Car Battery Saver

Prevents the complete discharge of the battery

when the door is left open accidentally

Circuit diagram:

Car Battery Save


R1______________10M  1/4W Resistor
R2______________10K  1/4W Resistor

C1______________47µF  25V Electrolytic Capacitor

IC1____________7555 or TS555CN CMos Timer IC

D1___________1N4148  75V 150mA Diode

Q1____________BD681 100V 4A NPN Darlington Transistor

LP1___________Existing Lamp Bulb, usually 12V 5W

SW1____________SPST  Existing Door-Switch

SW2____________SPST  Existing Bypass Switch


I recently forgot to close the door of my car after parking in the garage and I found the battery completely exhausted after the week-end, when I tried to start the engine on Monday morning.
This inconvenience prompted me to design a simple circuit, capable of switching-off automatically after a few minutes the inside courtesy lamp, the real culprit for the damage.

Circuit operation:

When the door is opened, SW1 closes, the circuit is powered and the lamp is on. C1 starts charging slowly through R1 and when a voltage of 2/3 the supply is reached at pins #2 and #6 of IC1, the internal comparator changes the state of the flip-flop, the voltage at pin #3 falls to zero and the lamp will switch-off.
The lamp will remain in the off state as the door is closed and will illuminate only when the door will be opened again.
The final result is a three-terminal device in which two terminals are used to connect the circuit in series to the lamp and the existing door-switch. The third terminal is connected to the 12V positive supply.