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Hi, the problem with Bipolar Transistors is their typical Threshold Voltage of 0.6 volts, while the IRF540 has a typical Threshold Voltage of 3 volts... The LM555 had issues when its pin 2 was at 0 or 5 volts, in the first case it didn't triggered as expected while in the second case the LM555 depends on a new negative beat to restart the C1 charging each time it's discharged, in other words, it breaks the cycle!
BTW, that's why I used the resistors R3 and R4, to center the heartbeat voltage at 2.5 volts, avoiding this way the problematic chip behaviour at 0 and 5 volts!
LM555 Watchdog for Arduino!View Instructable »