Room light-on indicator

Main project requirements and background

An indicator (LED) should be installed outside the closed door to show that the light is on inside the room. This may mean that someone is inside or that the light was left on.
The circuit must be powered by two AA cells and consume as little power as possible.
The circuit should be able to operate from discharged AA cells (1.2-1.3V at rest) that are no longer suitable for other devices because of their low voltage.

Device schematic

How it works and the components used in the project

A photoresistor is placed inside the room as a light detector. It is best placed closer to the light source so that other objects cannot block the light reaching it.
The red indicator LED is located outside the room.
Place the remaining components where the connecting wires will be least intrusive.
The photoresistor and resistor R1 = 6k8 form a voltage divider. By changing R1, you can set the light-level threshold at which the LM393 receives enough supply voltage to operate, turning on the LED indicator.
The LM393 is used in an unconventional way in this circuit: its open-collector output acts as a low-side switch for the LED, while the IC itself draws little current.
When the light level is too low, the photoresistor has high resistance and only a small current flows through it. The LM393 does not receive enough supply voltage to operate, so the LED indicator remains off.
At a sufficient light level, the photoresistor's resistance decreases, the LM393 receives enough supply voltage, and the open-collector output on pin 1 completes the LED path to the negative supply rail. The LED lights up.
A current-limiting resistor should be installed in series with the indicator LED. Choose its value based on the supply voltage, LED forward voltage, and desired current. In my unit, the circuit runs from two discharged AA cells without a separate resistor and the LED is not damaged, but this should not be considered universally safe. With fresh 1.5V AA cells, a 50Ω current-limiting resistor is recommended; the exact value should be checked against the specifications of the particular LED.
Even with such discharged cells, the device runs for several months with multiple activations per day.

A list of components used in this project, with links, is available in the Google document ("Light on indicator" sheet).