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This unique
circuit makes your dome light look cool. Usually when the car door
is closed, the dome light just goes OFF. With this circuit, you
can have our dome light fade slowly in brightness and finally go
OFF. This slow dimming of the light gives a very good feeling at
night. It looks very romantic!
The circuit can be explained as follows: When the car door is
open, the push to off switch of the door is ON and hence it
charges the 22uF capacitor fully. The opamp is acting as a voltage
follower and its output is same as the voltage across the
capacitor, which is 12V when the capacitor is fully charged. Due
to a high voltage at the output of the IC, the transistor
saturates, turning ON the bulb to full brightness.
Now when the
door is closed, the door switch is pushed in and hence the switch
goes OFF. When the switch is OFF, the capacitor starts discharging
slowly through VR1 and the 10K resistor and the voltage across it
decreases slowly. Hence at the output of IC 741 also the voltage
decreases gradually, hence decreasing the base current to the
transistor. This produces a slowly decreasing current through the
bulb and the bulb fades out and finally when the capacitor is
fully discharged, the bulb goes OFF.
After building the circuit, with the push-to-off switch in ON
position (not pushed in) i.e. the car door open, adjust the preset
VR2 to the required initial brightness of the bulb. Then push the
switch in to turn it OFF(or close the door) and adjust VR1 for the
time to bring the bulb from full brightness to OFF.
I would suggest you set VR1 and VR2 to their maximum values.
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