Here is a simple battery monitor circuit in which the LED will continue flashing until battery voltage is above the level. Transistors Q1 and Q2 are wired as an astable multivibrator circuit. This flashing battery monitor circuit can operate from a variety of voltages from 6V to12V.
The voltage level at which an LED stops flashing can be set by adjusting R4. When the a preset battery voltage reaches threshold flashing frequency decreases andwhen the voltage drops below the threshold of the diode D1 LED turns OFF. This is used to compensate for changes in voltage of the base emitter Q1 due the changes in temperature.
Flashing battery monitor parts list :
R1 : 220 ohm
R2 : 120K
R3 : 5.6K
R4 : 4.7K variable
R5 : 33K
R6 : 680K
R7 : 220 ohm
R8 : 100K
R9 : 180 ohm
C1 : 4.7uF
C2 : 4.7uF
D1 : BAT46
D2 : LED
Q1 : BC547
Q3 : BC557
