DS1307 Real Time Clock Breakout Board This real time clock (RTC) with battery backup allows your microcontroller project to keep track of time even if it's reprogrammed or if the power is lost. It's perfect for data logging, clock-building, time stamping, timers, alarms, and more. 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). The DS1307 is simple and inexpensive but not a high precision device. It may lose or gain up to 2 seconds a day. For a high-precision, temperature compensated alternative, take a look at the DS3231 precision RTC . It works great with an Arduino using our RTC library or with a Raspberry Pi (or similar single board linux computer) . Note: This product does not come with a CR1220 coin cell battery. We recommend you purchase a CR1220 coin cell battery to use with this product. Features PCB & header are included Plugs into any breadboard, or you can use wires Two mounting holes Will keep time for 5 years or more Specifications Length: 25.8mm/1.02in Width: 21.7mm/0.85in Height: 5mm/0.2in Weight: 2.3g/0.09oz Mounting holes are 2.2mm(0.086in) diameter, 25mm(0.98in) apart This board/chip uses I²C 7-bit address 0x68 . Made in China.