Maker Factory ELECTRONICS EXPERIMENTS BOX Manual page 316

Circuit 181 and 182: Resistors R3 and R4 (R5) form a stable voltage divider. The lower battery voltage is fed
to the base of T2. At threshold, voltage is fed to base T2. T2 turns on, T1 is blocked, LED1 is off, if and for as
long threshold voltage is exceeded (battery voltage is sufficiently high). T2 opens, T1 is electrically conduc-
tive, if threshold voltage is undershot. LED1 is lit and signals that battery voltage is too low.
318
Fig. 3.181: Circuit 181: Simple 7.5V
battery monitor
Fig. 3.182: Circuit 182: Simple
8.2V battery monitor