Hey there, I'm a rookie here, and I'm tinkering a bit with the 9th project. My question is:
What would happen if I change the 9v battery (the stated in the project), for a 7.2V - 1500mAh? Will I burn something? In theory the voltage is lower, but, typically a 9v 'transistor' battery is around 400mAh. 
Thing is, I'd like to use a r/c car's engine ( lrp s10 blast high torque motor) and it needs the 7.2v battery to work fine.
Thank you
Hi there
I think it should be no problem to use your battery-pack as a power supply (with the included motor).
As the mAh specifies the capacity of the battery-pack not the current that will flow.
However, if you use the r/c car motor, I am afraid you will destroy the Mosfet transistors, that are included in the set (Maybe the Arduino as well).
The transistors are rated for a continuous current of almost 10 Ampere.
But the current that flows through a modern r/c car motor is much higher.
(At least thats what I heard...I'm not up to date on the subject of r/c car motors)
Hope im not to late 