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I would like to use a 555 circuit as an external watchdog for my Arduino project. The project is a cellular tracker that goes to sleep for an hour, wakes up and does some measurements, uploads them and then goes to sleep again. The 555 should take care that the active period does not take too long because the systems hangs, and in that case trigger a reset. My first thought was to power the 555 circuit with a PIN of the Arduino so that it only goes on when the system is awake and is turned off while sleeping. But this does not work as the reset line has to stay high even while sleeping and if i turn the 555 off reset would also go low. Could a Mosfet help? Any other ideas?

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    Why can't you use the internal watchdog for this? – Ignacio Vazquez-Abrams Jan 25 '15 at 14:57
  • Because it is not reliable and way too fast. I am running on reduced clock speed which means maximum watchdog cycle would be 3-4seconds. GSM communication takes time... – Stephan Noller Jan 25 '15 at 17:02
  • GSM communication shouldn't block the arduino code. You should at least be able to work around any blocking code that is in the library. Something like while(!Serial.available() && millis()-starttime<10000){wdt_reset()}. – Gerben Jan 25 '15 at 20:15
  • Will give it a try - but nevertheless - would feel saver with an external fallback, still. Any ideas on this? – Stephan Noller Jan 25 '15 at 20:52
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ok guys - found the answer: this post here at electronics-stackexchange gave the right hint: Watch-Dog Timer. And my gut feeling was right as well - i simply use a NPN-Mosfet that is being triggered by an Arduino-PIN (which easily can be done while sleeping as well as sleeping means waking up every 8seconds anyway) that connects the capacitor to ground and therefore discharges the guy. Awesome. This solution can be used as a real fallback in case of whatever happens, while operating the system with built in watchdog-functions while everything is ok.

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