Why can't we use a 5mm LED as the flasher in this circuit? What's the difference between a 3 mm and 5 mm led? How do you identify one?
Note that the paragraph that accompanies the circuit specifies the LED type as a flashing LED. It says that a normal LED will not work. The paragraph does state that the LED in the picture is 3mm, so I can understand the confusion. However, a flashing LED has a small circuit built in to make it self-flashing.
This circuit needs that internal circuitry of the flashing LED to drive the NPN transistor.
In summary, if you can find a 5mm flashing LED, you can go ahead and use it! Just make sure it is a flashing LED though! A quick Google of flashing LEDs will give you numerous results with different sizes.