Circuit assistance: Low power timer and trigger

I am working on a small project that needs to be really low power (again).

This time the system only needs to wake up:

  1. twice a day to send an sms with some status in it -
  2. and if a little micro-switch is pressed send another sms.

Very simple indeed :wink:

I am considering a 555 timer circuit with a relay on it, to switch on the system.

Any ideas?

555 likely won’t be sufficient to wake up twice daily, and you’re going to need a micro for SMS. I’d 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 “system” 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 “standard” ? If that is the right word? But a really small timer circuit with a low-power mcu would make sense.


In the spirit of NETMF the question is how low the G30 TH/Medusa II could go for lowest power?

The G30 is about 1.5mA in hibernate. You can use an alarm with the real time clock.

@ hagster - Brilliant, that should do. Do you know if the documentation on this is up to date?


@ njbuch - yes I think it works now. I had some issues a year or so ago with the cerbII40, but I think they were fixed.

@ hagster - Thank you so much!

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, you’re probably never going to be short of power :slight_smile:

The accuracy of the RTC would be the thing I’d be worried about… not very worried, but worried none the less, especially if you expected the SMS to happen at a very particular time. You’re likely to get drift, and could adjust for it once you get an idea what it’s performance is like.

You might consider an RTC with an alarm feature, triggering the power-on circuit.

@ Brett - hmm, might not be relevant but what kind of chips/circuits are you recommending?

Could be anything. MSP430’s 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)

This might not be such an issue as he does have a modem so could request the time from the network or a time server if he suspects there is any drift.