Edit: Below is an ASCII art rendering of the circuit. Beware that the motor is ON when the Arduino output is LOW.
+-----------+
| 5V|-----------------+-------------------+
| | | |
| Arduino | | |
| | ,-----. B |< E ___ +
| D13|----| |----| PNP _
+-----------+ `-----' |\ ___
|C _
| ___
+---------+ _
| | ___ 9 V batt.
| | _
--- | ___
^ motor _
/ \ | ___
--- | _ −
| | |
| | |
+---------+---------+
Edit: Below is an ASCII art rendering of the circuit. Beware that the motor is ON when the Arduino output is LOW.
+-----------+
| 5V|-----------------+-------------------+
| | | |
| Arduino | | |
| | ,-----. B |< E ___ +
| D13|----| |----| PNP _
+-----------+ `-----' |\ ___
|C _
| ___
+---------+ _
| | ___ 9 V batt.
| | _
--- | ___
^ motor _
/ \ | ___
--- | _ −
| | |
| | |
+---------+---------+
When connecting together several circuits that have different supply voltages, common practice is to make them all have a common negative supply rail. This is called "common ground". As explained in Majenko’s answer, you would need an NPN transistor for that.
If you want to use a PNP instead, then you could forget the common practice and do everything "backwads": if you connect together the positive ends of the power supplies, you will have a "common VCC" instead of a common ground. Relative to the Arduino ground, your 9 V battery will have one pole at +5 V and the other at −4 V.
The wiring is the following:
- connect the emitter to +5 V (Arduino +5 and battery +)
- connect the base to an Arduino output through a resistor
- connect the collector to the motor
- connect the other end of the motor to the battery −
- put a free wheeling diode in parallel with the motor.