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Registered Member #1822
Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300
Im not sure what the limitations are for the 555/556 but, I want to use a 556 timer to flip a relay after 7 hours and 50 minutes and then have the other half of the timer turn it back off. Is this within the capabilities of the 556?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
No, a time constant that long is essentially impractical. A big R is going to be challanged by board leakage, a big C will be too costly or too leaky, depending on technology.
The 4040/4060 type CMOS ICs have been built for this sort of application, with 12/14 bit counters in them to reduce the RC time constant needed when making long delays.
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