hi
i build a bcd clock shown in the page
http://ourworld.compuserve.com/homepages/Bill_Bowden/clock.htm
and all the circuit works fine and i am using 5v dc source voltage.
and my question is how can i add a memory circuit to the clock
to remeber the time the clock stops after the power is on other time?
and how can i calculate the duration of the 5v battery in the circuit?
thank you in Advance
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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