Hello,
I want to start a project where I want to make a circuit that will notify me with a beep once a year. This will also be powered by a small battery.
Any suggestions on what chip to use that is really low powered? I was thinking about the attiny85
Also, is there a library that is good to use on timers? As I want a beep once a year the millis() is not the best to use?
My project will have one button, one led and one buzzer so the chip should have 1 input and 2 outputs 
The big problem here will be measuring a 1 year period accurately without the use of an RTC (real time clock) module.
You can buy little DS3231 RTC modules that have a CR2032 time/date backup battery that should last a few years. Another option would be to use a Wifi enabled MCU that can get NTP time but Wifi is power hungry and not necessarily available.
For the MCU you might consider something like the nRF51822 or nRF52832 as they are very low power standby and can potentially work on a CR2025 for > 1 year but they are not so easy to setup and program in the Arduino IDE.
It is not that necessary to measure a year accurate down to the minute, but a periode of say 5 years would be great to get down to not miss by more than a few weeks, will that be possible w/o the use of a RTC as that will add to the power consumption..
kjetilhansen:
It is not that necessary to measure a year accurate down to the minute, but a periode of say 5 years would be great to get down to not miss by more than a few weeks, will that be possible w/o the use of a RTC as that will add to the power consumption..
If your just relying on the MCU's built in clock then it's frequency will fluctuate depending on temperature and may be giving the wrong PPM in the first place. Over a year you will probably be weeks out unless you can tune the frequency for better accuracy.
The power trade-off of using an RTC can possibly be recovered because you should be able to put the tiny85 MCU into deep sleep for several hours/a day at a time and use the RTC alarm to wake the MCU. Without using an external trigger source your going to wake the tiny85 every 8 seconds with the watchdog timer.
Someone may have tested the relevant power requirements of powering a tiny85 every 8 seconds compared to powering it once per day + the RTC power needed.
Can't remember the anniversary (birthday, etc.) eh? Great idea! The "beep" for time's up should repeat every minute in case you aren't right there when it happens. When it does, you should also change the battery, and tell the device to start timing another year. A single acknowledge button should do it. You can power the whole thing from a 3 volt coin cel. Use an RTC, which should wake the tiny once a day (at 12:01 perhaps) to check if today is the day, and if it is, signal you. Currently I like this one from Sparkfun, because it has an onboard supercap so changing the battery won't be a problem. You can make this thing really small - the coin cell will run the RTC, and for a second or so every day, the microcontroller. Or maybe build it into something big, so you don't lose it!