of the picaxe at 3V will still be good enough to drive my motor.

The data sheet for them (IRF7105) has been included in the attachments.























I've also found a dedicated LiPo charger IC (MAX1811) in the same 8 pin SMD package. This will let me charge my battery from a USB port or any other voltage source between 4.5V and 6.3V



After some thought I've decided to abandon the ultrasonic sensors in favor of IR. The two main reasons are simplicity and because the sonar sensors are too delicate.

This is the schematic for the time being. The actual charger circuit will probably be mounted at the end of a USB cable with just a small plug going to the car.Click on the schematic for a larger picture.

Once some more parts arrive I'll start construction and update this further.


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I want to see progress on this one. Now!
So do I. Still waiting on some parts.

You could use a Picaxe 14m2 they can run at 1.8-5.0 volts

Unfortunately I stopped this project when I moved to China. I hope to have another go at it when I get the time. Admittedly I would not use a picaxe now. If I build it again it will be with an SMD ATmega168 configured as an Arduino Nano.

"After 2 months parts are starting to arrive. Futurlec have been veerryyy slow lately"

So, my order that they claim they shipped six weeks ago may still be on its merry way and not lost in the nether-regions?

Very cool. And thanks for the pointer for a mini h-bridge! My bugbot will one day thank you for that too.

Have you considered LEDs as sensors? You don't get much range (maybe 50mm) but for a micro wall racer that might do the trick. With the added bonus that you can use them as lights...

OKay, you're thinking I've gone mad. No. It's a side-effect of LEDs. When you reverse-bias them, they become capacitors. The discharge rate is proportional to the incident light. You can time the discharge by making your output port and input port and timing how long it takes before it reads 0.

They're tiny and inexpensive and who cares if you smash one against a wall repeatedly?

I did not know that. now I may have to try something. :D

BOA when you say reverse-bias them do you mean just the reverse the polarity. Connect the + to - and - to +? Do you still need a resistor for this to work? And would the resistor still be on the + end? 

My very first prototype of the laser range finders used a LED spectrally matched to the laser as a sensor with the theory being that it would work as a filter, only responding to the frequency of light produced by the laser. Unfortunately with reflected laser light it never got more than 200mm range.

Considering the poor turning circle of this car (definatly needs to powerslide like Frits's) and the fact that the motor is running on at least 4x the power (allowing for voltage drop from the "H" bridge) I want about 1M range if possible.

I know the thing is weak, but regarding the stripped ultra-cones: I think there is a great chance that you are going to "slam"into walls while testing:)

(Don't put them in front) 

I agree, I've already broke one off accidently by bumping it with my hand. Supaglue is great stuff :D