By using two transistors and some of R and C, we can buid a flip-flop circuit (as seen at Figure 1). The meaning of flip-flop is one ON and the other is OFF and vice versa.

Figure 1. Flip-flop circuit using two transistors.

By adding a same transistor circuit (as seen at Figure 2), so the flip-flop circuit is changed to LED circuit that has mode like a running object, so it is called running lamp.

The speed of light on and off is depended on the value of Rb and C, both of figure 1 and 2.

Figure 2. Running LED circuit using three transistors.

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