Operate with 5 to 12 volt, this circuit can be useful to mainly monitor the capacity of the batteries, portable and easy to operate. LED D1 flashes to attire the user’s attention, signaling that the circuit are running. The circuit generate about two LED flashes per second with current consumption about 200uA. The sequence of LED flashes are generated with transistor Q1 and Q2 that wired as an uncommon complementary astable multivibrator. Both are off 99% of the time, saturating only when the LED illuminates, thus contributing to keep very low current consumption.
R1,R7—————————————–220R 1/4W resistors
R2———————————————120K 1/4W resistor
R3———————————————5K6 1/4W resistor
R4———————————————5K 1/2W trimmer cermet
R5———————————————33K 1/4W resistor
R6———————————————680K 1/4W resistor
R8———————————————100K 1/4W resistor
R9———————————————180R 1/4W resistor
C1,C2—————————————–4u7 25V electrolytic capacitors
D1———————————————BAT46 100V 150mA schottky-barrier diode
Q1———————————————BC547 45V 100mA NPN transistor
Q2———————————————BC557 45V 100mA PNP transistor
B1———————————————5 to 12 V battery
This circuit will work with battery supply voltages in 5-12V range. When the battery voltage approaches the exhausting value, the LED flashing frequency will fall suddenly alert the users, and when the battery voltage fallen, the LED will remain permanently off. Range and the LED flashing can be stopped at the desired battery voltage by adjusting trimmer R4. This can be modified by changing R3 and R4 value slightly. The current consumption of the circuit can be reduced further on by rising R1, R7 and R9 values.
source : www.redcircuits.net