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Old 20th July 2005, 08:31 AM   #1
nickch is offline nickch  New Zealand
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Default 555 timer latching

hi, i'm looking at using a 555 timer circuit activated by a LDR to get a delayed step function. ie, when gets dark, voltage divider + LDR causes the 555 give a step function 5 seconds after the LDR senses that its dark.
i'm quite new to electronics and haven't played with 555s b4 but would like to use it in one of my current circuits. could anyone pls help supply me a simple schematic or something?
thanks
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Old 20th July 2005, 08:48 AM   #2
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A 555 timer won't do that. It will simply change state upon detecting dark then latch for 5 secs.

What you need is a comparator followed by a delayed turn-on circuit. A resistor-capacitor network will provide the delay. I'm not going to hold your hand all the way and give you circuits because any half decent textbook on op-amps and basic electronics out there will have plenty.
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