The circuit in which an LED will continue flashing as long as the voltage of the battery to be monitored is above the set level. The transistors Q1 and Q2 are wired as a complementary Astable multi-vibrator. Operating voltage for this circuit is 9v (or 6v can be used). The voltage level at which LED has to stop flashing can be set by adjusting the preset R4. When the battery voltage reaches the threshold the flashing frequency decreases and when the voltage goes below the threshold the LED becomes OFF. The diode D1 is used to compensate the changes in the base emitter voltage of Q1 due to temperature changes.
Transistor Q1 = BC547, Q3 = BC557
R1 = 220ohm, R2 = 120K, R3 = 5.6K
R4 = 4.7K, R5 = 33K, R6 = 100K, R7 = 220ohm, R8 = 100K, R9 = 470ohm
C1 = 4.7uf, C2 = 4.7uf
D1 = BAT46 (Schottky Diode)