by pebe » Sat Apr 26, 2014 5:20 am
Your first circuit will not work because the current the 39ohms base resistor of the BD129 would draw would be on the maximum limit of the 555, and a FET makes a better switch than a bipolar transistor.
The second circuit is much better. The 555 oscillator is wired in the configuration I would use. You don't need the 470µF cap across the IRF520, - it defeats the object of using PWM. The diode across then motor may not be required because the FET has an internal diode which serves the same purpose.