date/time triggered DC switch - going small

Thread Starter


Joined Apr 4, 2016
Hi all,

First I'm new to all this (newb), so fill free to bash on my idea, but keep it constructive please.

So the (sub)project seems simple, but is a challenge to me:
Make a date/time triggered DC switch that will not be powered always, so it needs a battery at least to keep the RTC running.
It is not of importance that the switch (dc latch relay) is triggered at the exact time/date but when the MC is powered and not on backup battery and the time marker has passed it should be triggered.

So the idea is to keep it on a 5V line and a 3V backup battery. As imperative is to have it as small as possible, I am thinking of going with a small MC with an inbuilt RTC and an option for keeping the RTC part on backup power when 5v is not available. As for battery, I would go with CR927, unless I have more space on the PCB.

When the time/date marker has passed MC should change the latch relay controlling DC power (5V or 12V w/ 2-3 A).

Having separate RTC solution from MC will demand more space on PCB (correct me at any point if I'm mistaken), so the PCB size will be determined by the chosen MC and relay. I have a possibility of going both sides on the PCB with elements, and going elongated rather than square. The smaller it is the better, as space is tight... also price should be low :)

Recommendations for the microcontroller are highly welcomed!
Also any other suggestion...



Joined Sep 13, 2015
Your description isn't terribly clear. But something like this will work: set up an alarm in the rtc. Once power on, the mcu checks the rtc to see if alarm has passed. If it does, disarm the alarm and do its thing and goes to sleep.

The MCu is also configured to wake up from the alarm.

Inboard or outboard rtcs will work. Just need to make sure that the rtc has its own power domain.