As per Duffy, use a 555 timer ic.
**broken link removed**
Google for: 555 PWM circuits
Hi,
Looks like the output protection diode is missing from that circuit
diagram. There should be a diode or zener or diode/resistor from
the transistor drain to V+ to make sure peak voltage stays
at or near the V+ line to protect the transistor, or at least
so that the drain voltage stays under the rating of the transistor.
For a diode, the anode connects to transistor drain, cathode to
V+.
For a diode and resistor, the anode connects to transistor drain,
resistor to diode cathode, other end of resistor to V+.
Without this diode the transistor drain can reach voltages as
high as 1000v or more when the transistor turns off due to
the inductive effects of the motor winding.