My line follower robot won't move forward

[white_fang]

Hi there,
just for clearance I am IT college student and I am still new in the field of electronics, please correct me if I have something wrong so I can learn more and thx

So few days ago I've been working on my line follower robot using the Arduino, I used the TCRT 5000 KY 033 as line detection sensor and two gear motors with two NPN transistors to move the robot, my circuit is connect like this scheme below (I can't find my sensor in the simulator so
I used another similar type of IR sensor) :

The problem is that when I use the laptop USB as power source (5V) the robot works fine and the robot moves fine but when I use external power supply (9V battery) the motors move in very low speed so that the robot won't move.
I can't know whether the problem is from the low power of the battery (still new) or because of my circuit connections.
Any Ideas?

[groundFungus]

To start the motors are not wired properly. No base resistor and you are missing the flyback diode that protects the transistor from the high voltage kickback when the motor is de-energized.

D1 is a silicon rectifier diode like the 1N400x.

The PN2222 is good for 800mA. Choose a transistor that will handle the motor stall current.

Do not power the motors off of Vin. The Arduino is not a power supply and Vin is not an output.

[jremington]

9V PP3 batteries are for smoke alarms, not motors. If the motors need 9V, use a 6xAA battery pack, or larger.

[white_fang]

According to my simple knowledge, the AA battery has 1.5V so the 6xAA battery pack should have :
1.5 * 6 = 9V
isn't it the same amount of voltage of the 9V PP3 ?
can you please explain for me more the difference between these two?

[jremington]

Correct, the voltage is the same. The maximum current delivery and battery capacity are very different.

[white_fang]

okay,
soo the 6xAA battery pack has more maximum current delivery I suppose?
(enough to run 2 5V gear motors)?

[jremington]

You can look those things up on the web.

[johnerrington]

@white_fang

can you please explain for me more the difference between these two?

yes

The amount of electricity stored in a battery is measured in Amp-hours (or usually mAh) at the 0.1C rate (which means a discharge duration of 10 hours)
a pp3 battery typically has a capacity of around 500mAh. so it could supply 50mA for 10 hours.
a decent AA cell typically has a capacity of around 2000mAh so it could supply 200mA for 10 hours.

The 0.1C rate gives a good indication of the maximum current you should expect to draw from the cell. So it would be unreasonable to expect a 500mAh PP3 to provide 500mA for 1 hour - or even a few minutes.

However a set of 6 AA cells could easaily provide around 500mA for a while. I test AA NiMH cells with a 2.2 ohm load, giving a discharge current of 0.5A

Its not great to use cells connected in series to power devices as the "weakest" cell will give out first. This can damage rechargeable cells.