[velogreg]

Gontran is on to something.
Try using the blinking LED's from RS hooked up to two AA's. I did this a while back for my son for one of his projects and the battery drained in less than 24hrs. So multiple LED's should get you in the timeframe you desire. As for time to drain, RS had somewhere a light sensing meter or probe, similar to the on at dust lanscape light sensor. Just place the LEDs and sensor in a closed shoe box, run the wires through the wall of the box to the outside. When the LEDs run out of juice, the LEDs go dark the sensor goes on, note the time.
Voila! You get to keep your job.
Good luck.
Greg


Quote:

Originally Posted by Gontran /img/forum/go_quote.gif You could also put a power resistor in paralel with the fan or motor. You can find blinking led's at mouser, digikey, maybe even rat-shack, or one of those pretty multi-color flashing LED novelty things people hang from earrings and other body piercings...(I'm sure there's a name for those, I don't get out much...) or those spinning ever changing LED thingies they sell at the amusement parks (around central FL anyway...)

 

[billybob_jcv]

You guys rock! And thanks to OtterMarc for PM'ing me that this thread was still alive - you are right, I hadn't checked since 1/13 - I thought the thread had died!

 

[Gontran]

OtterMarc probably has the best idea. A relay configured as described would consistently open at the same voltage....and open completely, stopping the clock dead. It's the simplest solution, it's likely to draw a couple hundred miliamps so it won't take forever to work, and with a few blinkey lights, it will be attention getting.

Not too technical and an easy project for a 10 year old.

 

[billybob_jcv]

Doh! I am such a Homer...

I went to Fry's today looking for a relay. Unfortunately, I got confused on the coil voltage ratings, and bought a 24vdc relay. Of course, the darn thing won't trigger with only two AA batteries.

Tomorrow I'll try to find a lower coil voltage relay. Does anyone know if a 6vdc relay coil will activate on 3 volts? Or should I also get a 4-AA battery pack? I really don't want to use 9v cells in the experiment, my daughter wants to use AAs 'cause that's what she's most familar with.

 

[Gontran]

Relays have a turn on voltage (required to actually pull the contacts closed) and a holding voltage, which is (gross estimate from memory) around 2/3 the turn on volts.

So with a 6V relay, you'll need 4 AA's to start, and it will probably stay on until the batteries drain down to four volts or so.

 

[billybob_jcv]

So, in this datasheet:
http://www.nteinc.com/relay_web/pdf/R16.pdf

There's a line that says: "Pick−up Voltage: 80% of nominal or less"

Is the "pick-up voltage" the same as the "turn-on voltage"? If so, then for the nominal 6vdc relay that would be 4.8v - right? I don't see a spec for the "holding voltage" - or do I have this completely backwards and 4.8v is the "holding voltage"?

And, for those who think I'm doing her project for her, please remember that her project is not to build a battery tester - her project is to make a hypothesis about which type of batteries last the longest, collect data, then make a conclusion about the validity of her hypothsis. Maybe I'm justifying having some fun, but I don't consider supplying her with the test apparatus as "doing her project", I also wouldn't expect her to build her own LCD clock in order to test the output of a potato battery.

 

[Gontran]

Take a look at this Maxim page. It describes a power saving circuit that applies a nominal voltage to trigger the relay and then lowers that voltage to keep it on economically.

Pickup voltage is the minimum required voltage to turn it on. (So I think your data sheet should read "80% of nominal or more")

Drop-out voltage (what I called holding voltage) is the voltage below which an energized relay will turn off. Which is the same as saying the minimum voltage required to keep it on.

In the case of this sample 5V relay, it will turn on with voltages as low as 3.5V, and once energized, will remain so until voltage drops below 1.5V

 

[billybob_jcv]

I think we have it! The only low-voltage relay I could find locally was this 5 vdc SPDT at Rat Shack:
http://www.radioshack.com/product/in...entPage=search
I have no idea what the specs are for the relay, the package doesn't tell me what the drop-out voltage is.

Once I finally figured out how the relay needed to be wired (that only took us about 2 hours!), we hooked the output to the clock and a cheesy LED message fan I got from a trade show - it looks similar to this one:
LED Message Fan
Everything is powered with 2 AA. We had trouble getting the clock to run reliably, I guess because it wanted 1.5v not 3v. I put a resistor on the clock, and it seems OK now. Unfortunately, the fan really wants 4.5v, so it's a bit anemic on 3v minus the drop through the relay. We're running our first full test with a pair of fresh AA to see how long it goes - my only concern is whether the relay will really open before the fan shuts down. If that happens, I guess I'll either get a 3-cell battery pack, or try to find a 2-cell fan.

Wish us luck!

 

[billybob_jcv]

My daughter has been running her tests (all by herself, I might add) for the last couple of days, and so far, so good! Typically, the fan stops first, but the light and the clock continue and within a fairly short time the relay kicks off. That works good enough! The batteries are also performing pretty much as I expected - the "Heavy Duty" batteries are pitiful (~1.5 hrs), the various brands of "normal" alkalines all last about the same (~2.5 hrs) - with the exception that the Costco Kirkland alkalines seem to kick butt, the "max" alkalines last longer still (~3+ hrs), and the 2300 mAH nimh last the longest by a big margin (over 5 hrs). I haven't got any of the newer 2700/2800/2900 mAH nimh, or the rechargeable lithiums. She still has more to test - including some non-rechargeable lithiums. She's doing a great job - she is recording all the test data in a spreadsheet, and she's calculating the cost per battery and per minute of charge. She's also planning to say something about the rechargeables being good for the planet, yada-yada.

You guys rock!

 

[SysteX]

Just as a heads-up for when you start dealing with lithiums. Make sure that you don't mix used cells with new cells; it's even more important for lithium batteries than it is for other types because the fumes released are VERY toxic. I had a titanium flashlight blow up in my hands about a year ago when I accidentally put a nearly discharged lithium and a full lithium in together--it's something I hope to never experience again.

Sounds like a neat little experiment, though.

EDIT--
This is not me, but my experience was similar. See here.