The_MagPi_issue_8.pdf

ISSUE 08 - DEC 2012

Visit our Kickstarter

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A Magazine for Raspberry Pi Users

Catch Santa using

home automation

This Issue. . .

Win a 512MB

Raspberry Pi

• Skutter

• Nanpy

• Pi Gauge

• Pibow

• CESIL Pi

• C++

• Ada

• MySQL

• Python Pi t

Merry Christmas

From The MagPi

h t t p : / / www . t h ema g p i . c om

Raspberry Pi is a trademark of The Raspberry Pi Foundation.

This magazine was created using a Raspberry Pi computer.

Welcome to the eighth edition of the MagPi magazine,

It’s Christmas! In this issue we hope to entice you into some festive projects to try after gorging yourself to the brim with

Christmas pudding.

In this month’s edition, we introduce you to a simple home automation project allowing you to control lights and

appliances in your house using the power of the Pi! Just in time to catch Mr Claus! We get your Skutter project in motion

with Morphy’s article on adding wheels to your base. Gordon teaches us how to light up a Xmas tree, we have more on

using the Pi to control an Arduino and Ben describes how to control servos attached to the Pi using the Internet! If this

isn’t enough we have more of the old favourites plus an introduction to SQL.

As per always, we have some great gifts for you to win in our monthly magazine. The MagPi would like to say a big

thank you yet again to PC Supplies who this month has outdone themselves by offering up a 512MB Raspberry Pi for

grabs!

In addition to this we have some exciting news for you this month. As of December 1st, we at the MagPi, are so excited

to be able to offer our readers the possibility of a printed version of all eight issues of the magazine! This is something

which gets constantly requested of us from our readers. All eight issues will be beautifully wrapped up in limited edition

MagPi binder making it a great gift to yourself or any of your loved ones of any age. For more information on this please

visit www.kickstarter.com/projects/themagpi/the-magpi-magazine-from-virtual-to-reality

On behalf of the whole team, thank you again for all your support. We hope you have a fantastic Christmas and we will

see you in the New Year (1st of February). While we have not managed to squeeze it into this issue, you might be

interested in www.xmas4all.co.uk from which you will be able to control their Raspberry Pi powered Christmas lights!

Ash Stone

Chief Editor of The MagPi

Contents

Dig out the toolbox for the next thrilling installment, by Bodge N Hackitt

Control your home with a Raspberry Pi and catch Santa in the act! by Geoff Johnson

Win a 512Mb Raspberry Pi Model B, from PC Supplies UK

The power of Raspberry and the simplicity of Arduino using Nanpy, by Andrea Stagi

Control servos over the internet, by Ben Schaefer

Simon Monk's new book covering basic to full GPIO python examples.

An interview with the designers of the PiBow case, by Chris Stagg

Christmas from the 70's using the CESIL programming language, by Gordon Henderson

Using basic variables and STL strings, by Alex Kerr

The second installment in our Ada programming tutorial, by Luke A. Guest

Get your teeth into some Structured Query Language (SQL), by Richard Wenner

Raspberry Jams and other community events

Creating multiple desktop widgets, by Colin Deady

Adding a motorised base

Part 2

This diagram shows a simplified version of

such a circuit. Closing switches 1 and 2

effectively connects the positive rail of the

power supply to the + terminal on the motor

and ground to the – terminal and the motor

runs forwards. Alternatively, closing switches

3 and 4 connects the ground to the + terminal

and the positive rail to the – terminal and the

motor runs in reverse.

In the last article we looked at some physical

means of adding motors to a robot and

investigated adapting some motorised

electronic toys as a potential source for robot

bases.

In this article I will begin to explain how you

can build your very own DC electronic motor

driver module and write a basic control

program for it.

There is a potentially dangerous situation if

switches 1 and 4 or 3 and 2 are closed. This

would create a short circuit between Power

Supply + and Ground which can be very

problematic to say the least. Care must be

taken when controlling this circuit to ensure

that this situation can never happen.

We will start by re-examining the standard DC

motor that was covered in the previous article.

To make the motor run forwards we apply a

power source between the + and – terminals

on the motor and to make it run in reverse,

simply swap the power source terminals

around.

In reality we can’t have four physical on / off

switches like this as we need to control the

circuit using the GPIO on the Raspberry Pi.

There are electronic solutions to this. One

possibility is the use of electromagnetic relays

to close these “switches”, however the

Raspberry Pi is not able to deliver enough

power from the GPIO to directly activate such

a relay without having something in-between

such as a transistor. Thid leads us to the

second possible solution which is to simply

use some transistors as switches.

The motor driver module we are going to

create will need to be a circuit which is able to

do this swapping around of the power supply

terminals

electronically.

This

can

accomplished using a “H bridge circuit”.

The transistor is arguably the most important

electronic invention ever created. Its

development is responsible for everything

from portable music players to the processor

used in the Raspberry Pi.

We will be looking at NPN type transistors.

This device has three terminals called base,

collector and emitter.

draw when our robotic base is trundling along

the floor. One way to accurately obtain this

measurement is to make the base move on

the ground and measure the current that is

being drawn. Here is the method I used with

my Big Trak:

Connecting a power supply across the

collector and emitter allows the transistor to be

used as a switch. Without a connection to the

base, the “internal resistance” of the transistor

is extremely high and the “switch” is off.

Connect the multimeter in series between the

battery/power supply and one of the motors in

the Big Trak.

If we apply a current to the transistor base

then the internal resistance will drop by a

corresponding amount and more current will

flow from the collector to the emitter.

The transistor is able to vary its internal

resistance very quickly, tens of thousands of

times per second. (It’s this feature that allows

transistors to be used as amplifiers).

The amount that the current affects the

internal resistance of the transistor is defined

by a ratio known as the “DC current gain” and

is referred to as “h FE ”.

In our case we want to supply a current to the

base that will make the internal resistance be

zero – just like a closed switch. This is called

“Transistor Saturation” and there is an

equation which tells us the current we need to

apply to the base to make this happen,

I B

= I C

/ h FE

where I C is the collector current and I B is the

base current. In order to find out what this

current is it’s necessary to measure the

current that’s drawn by the motor. This means

an experiment is needed!

The second motor must also be connected to

the power supply and active otherwise only

one motor will try to drive the whole Big Trak

which will result in an inaccurate

measurement. However we only need to

measure the current drawn by one of these

two identical motors.

For this you will need your motorised base (in

my case it’s the modified Big Trak), a power

supply (some batteries) and a multimeter.

Add some weight to the Big Trak which

approximates the expected overall weight of

the finished robot. In the case of the Skutter

this includes adding the robot arm.

If you don’t own a multimeter yet, they are an

essential tool for anyone who is involved in

electronics and allow you take a wide range of

measurements including voltage, current,

resistance, capacitance and h FE . Maplins sell

one for £7.99 (CODE: N20AX).

Complete the circuit between the batteries and

motor, including the multimeter in series as

shown. As the Big Trak rolls along the floor,

take a measurement of the current which is

being drawn. Under the expected load for the

Skutter using this method, one of the two big

trak motors will draw a current of 2.5 Amps.

It is possible to obtain a good multimeter for

under ten pounds from a variety of retailers.

DC motors draw different currents under

different conditions. If a motor is “free-

wheeling” then the motor will draw a

comparatively small current.

CAUTION: When motor stalling was tested

the current drawn was approximately 20

Amps.

Alternatively a “stalled” motor (a motor that is

prevented from turning) will draw an extremely

high current. The harder we make a motor

work, the more current it will draw. In our case

we want to measure the current the motors

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