This circuit connects to the serial port of many cellular phones. It provides an input and an output port capable of being remotely controlled using another mobile. When the mobile receives a predefined SMS text message, like Activate burglar alarm or Start backup pump, the circuit automatically recognizes it as a command and switches the output accordingly.
The software implements low-power techniques, which extend battery life to more than a year of continued use using AA cells. The TINY12 is perfectly suited to run on batteries. Most of the time the controller is in sleep mode, with only the watchdog on.
In this condition, power consumption is only 200 µA. Control is resumed when the watchdog expires (a couple of seconds), or when the input pin changes, thanks to the “wakeup on pin change” capability of the TINY12.
When running the required current is about 1 mA, for a period as short as 20 mS. Every 10 short breaks (about 20 sec) the micro controller starts a longer dialogue with the mobile, requiring an extra 840 mS at 1,2 mA.
As the circuit run on the internal oscillator, the clock frequency varies over battery lifetime. The serial port routines implement an auto-baud algorithm that adapts its timing as clock changes. To save another nibble of energy, the calibration value is stored in eeprom and the calibration is repeated only when needed.
Last but not least, the circuit must behave correctly when still connected to a discharged battery. The prototype passed the test brilliantly: the internal brownout halted the controller as expected.
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