LM555 Watchdog for Arduino!

This is a simple schematic that allows the use of the chip LM555 as a Watchdog for the Arduino board.

• 1 x LM555
• 1 x MOSFET IRF540
• 3 x 100k Resistors
• 2 x 10k Resistors
• 1 x 100uF Capacitor
• 2 x 1uF Capacitor
• 1 x 0.01uF Capacitor

• R1 and C1 define the timeout accordingly to the LM555 datasheet;
• R2 avoids undershoot that otherwise would disrupt the LM555;
• C3 guarantees that only transitions (beats) are considered as Hearthbeats;
• R3 and R4 centers the heartbeat voltage at 2.5 volts, low enough not to trigger the MOSFET and high enough to enable the LM555 trigger;
• The MOSFET IRF540 discharges the capacitor C1 whenever receives a positive beat, avoiding this way the LM555 triggering that would reset the Arduino;
• C4 and R5 guarantee that the reset pin doesn't remain low when LM555 is triggered and its pin 3 goes LOW.

• In the Arduino loop function, insert an if statement that will switch the Pin D4 HIGH and LOW alternately;
• If for some reason the Pin D4 remains HIGH or LOW for more than the timeout, the Arduino RESET will be activated;
• After the previous RESET activation, this Watchdog will remain inactive until a new low beat is received.

Here is the Schematic in PDF.