Here's a possibility
battery | | halogen light | _| | pwm_____10R_____|| RFP30N06LE |_ | | ground
formatting link
The rfp30n06le is a logic level mosfet, so your 5 volts will turn it on. The mosfet won't draw any current from your pic or555. The mosfet has about a twentieth of an ohm resistance, so it won't get hot.