Micro Wall Racer
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.
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.