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ocrdu
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How to put an ATmega328 into sleep mode and wake it up without a button, without WDT

I am building an IR based faucet. I need to include a battery backup. I am using only an ATmega328 with a crystal and a capacitor, I am not using a regulator or LED.

I need to increase the battey life. I am using 4AA batteries in series with a capacity of 2000 mAh and thd device is currently consuming 15 mA when no hand is detected.

When there is no hand in front of the IR receiver it should be in sleep mode and when a hand comes near to the sensor it should wake up and energise a solenoid.

Instead of waking the Arduino every 2 s is there any way that it could wake up only when a hand is near?