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A "watchdog" circuit using a TS555. Pressing the button brings the Trigger LOW which will start the timer and also acts as the "pat" for the watchdog resetting the timer each subsequent time it is driven LOW.
When the timer is active, the relay will be engaged for roughly 10s unless the button is pressed before the 10s elapses, in which case another 10s extension is granted.
The push button is also hooked to a 1uF capacitor to make it edge triggered. Without this, if the button was held LOW, the circuit would never time out as the 555's capacitor would always be drained. The timing of this capacitor, the 10K, and 1M resistors gives the trigger enough time to fully reset the 555 if pulsed roughly every five seconds.
Note, the 10uF cap connected to the RES pin prevents instant trigger when the 555 is first powered on.
This circuit was designed to act as a watchdog on sprinkler equipment, in essence cutting power to zones if the watchdog is not pet via code within a 10s window. This acts as a backup fail-safe and prevents a software crash from causing watering to continue indefinitely.
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