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Pololu 3pi Robot

Pololu item #: 975

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The Pololu 3pi robot is a complete, high-performance mobile platform featuring two micro metal gearmotors, five reflectance sensors, an 8×2 character LCD, a buzzer, and three user pushbuttons, all connected to a C-programmable ATmega168 microcontroller. Capable of speeds exceeding 3 feet per second, 3pi is a great first robot for ambitious beginners and a perfect second robot for those looking to move up from non-programmable or slower beginner robots.

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	Description		Specifications (10)		Pictures (15)		Resources (20)		FAQs (5)

How does the battery charger connection work? Is there a charger included?

The 3pi robot does not include batteries or a charger. The battery charger connection provides a direct connection to the batteries so that if you use rechargeable batteries, you can recharge them without taking them out of the robot. You will need a charger capable of charging four NiMH or NiCD cells (depending on what you’re using) in series. Such chargers are readily available in hobby stores for charging electric model airplane battery packs. Please note that rechargeable batteries are not required as the 3pi can use regular alkaline cells, but we strongly recommend investing in some NiMH cells and a charger.

I’m ready to order a 3pi, but am wondering what sort of battery charger I need. Any recommendations?

You will only need a battery charger if you plan on powering your 3pi with rechargeable cells. The features you want are ability to charge 4 NiMH cells in series (they are usually in battery packs, not in battery holders like on the 3pi) and to be powered by AC (a wall outlet). In general, having more flexibility (such as the ability to charge 1-8 cells) is nice for future projects. Though we don’t have experience with these particular models, the following chargers look like they should be nice:

Tower Hobbies AC/DC Digital Peak Charger w/LCD
Triton Jr (customer-recommended)

These chargers can be connected directly to the 3pi’s battery charge port, allowing you to charge the batteries while they are still in the robot.

Since the 3pi just uses ordinary AAA batteries, you can buy battery chargers (into which you stick the batteries) at most general electronics stores. For example, a quick search of the Radio Shack web site yields:

4-6 Hr. Battery Charger + 4 AA & 4 AAA Ni-MH Batteries

The downside to a charger like this is that you have to remove the batteries from the 3pi to charge them.

The green (PD7) user LED on my 3pi flickers even though I’m not doing anything with it. Is it malfunctioning?

No, this behavior is normal. To get the most out of the ATmega168’s I/O lines, one of the LCD’s data lines (PD7) doubles as the control line for the green LED, so this LED might flicker when the LCD is updated. As such, the amount of flickering and effective brightness of the LED will generally be a function of the rate at which you are updating the LCD. Note that you can change the state of the green LED without affecting the LCD at all, and using the LCD via the Pololu AVR library will only very briefly change the state of the LED line as needed before restoring it to its previous state.

Can I augment my 3pi by adding my own electronics/sensors?

Yes. The easiest way to augment your 3pi is through an expansion kit, which can comes either with cutouts that let you see the LCD below or without cutouts. The version without cutouts replaces the LCD, giving you access to more I/O lines and more prototyping space. An expansion kit is not required for addition of your own electronics, however.

The 3pi robot has a limited number of free I/O lines that can be used as inputs for additional sensors or to control additional electronics such as LEDs or servos. These free I/O lines can be accessed through the pads at the center of the 3pi, between the two motors, labeled PD0, PD1, ADC6, ADC7, and PC5.

PD0 and PD1 are unused digital I/Os, and connect to the ATmega168’s UART module if that module is enabled. This means you can freely use PD0 and PD1 for general-purpose digital I/O, or you can use them for serial communication with another microcontroller, a serially-controlled device, or a computer (note that you will need to convert the signal to RS-232 levels or USB to communicate with a computer).

PC5, ADC6, and ADC7 are all connected to 3pi hardware via removable shorting blocks. By removing the shorting block, you can use these pins for your own electronics. PC5 can be used as either a digital I/O or an analog input. When its shorting block is in place, it controls the emitters for the IR sensors; when its shorting block is removed, the emitters are always on.

ADC6 and ADC7 are dedicated analog inputs. ADC6 connects to a voltage divider circuit that monitors the battery voltage when its shorting block is in place. ADC7 connects to the user trimmer potentiometer when its shorting block is in place.

Please see the 3pi pin assignment table for more information.

I’m adding peripherals to the 3pi that require 5 V. How much current can the 5 V (Vcc) power bus supply?

Because the 5 V goes through two power stages, the answer is not completely clear-cut. The 5 V regulator itself has a 900 mW power dissipation limit, so with a 4.3 V drop from the 9.3 V boost voltage to 5 V, we get just over 200 mA. The stock electronics on the 3pi typically use under 50 mA (however, this depends on what your program is doing, if you are making high-frequency noise with the buzzer, and so on), so you could figure an absolute max of 150 mA, with 100 mA being a more comfortable guideline.

However, the boost voltage has a limit of its own of around 1 A, which is dependent on your battery voltage. The motors and IR LEDs also use this supply, so using a lot for your 5 V will affect what is available for the motors. You can almost stall the motors and still have the full boost voltage on the motors in the stock configuration; if you’re also drawing an extra 200 mA for other electronics, the boost voltage will start dropping as the motors approach stall, though this is not necessarily a bad thing since it will limit the stress on the motors and lower the voltage drop on the linear regulator.