Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 247

Circuits 19 and 20: A strong base current flows through T2, if a button is activated. The LED turns on when
the connection is made. At that moment, R4 blocks transistor T1. Upon releasing the button, a small amount
of holding current flows through R3 and on to the base of T2 keeping it electrically conductive. If the reset
button is activated, this holding current is discontinued and T2 blocks and LED1 turns off.
Fig. 3.18: Circuit 18:
Boring double turn
signals
Fig. 3.19: Circuit
19: Electronic
brain cell
249