Better you use a transistorised astable multivibrator with NPN & PNP transistor and a POT in series with feedback Capacitor. This is the cheapest circuit and the simplest one.
**broken link removed**
Use left npn biasing resistor as 1m, pnp collector res 1 k, feedback capacitor (1-10uf) & pot 220e to 1k. Use led biasing resistor as per supply & led current.
The wrong supply and the supply node is emitter of PNP transistor. Please excuse me as I draw in hurry.