I was given the challenge to build
a class 1 robot for as cheap as possible using new parts and tools most
folks have on hand. This is the resulting robot I whipped up in about 2
hours (would have been shorter if I did not have to take all the
photos). Note there would have been a bunch of things I would have done
differently if I was not trying to cut as many corners as I could to
keep costs down.
(Note if you don't have some of the following or know how to use them
check with the list someone more than likely will be willing to give
you a hand. Also always wear safety protection.)
To disassemble and reassemble
the RC car. Mine just required a medium philips and a flat blade to pop
some clips open.
Wire stripper / cutter
To cut and strip the RJ45 cable
Soldering iron / solder
To attach the RJ45 cable to the
RC car PCB
Hot glue gun
Used to reinforce the wire
connections where they meet the PCB in the RC car
Dremal with fiberglass cut off
To modify the body of the RC
car. (Also known as the poor mans universal milling machine. Very
useful to have)
To cut the aluminum sheet
Drill and bits
Hand or electric will work.
Needed to put mounting holes in the aluminum sheet and holes in the RC
car for the zip tie to secure the RJ45 cable
Sandpaper / file
To smooth out any sharp edges
left on the aluminum.
Bill of materials
Cheap RC car
3' RJ45 cable
2x RJ45 coupler (note these can
be shared between multiple robots)
Aluminum sheet 6"x18"x0.025"
thick (make sure to use aluminum not steel) this is enough for multiple
2x 6-32 screws (had in a drawer.
You can buy a bag full for a buck at home depot.)
2x zip ties (again had a full
bag at home usually a few cents each when you calculate the cost out)
total does not include tax or
batteries so will be slightly higher.
Note if you have a junk box or shop at garage sales / thrift shops you
should be able to build this same robot for almost nothing.
And the starting pile of parts:
1.) First remove the RC car from the box (can be exciting) and verify
it functions correctly.
2.) Next remove the two screws marked by the red arrows to remove the
top of the RC car
3.) Flip it over and remove the one screw marked. Once that screw is
removed there are 2 clips you need to unlatch to remove the cover over
4.) Next there are 2 clips holding the PCB down to the body. Carefully
unclip them and slowly lift the PCB up. You will need to compress one
of the battery terminals to get it to pass though the batter
compartment. Also the motor leads are plugged into the PCB there is a
small latch on the bottom side of each that you need to squeeze in to
release them. Note if your careful with the soldering iron you should
be able to leave the PCB in the RC car and avoid a lot of work.
5.) And now the PCB is out. If i was not cost limited at this point I
would have simplified the design by just running the two motor leads
out the tether cable and putting a pair of double pole double throw
on-off-on momentary switches in the control box along with a battery
holder. Greatly simplifies the design but would have added another $10
-$15 for the switches and $5 - $10 for a battery holder.
6.) At this point these parts can be tossed (unless you want to put
them back on for looks)
7.) And these parts you want to make sure you keep. (battery cover and
PCB cover are in the background.)
8.) These cheap RC cars cannot easily be switched between frequencies
so the RC transmitter and receiver are about worthless for competition.
So we are going to convert this from a RC car to a wired car. Now a
little bit of electrial theory. These cheap RC cars have a
pair of simple H bridges driving the front steering motor and the rear
drive motor. The circuit diagram looks a little like this for one motor.
To get the motor to run forward the forward input is pulled high (V+)
so Q4, Q5, and Q1 turn on. This causes the upper motor terminal to be
grounded and the lower one to be at V+ running the motor in the forward
direction. In the case of reverse the reverse input is pulled high (V+)
which turn on transistors Q3, Q6, and Q2. This causes the top motor
terminal to be at V+ while the lower one is grounded. This causes the
motor to run in reverse. Note both input CANNOT be high at the same
time. If they were this would short V+ to ground through the
Normaly the forward and reverse inputs go to the RC reciever chip. So
to make this cable controlled we are going to remove that controller
chip (red box).
