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Light on indicator

 
 
Main project requirements:

  1. It is necessary to install an indicator (LED) in front of the closed doors to the room, which would signal that the light inside the room is on, signaling that there are people inside or who forgot to turn off the light.

  2. The circuit must be powered by two AA batteries and be as energy-efficient as possible.

  3. The circuit should be able to operate on discharged AA batteries (1.2-1.3V at rest), which due to low voltage are unusable anywhere else.

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Schematic of the device.
 
Light on indicator
How it works and components used in the project.

  1. A photoresistor is placed inside the room (preferably closer to the light source so that it cannot be blocked by objects) to act as a light detector. 

  2. The indicator LED (LED, RED) is located outside the room.

  3. It makes sense to locate the other elements where the connecting wires will be less obstructive.

  4. The photoresistor and resistor R1=6k8 function as a voltage divider. By changing the resistance of resistor R1 it is possible to achieve the level of illumination at which the LM393 will be powered, thereby turning on the LED indicator.

  5. The LM393 comparator in this circuit is used non-standard as a transistor with low current consumption to simplify the circuit.

  6. If there is no light, the photoresistor has a high resistance, practically does not conduct electric current, and the voltage to supply the LM393 comparator chip is insufficient, switching on does not occur, and the LED indicator does not shine.

  7. In case there is a necessary level of illumination in the room where the photoresistor is located, LM393 is switched on and the LED indicator through pin 1 of LM393 chip is connected to the negative pole of the power supply, which leads to its glow.

  8. It might be necessary to install a current limiting resistor in the power supply circuit of the indicator LED because 3V is a high voltage for the red LED, but I use as a power supply circuit discharged AA elements 1.2-1.3V voltage on which falls even more when the circuit is working, which does not lead to damage to the LED element. If fresh 1.5V AA cells are used, it is recommended to use a 50 ohm current limiting resistor.

  9. Even with such discharged batteries, this device works for several months with daily multiple switching on.

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A list of the components used in this project with references can be found at Google-документе ("Light on indicator" sheet).

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