hi i build a bcd clock shown in the page
thank you in Advance
I _think_ you mean "how can I add a memory circuit to remember the time when the power goes out"?
And do you want the clock to _stop_ when the power is down, or do you want it to keep track of time just without the display running?
If you want to do the latter, then split the circuit into a battery (or supercap) powered section that keeps track of time, and a display section that lights lights. Charge the battery for the first section from the second, protect it with diodes, make sure that when the power goes off the first section's outputs go to 0V so they don't have excessive power drain, and see what you get.
To figure out your battery drain, measure the current. To figure out your battery _life_, find the capacity of the battery and divide it by the current.
-- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
The obvious solution is to add steering diodes to the 5 volt line. When 5 volts is present your clock is powered by the 5V when it is not present another diode isolates the 4.5 volt battery supply (any voltage lower than 5 (plus one forward diode drop voltage) to keep the battery from supplying voltage when not needed).
The downside is all those LEDs being driven directly from the ripple counters. Presumably you won't know to need the time if power is off? (the clock needs its 60 HZ AC for the time base so won't count when power is gone). The light emitting diodes will eat more power than the rest of the circuit by an order of magnitude so will use battery power - how much depends on how many leds are on when power is off.
It would require removing the ground return for all the leds when power is off - tying all the led cathodes to a common bus, then using something like a mosfet to only connect the ground (and light the displays) when power is there.
There is/was a nice little circuit floating around using a 555 timer that would supply an ersatz 60 cycles when the time base from the power line was removed. Reset line inhibited the 555 astable when power was present - so the clock could count with good enough accuracy. A few minutes a day of power outage wouldn't be noticed in the time setting.
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