Who This Is For
This is targeted at folks who have never built a
combat robot before. It is not a technical document.
Folks who have built a combat robot before will be
bored silly by this. This will not help folk who
have access to mills, lathes, CNC machines and
degrees in electronics (you don't need my help-go
build something cool).
If you are asking yourself the above question, the
answer is that you probably shouldn't. I am not a
heavyweight in the robot combat world (or any other
world). What I am is an intermediate level builder
who really enjoys a hobby and would like to get
others involved. I don't have lathes, mills, or CNC
machines (OK, there is a degree in electronics but
that isn't going to come into play here - promise).
What Is A Combat Robot?
For the purposes of this exercise, a combat robot is
an R/C car in a very bad mood with the desire to
find other R/C cars and kick them into scrap.
Purists will tell you that to qualify as a robot, a
machine must be autonomous. Fine. Good. Go read
This is one if the few times in this little article
that I am not in any way kidding. Combat robots are
inherently dangerous. You do this at your own risk -
I accept NO liability for ANYTHING. This hobby, in
both its construction and execution, has the
potential for fire, explosion, trauma and other
injury as well as death. Be smart, be safe. Read
manuals. Read warning labels. Ask questions. THINK.
The Basic Parts
To get a combat robot to do anything (other than be
an vaguely amusing sculpture) you need six
fundamental elements (this is the order we'll be
Controller - accepts the commands from your
receiver, and translates those commands between
the batteries and motors.
2. Transmitter - issues your commands to your
3. Receiver - receives the commands at your
4. Motors - makes the robot go (and hopefully
kill the other robot).
5. Battery / Charger - provide the power for
6. Chassis - holds everything but the
Transmitter in one snug package.
The transmitter, receiver and the speed controller
are the items that most folks just can't do for
themselves. (If you are one of the folks that can
build your own electronics hooray for you! Now go
bask in your own superiority-QUIETLY, while the rest
of us get on with our pitiful ignorant lives.) The
bad news is that these items are relatively
expensive and absolutely indispensable. The good
news is that in recent years these items have become
essentially commodities. You are encouraged to spend
your money here, buying for the future. Investing in
good quality radio equipment and high quality speed
controllers is not only less frustration in the long
haul, but much cheaper. With care, these items will
fight on in multiple different chassis.
For this project, we're taking a short-cut - the
selection of our source of motors, specifically the
cheap & nasty battery operated drill. This is the
source of the motors, the batteries & the charger.
Outside of this project, how to try and save money
on your robot is the subject of a fair bit of
My personal spending priority is:
2. Transmitter / Receiver (a.k.a. Radio system
3. Battery / Charger
Especially the battery vs. motor priority portion
can be the subject of disputes no less vicious than
Mac vs. Wintel or any of the innumerable schisms
that make religious studies such a side splitting
(or slitting, these folk get SERIOUS) hoot. Well,
I'm the one writing this, those are my priorities,
and the rest of you can go schism among yourselves
The Speed Controller (ESC) is really the beating
heart of a combat robot. It will probably be the
most expensive item on your shopping list. Do not
skimp here. Many of the items that can blow up or be
damaged during a fight can be often be borrowed from
other competitors in the pits (No really, one of the
strange things about a robot fight is that your
opponents are almost as interested in your bot being
able to fight as you are. Otherwise, how else can
they beat you to scrap? Oh yeah, the pits are where
to find the bot builders who aren't fighting at the
moment. Look for tables off to one side that look
like a hardware store mated with an electronics shop
& then exploded.). There are frequently enough
spares floating around the pits to replace most of
the items that die during a fight-but the ESC is
expensive & consequently rare. Make protection of
the ESC a basic criterion of your design.
The basic "buy, don't buy" elements for selection of
an ESC are:
Type of Motor
there are two basic motor types out there to be
controlled; Brushed & Brushless. Most ESCs will
only work with one type or the other. Make your
system match. For the cheap 12 lb bot, get a
- all ESCs will specify the
maximum voltage that they can be supplied with.
This specification is not a suggestion. It is an
absolute maximum. Don't exceed the voltage; you
will not be pleased with the results.
- all ESCs will specify the
maximum continuous current that they may supply
to a motor. This specification is not a
suggestion. It is an absolute maximum. For the
cheap 12 lb bot, anything over 15 amps should be
fine. But, like I said before - try to buy for
- many of the ESCs designed for
R/C cars will not reverse. In combat robots,
this is bad. Unless you are blessed with
supernatural driving ability (you are not), if
you cannot backup, you will lose.
- there are quite a few ESCs out
there, mostly for R/C boats & cars, which do not
have electronic braking. This is a bit of
jiggery-pokery in the circuit that stops the
motor when your transmitter stick is in the
middle (neutral) position. This can be the
difference between stopping before you hit a
spinning saw of death and coasting obliviously
into it. There are some drivers out there
(disgusting youngsters with the reflexes of a
coked up snake, you know who you are) to whom
this feature is a matter of indifference. For
the rest of us, this should be a buy/ don't buy
- Battery Eliminator Circuit is a great
feature to look for on an ESC. This will supply
the power for a Receiver from the ESC,
eliminating the requirement to make alternative
arrangements. Keep in mind that if you have
multiple speed controllers in a bot (say a dual
channel brushed for drive motors & a single
channel brushed for a weapon), that you should
only use the BEC from one of them.
