Adafruit Industries Adafruit Industries DS1307 Real Time Clock Assembled Breakout Board
This is a great battery-backed real time clock (RTC) that allows your microcontroller project to keep track of time even if it is reprogrammed, or if the power is lost. Perfect for datalogging, clock-building, time stamping, timers and alarms, etc. The DS1307 is the most popular RTC - but it requires 5V power to work (although we've used it with 5V power and 3.3V logic successfully). Battery not included.
Adafruit Industries DS1307 Real Time Clock Assembled Breakout Board
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