Power supply watchdog for microcontroller systems
Microcontroller running 24hours/7days sometimes (very seldom, but possible) can come to a latch up condition. In some cases a reset is not sufficient to overcome this problem, only a power cycle can solve it.
The circuit attached switches the supply power of a microcontroller system. If triggerpules are missed for more than 10 seconds, the power is switched off for app. 5 seconds and automatically switched on again.
If the duty cycle of the triggerpulse is app. 20%...50%, a wide range from a few microseconds up to 5 seconds is possible.
For the watchdog the good old timer NE555 is used. The NE555 is in astable operation, but the capacitor is discharged before the voltage reaches the threshold-voltage level if triggerpuses are present and therfore the output pin is in high state, so the supplypower is switched on.
If triggerpules are missing for more than 10 seconds the output pin of the NE555 switches by meas of Q3 and Q1the supply power for app. 5 seconds off and the microcontroller and all sensors (...) connected to it will be reset.
The triggerpuses discharge the capacitor by transistor Q2.
This is a save way to bring a microcontroller system back to normal operation.