Click to enlarge
9.) Here is a picture of the board with the chip removed.
10.) Next add wires to V+, Ground, Forward, Reverse, Left, and Right
inputs. Secure with hot glue to prevent the wires from ripping the
traces off once you have tested all the inputs work. To reassemble the
truck just revers the steps used earlier to take it apart. Note again
if I had more to spend I would also add pull down resistors to keep the
inputs from floating. But in my testing I did not see any false
triggers so things look to be stable when floating.
Click to enlarge
11.) Here is a photo of the test rig I used to verify everything was
working before adding the hot glue. Touching the solid orange wire to
any of the other input wires should trigger that input.
12.) First cut a slot for the cable in the PCB cover then reinstall the
PCB cover and retaining screw. Drill 2 small holes in the side of the
RC car and secure the cable to them using a zip tie. (see yellow item
in photo above and bellow) Note the 2 extra wires sticking out of the
cable. These are unused in this design and could be used to activate a
second motor to run a ball scoop or claw.
13.) At this point it it time to work on the controller. Since I am
cash strapped I am just going to reuse the controller that came with
the RC car.
14.) Flip it over and remove the battery compartment door and screw.
Once that is done you will need to unclip the 4 clips around the edge
of the controller to get it open.
15.) Once open carefully set aside the controll sticks and it should
look something like this.
click to enlarge
16.) Note the control chip in the lower left corner. Again that needs
to be disconnected. They easist way in this case is to use a dremal
tool with a fiberglass cutoff wheel to slice though the copper traces
running down into that corner. Once that is done you just need to
solder the wires in the RJ45 cable onto the switch legs. Note 2 legs on
all four switches are connected together. This is where your orange V+
wire should go. The other four input wires should connect to one of the
other 2 legs of their respective switches. Note the color code used on
the RC car end will match the wire colors on this end. The remaing 3
wires (2 spares and ground) are not used and can be tucked under the
PCB for later use.
Click to enlarge
17.) Next replace the 2 joysticks
18.) At this point we need to modify the top cover to make enough space
to pass the tether cable though. Easiest method is to cut some of the
antenna boss off.
19.) Next snap the top cover back in place while taking care not to
pinch any of the wires. Once in place replace the screw and battery
door. This controler will NOT need to have any batteries in it. Just
the 3 batteries in the lower unit are needed. Also zip tie the tether
cable to the rear antenna boss.
20.) At this point we have achived turning a cheap RC car into a even
cheaper looking corded car. Now it is time to add a ball scoop. First
cut off a 3" x 6" strip of aluminum. Note the tin sinips will leave a
sharp edge. Its best to sand / file the edge after every cut to dull
out this sharp edge. It is VERY easy to get nasty deep cuts on these
edges. (paper cuts on steroids.)
21.) This RC car happend to have 2 conveninet holes located on the
front edge of the bumper. A 2" wide by 1/2" tab was about perfict to
mount the scoop to these holes.
22.) Once that tab is cut and bent out you will need to tranfer the 2
hose and possibly add a notch to clear the steering adjusment. The
easiest way to tranfer the holes is to use a mechanical pencil or a
punch to mark the locations while holding the scoop in place. Once that
is done drill the 2 holes and verify they line up. After that you can
bend / cut the edges of the scoop to your preferance.
23.) Mount the scoop to the front bumper using two 6-32 screws (and
nuts if needed. The holes in the plastic were slightly undersized so
the screws held without nuts)
And there you have it a VERY basic bot for Class 1 competition.
Now you might be thinking if that is all there is where is the fun?
Like I said this is a VERY basic bot and has a lot of short comings.
For instance it is very light and has little traction. There are lots
of little tweaks like adding weight or coating the wheels that can
either help or hurt its performance. Also there is more advanced
improvements like replacing the flat scoop with a motorized grabber that
can pick up the balls for transport vs just pushing them along.
And here is a quick you tube video of the bot running around my shop
floor with some golf balls.