Arduino watchdog with 555 Timer and a MOSFET

Question

I want to protect an Arduino from stop running using a 555 timer. I saw some projects that use the Arduino RESET pin to do that but there are cases that my Arduino project still stop working even when I do a reset using the RESET pin.

So I used the circuit below to cut the 5v line of my Arduino. I choose the 5v line instead of the ground because I have only one 5v pin and more than one ground at Arduino.

My problem is how can the Arduino reset the 555 counter? How can I pulse the 555 to prevent it from disable the Arduino? If I connect the D2 Arduino pin to the Threshold/Trigger/Capacitor pin like I saw on some projects, and keep the capacitor charged, it works when the Arduino is running but if it disables it never turn on again because the Arduino don't allow the capacitor to charge again and it stays forever off.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

These microcontrollers used in Arduinos typically contain a so-called Watchdog, which fulfills exactly the role of your 555, but is

• already integrated, thus needs zero additional components (and thus, sources of failure),
• uses an amount of power that is, even with lab equipment, pretty much impossible to measure, whereas the classic 555 is a true power hog,
• can be way more accurate,
• will reliably reset your system exactly once until it is started again (which you typically do early on in your firmware),
• is designed and tested by experts for production in the millions to billions and I'd thus expect it to be way more reliable than your own analog circuit and
• leaves your Arduino in a defined state instead of violently disconnecting it from power, which leads to a brown-out situation with currents flowing in directions noone foresees.

So, abandon the 555 approach and simply learn how to use the watchdog. It's superior in any aspect I can think of.

Answer 2

A watchdog, as described by Marcus is the better solution than a 555 timer. The way that I use a 555 timer is by using a pulse (based on your program) out of the Arduino in your case into the reset pin of the timer. As long as your program is running the 555 gets reset before it "goes off". If you program stops running, then it doesn't get a reset and trigger the processor reset when it times out.

Answer 3

Some mentioned using the watchdog. I must say that while the watchdog is present it too has failed me multiple times. A hardware reset watchdog is the only sure way. I too am looking for a solution like this but my solution targets pulling down the reset pin. Now if a hard reset such as that doesn't reset the arduino then you may have another more serious problem.

P.S, the FET you are using is that logic level?

Answer 4

There are easier ways of doing this. There's the internal watchdog timer (WDT) in the Arduino, but there are also dedicated watchdog ICs. These are easier than using a 555 because they are very low power and have configurable time periods. One example is the TI TPL5010 but there are many. When working on a very low power applications sometimes the external watchdog IC uses less current than the internal WDT.