The Atmel microcontrollers on which many Arduino and compatible boards are based are rightly famed for their low power drain, in particular when using interrupts and the chip's built-in sleep functionality. Sometimes, though, low-drain isn't enough - which is why Instructables user Hase has published a schematic for a zero-drain Arduino-compatible standby circuit.
Designed as part of a project to convert a small commercial van into a camper, to drive a custom dimmable 30W LED lighting system, Hase's circuit is designed to entirely eliminate the standby power drain of the Arduino. "The problem: onboard devices for vehicles (cars) typically draw some standby power. I hate that: my car is used very little and I frequently have the battery depleted," Hase explained. "So my own designs should do it right: zero standby power. Zero as in nil, zilch, nothing; not 'very little' or 'almost nothing' but actual zero."
Hase's design uses relays and transistors to shut the power off completely while the circuit is in 'standby' mode. Once a switch is pressed, the relays activate a buck converter to power the Arduino. While the Arduino is active, the power continues to flow - and the board's pulse-width modulation (PWM) output can be used to dim or brighten the in-car LEDs. When the button is pressed a second time, the Arduino triggers a shutdown sequence which cuts its own power - ready for the next time the circuit is active, and without even the tiniest of standby power drain you would normally have using the microcontroller's sleep functionality and interrupt pins.
The purpose of the power management is to save the battery when the LED is off (zero standby). The push button turns it on, the rotary encode operates the dimmer function and the push button also tells the firmware to kill power again.