“Flashing a LED is certainly among the first set of problems any burgeoning electronics specialist is tackling, may it be by using an ancient NE555 or, more recently, a microcontroller to control the LED. As it turns out, we can turn any trivial problem into a harder one by changing its constraints.
So, what about the challenge of flashing a LED from the charge of a single battery as long as possible? Of course, also this is not a novel problem. Two interesting approaches that I came across in the past: 1) Burkhard Kainkas Ewiger Blinker (“Eternal Blinky”) and 2) Ted Yapos TritiLED.
B. Kainkas project is a n LED flasher circuit made from discrete transistors that consumes about 50µA and is able to run for years from a single AA cell. Ted Yapo raised the bar a bit further and investigated in intricate detail how to make a LED shine for years at very low intensity from a CR2032 coin cell. His project logs are worth a read for certain. One very interesting detail is that he concluded that using a lower power microcontroller to control the LED is actually the most efficient option. This may be a bit counterintuitive, but appears more obvious when looking at his attempts of building a discrete version.
Many microcontrollers offer highly optimized low power sleep-modes that can be used to wait between the flashes. The microcontroller will only be active when the LED needs to be flashed. At that point it does not really matter how much the active power consumption of the microcontroller is, because the LED will need several mA to emit light at a sufficient level.
Enter the infamous “3 cent” Padauk microcontroller family that I used for several projects before. To my surprise, these devices offer very competitive low power sleeping modes that seems to be on par with several “low power” 8 bit microcontrollers that cost ten times as much. I investigated how to implement an ultra low power LED flasher on the PFS154.
Implementation
The first step in reducing the power consumption of the MCU is to use the low speed oscillator as a clock source. In the PFS154 this is called the “ILRC” and provides a clock of around 52 kHz depending on supply voltage. One oddity I found is that it was necessary to activate both the high-speed and low-speed oscillator as a first step and only disable the high speed oscillator in a second step. Directly switching to the ILRC halted the MCU. The code example below is based on the free-pdk includes.”