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About four years ago I was having trouble controlling a model aircraft. You know the: when it came towards I couldn't think myself into the model and would steer the wrong way. me luckyto get a new cheap NEW BRIGHT F150 monster truck just when I needed to learn how to control an RC vehicle (I was having trouble controlling a model aircraft when I came towards me). They were normally $120 but was reduced to $35 as return because no-one would touch ti. However the control is very bang bang difficult. If you make the truck go forward it dashes off so fast that the rear wheels skid for the first 20 feet - by which time it is nolonger pointing where you want it to go. As soon as you touch the steering it does a 360 degree spin. Stopping also results in a spin. Basically near impossible to control.
Taking the plunge I removed the switching gear from the steering servo and replaced it with the inards of a linear pot. This is proably the hardest part of the upgrade. See photos.
I have an old 29Mz 3 Channel Sanwa proportional radio control system, so now all I needed was a reversing speed controller with brake and a servo driver decoder. The PicAxe28X seemed ideal for the job. The only MosFets I had were a bit old and required +10V gate voltage to get the low resistance required for the 10A motor in the truck. For the steering servo drive I originally thought that this would draw less than 200mA, but after trying out the single transistor drive I discovered this was very unreliable - hence the addon BD140/139 transistor driver. See the attached circuit.
The source code for the pixaxe is found here. Also provided is a motor only test and a steering only test.
The software has some unique features. Move the thottle forwards and the truck will accelerate proportionally. Pull the thottle back and the truck will brake proportionally. Release the thottle and then move it back a second time. The truck will now reverse proportionally. Move the throttle forward and the truck will brake. Move the throttle forward a second time and the truck will go forward again.
A little slow motion movie of the cheap truck in action.