An important consideration in the ESC of your dreams
could be number of channels. There are many ESCs
available that will control 2 motors (Left & Right
Channels) from the single board. This can simplify
your layout considerably. Some ESCs allow for
control of 3 channels, Left Right & Aux. This is
very handy if you are interested (now or in the
future) in having an active weapon on your robot.
Mixing is a feature found on many ESCs. There are
two steering techniques available to the combat
robot; Tank & Mixed. In Tank steering, the left hand
stick controls the left side motor(s), and the right
hand stick controls the right side motor(s). Both
sticks forward= forward, move one stick back and the
other forward= a very fast turn. In Mixed steering,
the ESC places Forward/Reverse on one stick axis (Y
axis, up & down) & R/L on the X axis of the same
stick. This results in a one handed drive, leaving
the other hand and stick available for weapon
control (BWAH_HAH_HAH_HA!... sorry).
Once you have an ESC- READ THE DOCUMENTATION! Keep
it around. Download a copy & store it in your hard
drive (you know you'll lose it one day).
Radio Systems (R/C)
You need a Transmitter (TX) & a Receiver (RX) that
match each other. A handy guide to concepts in R/C
can be found here
while a guide to R/C terminology
can be found here.
Radio System Types
Best - 2.4 GHz
Spread Spectrum: This tech will be the most
glitch free and avoids Channel conflicts.
Better - FM 75 MHz (channels 61-90): Good
range & resistance to glitches.
Allowed -AM 75 MHz (channels 61-90): Cheap
Avoid - 27 MHz: Not allowed in most rule sets.
A note on channels:
If you decide on an FM or AM system, you will have
to also purchase channel crystals (XTAL). Crystals
cost anywhere from $7-15 each. You need matching
XTALs for both TX & RX (example: a channel 65 for
your TX AND a channel 65 for your RX). Plan on
having at least 3 channels (more is better) that
work with your system to go to a fight. The reason
for this is that you want to control only your bot-not
other folks'. Of course the reverse of this is
true-you REALLY don't want somebody else to have
control of your bot! Having multiple channels to
select from is frequently a requirement of the rule
sets. Be prepared for this; it is really
disappointing to not be allowed to fight because of
channel conflicts. It is also a very good idea to
test each channel pair in your completed bot BEFORE
you show up at the fight. Like anything else, XTALs
To completely avoid channel conflicts, consider a
2.4 GHz spread spectrum system. While the initial
investment is higher, these radios will bond with
their receivers, creating a unique pair that that
does not conflict with other systems. The result is
fewer giblets to cart around and less scrambling in
the pits to resolve radio issues.
Another consideration is the NUMBER of channels a
TX/RX pair can control. For example, the GWS system
cited for a FM system will control 4 separate
things; say Left Motor, Right Motor, and a Weapon
motor, with a channel left over. The example cited
for an AM Radio only has 2 Channels of control which
will be generally used up by Left Motor/ Right
Motor. Try to not only consider what you build
today, but what you may wish to build tomorrow.
If the ESC you select does not have BEC, you will
need to provide a 5v source to the Receiver.
You need at least two motors for your bot, one for
Left side drive & one for Right side. The motors by
themselves are not particularly useful; they spin
too fast and with little real power. Some sort of
transmission is also called for. For your first
combat bot, allow me to recommend the quick
"one-stop shopping" technique of the battery
operated drill. This is a great inexpensive way to
get both motor & transmission in a single fell swoop
(there is an additional benefit to this that we'll
discuss in the next section-multitasking baby!). The
motors in the drill are the brushed type.
Keep an eye open at the hardware or "home
improvement" store for sales. Perhaps the easiest
way to get hold of very inexpensive motors is to
purchase on-line from a site like
After I wrote that sentence, I found an 18v drill
motor for $27 and a 4.8v for $15. An 18v drill motor
would be delightfully adequate for a 12 lb to 30lb
fighting robot (4.8v might be a bit anemic for a
12lb). Anything in the 9v and up range is a nice
place to start. These drills cost less than most of
the "specialty" motors alone.
Once you have the drill, there is a bit of surgery
required (we call it hacking). I am lazy, so this
will not be covered in this document. Instead, I
refer you to
a nice web article
by a builder of almost mythic stature in the Robot
Battles family of fighter/ builders, Dale
Heatherington (spend some time on this site; there
is LOTS of good stuff for the geek here. There are
lots of other drill motor hacks out there, as usual,
Google is your friend). Pay careful attention to the
July 2006 Update, those set screws are vital.
What you have after the hack (and say $60, including
shipping) is a pair of motors (transmission
included) with a shaft already attached (the threads
on that shaft are 3/8" 24, you can mount wheels
directly to this), batteries to run them, and
chargers for the batteries. This combo of materials,
purchased separately, can easily cost (no kidding)
hundreds of dollars.
