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28 day timer

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I've also been searching for a 28 day timer and hit this thread a few times. Nothing simple and economic turned up. Eventually I figured out a way to implement this, so I thought I'd share with everyone.

I needed a 28 day timer to turn on an appliance every fourth weekend. I have not made this yet but here is the plan...

I will get a 7 day digital timer. Every Saturday morning at 12:00am it will turn on for 3 hours. Then every Sunday night at 12 midnight it will turn on for 3 hours. Thus over a weekend it will turn on for a total of 6 hours, and over 4 weeks it will go a full 24 hours.

Now, a 24 hour mechanical timer has both the motor AND the output connected to the input, so the timer needs to be opened and modified to connect only the motor to the output of the 7 day digital timer. The simplest way is to connect a lead from the motor which plugs directly into the 7 day timer. The rest of the internals with mechanical switching of input to output can remain as is - just plug into a socket and plug in your load.

Since the timing of my application is not so critical I will offset the actual switching of the intended load by 1.5 hours - that is, it will turn on right in the middle of the second last 3 hour run time (18-21 hours) at 19:30, and off in the middle of the last 3 hour run time (21-24 hours) at 22:30. This will also ensure reliable operation since if one timer is switched even a fraction of a second before the other one then it could throw out the whole timing.

So that translates to a turn on time of 1:30am on the fourth Saturday and a turn off time of time of 1:30am on the morning following the fourth Sunday. Some adaptation may be needed to suit others' applications though, but it should be achievable. Perhaps another 24 hour timer (without modification) to switch on a load once a day for a short duration, a one-shot timer, or whatever.
 
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Update: I made the above system and it works like magic. I decided to refine the design further. I also bought an extra 7 day digital timer and will modify it so that a separate lead supplies power to the control circuitry only as per the 24hr mechanical timer above. It's a little more involved than the mechanical timer since tracks on the PCB need to be cut and wires moved (to isolate the mains active relay from the control circuitry) but I believe it is achievable. This will be cascaded from the output of the other 2 timers, and will be more finely programmed to give the precise timing I want. With this approach it would suit the OP's purpose perfectly - you could leave the output on for a whole day or weekend, or as little as one minute, or even multiple times depending on the capabilities of your digital timer.

Just for interest, it is for a charger connected to a battery in a ride on lawn mower which sits idle for much of the year. No flat batteries again.
 

AnalogKid

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How long do you want the charger to run once it is turned on?

28 days = 2.42 million seconds. The CD4521 is a single-chip oscillator-plus-24-bit counter. Starting with a watch crystal for no-adjust accuracy, you need a few more stages to hit the divider total.

1 - 32.768 kHz crystal
1 - CD4521 oscillator/divider
1 - CD4020 divider
3 - 1N914 diodes to decode the on and reset times
1 - relay driver transistor

or something like that.

ak
 

AnalogKid

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Back to post #1, a digital timer plugged into the wall with a mechanical timer plugged into it gives you a timed AC outlet with zero modifications. Just plug in the battery charger and you're done. $20 at Wal-Mart is a good solution.

Note that by definition there will be a slight difference between the electronic and mechanical timers' opinions of how long 24 hours is. Over time the phase of the two timers will slip to the point that the on and off cycles of the mechanical timer will jump from off for 27 days to on for 27 days. At only 12-13 cycles per year, it will take a long time for things to slip that far; but still...

ak
 
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