Low voltage timer

[Madscientist267]

Hey all,

Looking for a schematic (or at least a point in the right direction) for a timer circuit that can run at the ~3.7V range of lithium ion cells...

I converted a small (~1W) garden light variety panel assembly to use LiPo instead of 3 NiMH and the panel sits just inside a window. I'd rather not drill a hole for this - its a trivial project and would be a real PITA besides...

It runs 4 blue LEDs in a fiber optic 'spray' lamp, as a night light. But I want it to cut itself off after about 4 hours or so to preserve not only charge, but DOD also affects the lifetime of the battery in general. The protection circuit in the battery eventually will cut it off, but I've noticed at the low rates that LED lights use the juice, sometimes it's way below where I feel comfortable charging them again.

555 won't do it for 2 reasons; one, the chip wont do much of anything at 3.7V, and two, the RC cap would have to be magnaniginormous to get in the time frame I'm looking at.

Any ideas?

Steve

[Norm]

I'm thinking of making a water clock, water or

anti-freeze drips in a container with a float

with a switch to shut off the light after a few

hrs.

[DamonHD]

FYI: http://www.absoluteastronomy.com/topics/Water_clock

Rgds

Damon

[commanda]

Look at the CD4020/40/60 range of divider chips.  One of them has an input stage which can use an rc network to make it oscillate. Check the datasheets carefully, a lot of cmos will run down at 3 volts. It's also available in the 74xx4xxx range, like 74HC4040.

In my files, is 09590190.pdf, which is the timer for my dump load controller. This uses a single gate from a 4093 as an oscillator driving a 4040 counter to get long time delays. 2 other gates from the 4093 form an R/S flip/flop, for manual start and stop.

Amanda

[rossw]

A few months ago, I had a friend aproach me, he needed a simple to use timer for his caravan. Wanted a selection of times - 30, 60, 90 mins. It had to turn off a radio or something.

I used a little picaxe. One input for an illuminated pushbutton. One output for the LED in the pushbutton, one output for the control (I used a small relay).

The device I used was a little 8-pin PICAXE - about $4, with hardly any other bits required. The reason I mention it, is that these chips will aparantly work happily down to about 3V, so should work in your application.

My code was fairly simple, but effective. One press was the shortest time and gave one brief flash of the LED every couple of seconds and was a (preset) 30 mins time. Another press within the first second or so and it gave a 1 hour timer (two quick flashes). Three presses was 90 minutes. A 4th press turned it off. Pressing the button more than a second or so after a previous press reset the current time to its full period (eg, if it was down to 5 mins left, one press would reset to 30 mins).

If anyone is particularly interested, I'll dig the details out.

[dnix71]

It's much simpler to buy a mechanical wind up timer. I live on a canal, so the water is 'free' but the gov't limits watering times.

It isn't always practical to get up to turn the pump off before leaving for work, so I bought a cheap wind up at Home Depot.

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10051&langId=-1&ca
talogId=10053&productId=100166878&N=10000003+90401

$15 and up to 12 hours. It would look neater in a gang box with a pigtail.

[Madscientist267]

Excellent...

I've been looking for a reason to learn PICAXE - Was just talking to a guy about it the other day matter of fact. The 8 pin version seems simple enough... Looking through the manual it appears to be just about perfect for the job.

Any heads up for a beginner in these? Things to keep in mind from the git-go kinda stuff?

Steve