17 Feb '12, 9pm

Looking for a battery/self-powered circuit/chip that'll spit out UTC seconds via i2c. Like this but cheaper: Ideas?

I’d like to use the 1Hz oscillator and the “open drain” SQW pin with an LED and resistor connected in series to the 5V supply. So far, I’m using a 3K-Ohm resistor to limit the LED to about 1mA (fairly dim). Is this sufficient as a pull-up resistor? A lower value / brighter LED would be nice. A 200-ohm resistor did also work, but I didn’t leave it running for a long time. Should I be doing something else, like using SQW to drive my own transistor to switch the LED? Seems redundant, but I’d rather put in an extra part than kill the clock. I can’t find anything in the datasheet that refers to allowable current on this pin; presumably it has no problem with the +5V it uses itself. All the online references to open drains (mostly talking about open collectors actually) I can find seem to be talking about using values up to hundreds of kohm to try to minimize current or dropping...

Full article: http://www.sparkfun.com/products/99

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