Re: 555 countdown timer design question
Technically you don't even need a 555 to do a hold-for-60 sec delayed turnoff, just need a high gain amplifier as a comparator (op amp, or darlington even) driving a switching element, works just like the vacuum switch.
Push button -> charges capacitor. Charged capacitor is over threshold of darlington and thus turns on. Release button -> capacitor slowly discharges through a resistor. When it drops below the darlington's threshold of around 1.4 volts, it shuts off, turning the output off. Simple enough?
Oh just wanted to point out some shortcomings of this: no hysteresis. That "off" trigger point could be very messy if the impedance of the input is high, so you need to carefully pick values... might not be good for minute durations.
For long duration delays I personally would have to go with a real counting system much like what... a microcontroller... would do. Mostly to make sure timing errors are minimized due to stray leakage, tolerances, etc. 1 minute delays are just starting to get to the ugly point. 1 hour is definitely not something I would ever do via analog - must be done digitally.
Technically you don't even need a 555 to do a hold-for-60 sec delayed turnoff, just need a high gain amplifier as a comparator (op amp, or darlington even) driving a switching element, works just like the vacuum switch.
Push button -> charges capacitor. Charged capacitor is over threshold of darlington and thus turns on. Release button -> capacitor slowly discharges through a resistor. When it drops below the darlington's threshold of around 1.4 volts, it shuts off, turning the output off. Simple enough?
Oh just wanted to point out some shortcomings of this: no hysteresis. That "off" trigger point could be very messy if the impedance of the input is high, so you need to carefully pick values... might not be good for minute durations.
For long duration delays I personally would have to go with a real counting system much like what... a microcontroller... would do. Mostly to make sure timing errors are minimized due to stray leakage, tolerances, etc. 1 minute delays are just starting to get to the ugly point. 1 hour is definitely not something I would ever do via analog - must be done digitally.
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