Use LEDs as photodiodes
Figure 1 An LED’s resistance changes with ambient light, which changes a voltage that drives a logic gate.
This type of light detector is highly power-efficient and is ideal for battery applications. You can use the NAND gate’s logic output to drive an LED driver or a relay driver, or you can connect it to a microcontroller.
Place the circuit so that sufficient light falls on the green sensor LED. Doing so avoids any voltage buildup near the junction that could be close to the NAND gate’s threshold voltage. The NAND gate’s power consumption rises sharply at the threshold voltage. When the gate’s input voltage is within the defined limits for the logic state, its power consumption is extremely small.