This is a really simple split power supply Op Amp PWM, only seven circuits. The large heat sink is necessary. The parts are easy to get too.
Step 1: Building the Circuit
As you can see it is small and simple. The pot should be at least 20k, and the more powerful transistor you use the better. A 1nF cap will produce 8 kHz oscillations a 150pF makes 75 kHz but the lower the capacitance the less reliable the circuit is. You can find your happy medium. =] The yellow leads go to ground and the load. The LED's are for the diagnostic circuit.
Step 2: Keeping It Cool
Step 3: Specs and Troubleshooting
Duty Cycle: 0-100%
Power: -12 +12V
Freq: .5 Hz - 1 MHz
The red shows the changes to the circuit. The * is because the vale depends on the LED'S you use. I used 6V ones and only a 380 ohm. The 10 uF cap slows the circuit so you can see the cycle and pulse width. If its not oscillating check the feedback network and the pot. The LED's should not light at the same time but will appear to if to increase frequency.