You need something to power your bot. Many fights
don't allow Internal Combustion Engines (a.k.a. IC
or ICE) because they have an unreasoning fear of
fire (silly, yes, but the times we live in require
tolerance of many strange opinions). We won't be
talking about IC. For most of us, this leaves the
electric option; for the vast majority of this group
it means a battery (you mad scientists out there can
work out your own alternatives, again QUIETLY).
So where is your battery & charger for this project?
You already have it- it came with the drill guns you
acquired and hacked for their motors. The batteries
are low-end NiCads. You need a (possibly) new tool
for this next bit-the Multi-meter (a.k.a. DMM or
just meter). This will allow you to test the battery
and all kinds of electrical thingummies in a number
of ways. You don't need anything fancy (Though they
are nice). Radio Shack, Home Depot, any hardware
store is a good place to look for this item. Talk to
the nice person at the store & tell them what you
are doing (you're gonna blow their mind!). There are
instructions in the box with your new toy (and best
A mile-high overview of the battery harvesting
process goes like this:
1. Check how the
battery fits into its charger before doing any
surgery. This will ensure that the correct
terminals touch the charger (+ goes to +; - goes
to -. This is called polarity- Use that meter!).
2. Make notes as you go, marking polarity on
both the batteries and the charger (Use that
3. Carefully remove the cells from their plastic
4. You may be able to solder wire (length is up
to your requirements, at least 6") right onto
the connector that was on the plastic housing
(at least to the wires that connect to them).
Alternatively, you can solder right onto the
ends of the battery itself (especially if you
were not careful in the above step). It is a
good idea to use Red wire on the positive &
Black for the negative (at the very least try to
use different colors for this. Use that METER!).
5. Once you have the battery exposed & soldered,
make sure to wrap it up nice & snug in electric
6. Repeat this process for the battery from the
7. Use test jumpers to (wire with a clip on each
side-make your own or go to Radio Shack) connect
your battery to the charger. Be VERY careful to
use correct polarity (USE THAT METER!).
Here are some basic guidelines to the care & feeding
of your battery and charger. Please note that items
that include words in all capital letters (like
NEVER & READ) are not suggestions; these have the
potential of explosions, fire, injury and (I kid you
identical cells into batteries.
Only combine identical Batteries.
NEVER short circuit your batteries or cells
(a short circuit is when you connect a battery's
(or a cell's) own + to its own -).
REALLY NEVER short circuit Li type batteries
MATCH your charger to your battery type
(NiCad chargers will not work with Li)
MATCH your charger setting to your battery
DO NOT over charge your battery
READ the manuals & documentation for your
batteries & and your chargers (take them to your
first couple of fights so you can refer to them
USE THE FLIPPING METER!
This is where you get to use as much imagination as
you may possess. I have built robots that, aside
from the electronics, were constructed entirely out
of materials acquired from (multiple trips to) Home
Depot. There are bots made from R/C Cars. There are
bots made from wood. There are bots made from closed
cell foam. There are bots made from exotic materials
like Carbon Fiber & Titanium. The choice (and the
skills & tools to implement that choice) is yours.
Fair warning - odds are good that your first robot
will… and there is no nice way to say this… suck
(this is true for almost all of us. If you aren't
one of US, then you are one of THEM. And you should
know what happens to one of THEM that get mouthy.
Remember, QUIETLY). Don't sweat it. Your next one
will be better, and so on.
Some guidelines to ease you into the process:
1. Start simple.
2. Figure out how much you are willing to spend.
3. Target a first event (might I recommend Robot
Battles @ DragonCon in Atlanta GA?). Read the
event rules CAREFULLY. Keep these rules firmly
in mind while designing & building. It is really
disappointing to be disqualified from fighting
because your ignored a rule. (PS-the safety
rules are NOT dumb).
4. Spend some time looking at other robots
(Google is your friend)
5. Use what you know (example-if you can't weld,
don't count on welding your frame together).
6. Draw it first. You don't need to be Leonardo,
a sketch is fine. Chances are good that if you
can't draw it, you won't be successful building
it. Put the drawing away for a bit and look at
it later, critically. Revise in the cold light
of day. (see 1 & 2)
7. Make a materials list based on your drawing.
I find it helps to order everything at once (I
live on the outside of the galaxy and therefore
order most everything on-line).
8. Take the components you have collected and
lay them out in the approximate way you have
drawn. Does it all fit? Revise & repeat. (see 1
9. Budget your time as well as your money.
Finish construction BEFORE you show up to the
10. Practice your driving BEFORE you arrive at
11. Don't be afraid to change things.
That's all I have for you.
Good Luck, fight well.
- Brandon Davis, Team Found Objects
Sample List-12lb robot
(I'm picking inexpensive value here; make up your
own mind. These are OPINIONS):
1) Transmitter - GWS GWT-4A 75MHz 4-channel Radio System (~$60)
2) Receiver - GWS Pico 4channel 75 MHz 4 Channel
3) ESC - IBC Dual Speed Controller (~$299)
4) Motors, battery, charger - 2 x Harbor Freight
12- 18v Cordless Drills (~$60)
5) Chassis - Use materials you are familiar with