I am designing a circuit with a 555 monstable one shot timer to auto off the circuit power if the time expires. How can a 555 timer be reset to zero with an external positive pulse and automatically restart the count? The reset function on the timer only stops the count and the output goes low. I need the output to stay high and the count refreshed to zero and continue counting to a predetermined value, then if it achieves this value, the timer output goes low.
Any suggestions?
Thanks,
Jeff

 

The circuit below will reset and hold the voltage on the timing cap to VCC/3 so long as reset is high. VCC/3 is the voltage on the cap at the beginning of the timing cycle, so when you release reset, the length of the first cycle will be very close to all those that follow. The output (pin 3) will be high while reset is high.
A 555 doesn't actually have a count, its just an RC multivibrator.

Attached Images

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