A better way to do this is using an h-bridge IC. But can I buy one in Sydney? Only Altronics and Farnell stock them.
http://itp.nyu.edu/physcomp/Labs/DCMotorControl
http://lab.guilhermemartins.net/?p=66
http://www.toddholoubek.com/classes/pcomp/?page_id=394
There is an official Arduino motor controller shield.Â
And the all done solution from Lady Ada – an Arduino shield kitset (with eagle files if you want to DIY) that controls dc motors, steppers and servos for good measure.
Lady Ada – 19.50USD + 9.65/33.50PP
http://www.ladyada.net/
Fun Gizmos (v3) – shield & components 25USD + 13.85/28.85/72.52PP
http://store.fungizmos.com/
Little Bird (v3) – shield only 18AUD +7.80PP (+GST I don’t like it when they neglect to mention that until checkout)
http://www.littlebirdelectronics.com/
NKC (v1) – shield only/shield & components 4.99/10.99USD (incl. shipping comes to 17-25USD)
http://www.nkcelectronics.com/
Schematics for Motor Shield v3
http://forums.fungizmos.com/
Local store w/ Arduino & motor controllers
http://www.oceancontrols.com.au/
Parts from Farnell ~36AUD
Parts list from Fun Gizmo (v3)