Multiple timers with ultra low power

Question

I want to blink 3 different coloured LED's with different timing repetitions.

Red - every 2 minutes. Green - every 25 minutes. Blue - every 2 hours.

Components in the circuit:-

1. 3 different coloured LED's
2. Small vibration motor (heptic alert)
3. Small battery 150-300 mAh.
4. micro-usb to charge the battery

As I want to maximise the battery life I am confused which IC's or MCU's to use for this application. The vibration motor will function with all 3 LED's. To find a balance with cost and battery consumption is really difficult.

The timer IC I found with very low power consumption is TPL5010. But I will have to use 3 of these to control 3 LED's. If I use a cheap MCU like ATTINY9 it doesn't have a RTC for time keeping. To achieve lower power consumption the LED's can be turned on and off with quick intervals but is an MCU necessary for this?

What is the best way to achieve my application? Thank you!

Answer 1

You don't need that timer IC at all, nor an RTC. Modern microcontrollers have low-power timers, which you can simply program to wake you up in a few seconds, or a minute, or two. There's no advantage to not implementing this functionality on a microcontroller; you just program your microcontroller to set a timer to wake in two minutes, change the LED state, program the timer to wake in two minutes (or 1 minute, should this be an odd multiple of 24th minute, and you need to activate the 25 minute LED). Every time you wake up, you check which LED's state to change.

The total parts cost of this is probably 3 LEDs, a sufficiently low voltage source (battery) so that you don't need a series resistor for these LEDs, a microcontroller, and a decoupling capacitor; if your timing needs to be exact, a crystal oscillator. No other approach will be as precise, cheap or easy.

As long as they are on long enough that you can see the LEDs light up, the total power consumption (over a day) of the microcontroller will almost certainly be lower than that of the LEDs, significantly so. An LED takes ~ 10 mA, a modern MCU in low-power timer mode ~ 50 µA.

Answer 2

Current consumption in a micro controller can be dramatically reduced by using a lower clock speed.

For example, here is the "Active Supply Current" for the Attiny9 using a 3.3V supply at various clock speeds:

• 12 MHz - 1.5 mA
• 1 Mhz - 0.31 mA
• 32 Khz - 0.013 mA

(From: https://ww1.microchip.com/downloads/en/DeviceDoc/ATtiny4-5-9-10-Data-Sheet-DS40002060A.pdf pp. 131-133)

And you can save more by sleeping.

A 32 KHz external crystal is enough for accurate time-keeping -- RTC chips use them for their timebase.

So use a lower clock speed if you're concerned about power usage.