3pi robots kits parts list & more

I need a tiny bot to do the following:

  • track line
  • avoid obstacle
  • identify IR emitting objects
  • identify multiple colors
  • identify direction (with compass)
  • tell x-y axis with an accelerometer

I’m interested in getting the 3pi robotic kit, but I would like to get the parts to assemble one instead, and then add a couple of sensors on.

I just noticed that the 3pi kit comes assembled. I would rather to get the parts which I’ll do it myself. that also means a lot of soldering to do… Yes, I’ll have to learn it.

Would someone verify the following is correct?
Parts list with approximate cost?

Pololu Orangutan LV-168 Robot Controller (+ Pololu Orangutan USB programmer, $28), $53 OR
Pololu Orangutan X2 Robot Controller VNH3 + LCD, $140 (don’t need the usb programmer, correct?)

Pololu QTR-8A Reflectance Sensor Array, $15
Pololu MMA7260QT ±6g Triple-Axis Accelerometer , $20

Chassis & motors:
Pololu 5" Robot Chassis RRC04A Solid Blue, $8 (only need one right?)
Pololu Ball Caster Variety Pack, $28
Pololu Micro Metal Gearmotor Bracket $6 (3 of them)
Pololu 100:1 Micro Metal Gearmotor, $48 (3 of them)
Encoder for Pololu Wheel 42x19mm , $45 (3 of them)
Pololu BCM Wheel $6 (2 of them)

Is the list sufficient?


The part list that you are suggesting will not, when assembled correctly, be a 3pi. It will, however, be a nice round robot.

Here are some suggestions I have about your parts list:


The LV-168, with only 8 User IO lines, so is not going to have enough IO lines for what you want to do. The motor drivers on the X2 are going to be overkill for a small 5" robot.

We are getting close to releasing a new Orangutan called the Orangutan-SVP which would be perfect for the robot you are trying to build. It will cost around $100, have a built-in programmer, lots of IO, built-in hardware support for our encoders, fits on the RRC04a and does 2A continuous current per motor channel. We are hoping to release it some time next week.


You should get the QTR-8RC instead of the QTR-8A because the A version requires analog inputs, which won’t leave you any analog inputs for your accelerometer or other sensors. Also, we have found that the RC version tends to perform better than the analog version, and you can read the RC sensors in parallel (the analog sensors must be read in series if your MCU only has one ADC).

Pololu doesn’t currently sell a compass carrier.

Chassis & Motors

You only need one RRC04A chassis, unless you feel like your sensors will take up too much space. In that case, you can stack the chassis on top of eachother or add a prototyping board to the top.

I recommend that you get the Pololu 42x19mm wheel and encoder set instead of buying the parts separately. The BCM wheel does not work with the RRC04A chassis or with the motors you suggested.

I can understand getting an extra motor and encoder for backups, but I don’t understand why you want to get 3 gearmotor brackets instead of 2.

The 100:1 metal gearmotors are going to be slower than the 3pi’s 30:1 metal gearmotors. I’d recommend getting 30:1 or 50:1 HP micro metal gearmotors, which provide more speed and torque.


Ryan, thank you very much for you help.

I picked X2 ($140) because I want the multitasking capability. We use it to do robotic soccer or even maybe robo-geocaching in the future. With the robotics soccer, we did heavy in triangulation calculation, and constantly monitor compass , so I thought it get a controller with multi-tasking capability will be helpful.
Will Orangutan-SVP ($100) have the similar feature?

QTR-8RC array ($15) sounds great.
Without a carrier for compass is worrisome… what is your suggestion?

Chassis + motors:
RRC04a ($8)

Pololu 42x19mm wheel and encoder set ($40) sounds good. I want 3 motors because I need a mobile head carrying a sensor for detecting obstacle and walls. I guess I may be better off to get two sets of these. An alternative to this is to use 3 stationary sensors to this instead.

50:1 HP micro metal gearmotors ($16) sound great.

Other wires, etc. suggested?


The Orangutan X2 uses an auxiliary microcontroller to interface with the motor drivers and buzzer, so you get parallel processing in that regard. The Orangutan SVP uses an auxiliary microcontroller to give you five additional I/O lines, four of which can be used as encoder inputs, which gives you parallel processing of the encoders. The main benefit of the X2 is that it can deliver more power to your motors and it leaves more of the AVR hardware peripherals free for your own uses since so much of the on-board hardware is controlled by the auxiliary MCU. Note that the Orangutan X2 is not currently supported by the Pololu AVR library, though we are in the process of adding support for it as we add support for the upcoming Orangutan SVP.

You can use a gyro along with your encoders to keep track of orientation, though a compass module would eliminate the errors that will build up over time when using a gyro or encoders. A gyro is best at tracking changes in orientation while a magnetometer is best at letting you know your instantaneous orientation relative to a fixed axis. We don’t sell any magnetometers, but you could probably find one at Sparkfun. The X2 only has 8 analog inputs, so keep this in mind when you pick out your sensors.

For your mobile head, you might get the best results with a stepper motor (we sell a stepper motor driver, but we do not currently sell any stepper motors–you could try Sparkfun for this, too) or servo, though I’m sure you could make it worth with one of our micrometal gearmotors and an encoder for feedback. The encoder is designed to work with our 42x19mm wheel, so you would need to mount the mobile head on this wheel.

You will need something for detecting obstacles. Depending on what kind of range you need, I’d recommend one (or several) of our Sharp distance sensors, or maybe one of our Maxbotix sonar sensors. You could use a variety of them for a mixture of long-range detection and close-up detection (e.g. set up some Sharp digital distance sensors around the edge of your robot and mount a 120cm GP2Y0A02 Sharp distance sensor or Maxbotics EZ0 sonar range finder on your rotating head. If you get our analog Sharp distance sensors, I’d recommend getting some JST cables for them as well.

Unfortunately we don’t have a good solution for an IR or color detector.

In addition to the parts mentioned above, I’d recommend getting some prototyping supplies such as breadboards for making easy test circuits, jumper wires for making quick connections between components, and maybe some wires with pre-crimped terminals for making your own custom cables. You will also need to find a suitable power source (I’d suggest something between 6 and 9 V capable of delivering the current to power your robot over some acceptably long period of time). We sell some rechargeable NiMH AAA battery packs that might work well for you, but you would have to get the charger separately. We also sell rechargeable NiMH AAA and AA batteries and battery holders.

Lastly, don’t forget that you’ll want an accelerometer and ball caster.

- Ben

a lot of thanks to both Ben and Ryan…

About the IR, I thought the reflectance sensors will work fine with it… however, it probably does not have a good viewing angle though.

Researching on Sparkfin for parts suggested…

My goal is to create a bot with the capability of :

  • detect IR object with a good vieiwing angle
  • detect distance on a mobile contraption like a 180 degrees moving head.
  • detect orientation instaneously with a compass
  • detect incline with accelerometer
  • the whole bot must be within 22 cm (all sides like fitting in a 22cm-dia cylindar).
  • a controller allows some sort of multi-tasking, as we do a lot of triangulation and constantly monitoring orientation and existance of IR object.

With these said, I have very hard time to find anything except the Mindstorms… as strange as it sounds… ! However the processor brick and motors are SO huge too. I had to get a multiplexer to get all the I2C sensors that I need. Yike!!!

We are very good at robotic programming in C, often got hung on the hardware!!!

I am out of clue!!!