LED blinking sequence and free

If the resistor that charges the capacitor is too low in value (or if the power supply voltage is too high), the current through the transistor will not become low enough for the transistor to turn off. If the resistor that charges the capacitor is too high in value (or the power supply voltage is too low), the  capacitor will not be able to charge to a high enough voltage to enable the transistor to turn on.  This is because the transistor draws as small amount of current before switching on.