What you will need:
this circuit requires +3v min to +15v max input voltage. this circuit can have a seperate voltage source than the one used by the load (led), but as always they need to have a common ground.
Discharge pin on the 555 is the signal output.
Q1 can handle up to 200ma but anything higher than that and you will get magic smoke. always check your data sheet for your components limitations.
for example 200ma = 10 leds that require 20ma of current each
Q1 can be replaced with other transistors or even mosfet like tip31.
R3 and R4 act as a potentiometer as LTspice simulator does not have a potentiometer model. just use a 200k potentiometer. the potentiometer will act like a voltage divider.
D1 and D2 are the 1n5818, make sure they are pointing to opposite direction of each other.
as you can see in the pictures below, the circuit is producing PWM signals. fist picture is full duty cycle, second picture is half.