A previous Design Idea describes a circuit that uses an astable multivibrator to drive an LED (Reference 1). The circuit in Figure 1 uses a simpler alternative approach. The circuit uses a 2N3904 NPN ...

I remember making a flasher with a LM3909 back in the late 1970s. It was very efficient. In regards to this circuit, I can think of one that is even simpler, although not so stable.

This simple device can be used to create complex electronic circuits, and although there are other types of transistors, bipolar junction transistors are still widely used in digital circuits.