555 likely wonât be sufficient to wake up twice daily, and youâre going to need a micro for SMS. Iâd just find a low power micro that will do what you need (any low power micro will do, likely; MSP430? mBuino? ).
What power source, and how much autonomy do you need? That will drive a significant part of your solution as well.
@ Brett - The âsystemâ is a full-blown Gadgeteer system equipped with a cellular modem and much more. This little circuit is just to wake it up on a timer or trigger.
The whole setup is powered by a 12V car battery.
Did not think about using the mbuino, which I have in stock. Was maybe looking for something more âstandardâ ? If that is the right word? But a really small timer circuit with a low-power mcu would make sense.
OK, then you have the smart micro side of things sorted. My suggestion was to use a low power micro that could do it all - RTC time checks and regular activities all at once, but seems you have everything sorted except the low power bit. Low power sleep modes for netmf are always better than running flat out, but never as good as a few of these purpose built low power chips or even more bare-metal chips where you can control everything that is happening. With a car battery to hand, youâre probably never going to be short of power
The accuracy of the RTC would be the thing Iâd be worried about⌠not very worried, but worried none the less, especially if you expected the SMS to happen at a very particular time. Youâre likely to get drift, and could adjust for it once you get an idea what itâs performance is like.
You might consider an RTC with an alarm feature, triggering the power-on circuit.
Could be anything. MSP430âs have a reputation as low power, even an Arduino or ATTiny can probably do much lower power than will matter to your application (with a car battery behind it). As for RTC, perhaps something like this RTC Real Time Clock for Arduino Memory Module DS3231 At24c32 IIC Precision for sale online | eBay that has internal temperature compensated crystal and two time-of-day alarms (uses SQW pin for interrupt that could wake the micro)