On Fri, 23 Oct 2009 14:54:47 -0500, John Fields
wrote:

On Fri, 23 Oct 2009 12:02:37 -0400, "rt003119"
wrote:

Group.

I need a 24 hour countdown timer such as the CANUK118 - Digital Clock With
Timer (http://www.electronickits.com/kit/co.../canck118.htm). The
additional feature I need is the ability to shut off the timer circuit at
(say 13:00 hours), in advance of the final countdown time of (say 14:00
hours), and have the timer circuit reset automatically to the desired final
countdown time of 14:00 hours, 24 hours later. Said another way, if the
countdown time is set to activate the relay circuit at 14:00 hours and the
timer is disengaged at 13:00 hours, I don't want to have to wait until
14:01 hours to reactivate the timer circuit to avoid the relay from being
activated at 14:00 hours or 60 minuets later.

---
you'll need a few things: a SPST NO relay, (a reed relay would be fine)
a CD4001B, a 24 hour timer which you can build using three CD40103B
down-counters, an NPN like a 2N2222 and some resistors.

Here's how it would work:

First, you momentarily place SW2 into the TIMER OFF position and then
return it to the TIMER ON position. What happens when you put SW2 in
the TIMER OFF position is that it sets a latch which starts the 24 hour
timer and turns on the transistor.

The transistor, in turn, energizes the relay which connects the LM8560
TIMER OFF pin to the positive supply, disabling the timer until the 24
hour timer times out.

When that happens the latch will be reset, the 24 hour timer will stop,
and the relay will be de-energized, disconnecting the TIMER OFF pin from
the positive rail, thus enabling the timer.

Want a schematic?

---
Thinking about it a little bit more leads me to believe that the reed
relay can be replaced with a transistor, making the circuit smaller,
faster, better, cheaper.

Win, win, so I'm hooked...

I'll post a schematic tomorrow just for the fun of working through it.