The LDR should be fixed some where on the doom light of the car, make sure that the LDR will only receive the light of the doom light, for doing that use some kind of tape to cover all sides of the LDR so it will not receive the outside light and the front of the LDR will be placed on the car's doom light.
The circuit is using a 3V lithium coin battery as a power source that will last longer because the circuit will not use any current in standby mode. The 1M variable resistor is used to adjust the sensitivity of the circuit to detect light. The 100K variable resistor is used to adjust the frequency of beeps of the buzzer. A low power TLC555 timer is used in the circuit, do not use NE555 timer in the place of this IC it will not work on 3V. If you don't find TLC555 then use 7555 timer IC that is an another low power version of NE555 timer IC. Or if you want to use NE555 timer IC then use two 3V lithium coin batteries parallel to supply 6V to the circuit, and use a 6V buzzer.
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This simple contact less car door left open reminder alarm will give beep beep buzzer when any door of your car is not closed properly. The circuit is simple and using few easy to find parts. The whole circuit can be made on a very small pcb or veroboard and can be fixed in a small enclosure.
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