Welcome to Electro-Tech!
I'm assuming you meant a timer that would run and turn off automatically when the battery drains, correct? You could probably use a 555 timer, a BCD chip, and a 7-segment display driver chip (7447?) for the timer display. To run the timer and keep the numbers, you could use a secondary battery that is pretty much isolated from the battery under test. You could use a voltage comparator to determine when the tested battery is finally drained, and use it to switch off power to the 555 timer. The separate power supply would leave the display switched on. I think there is one problem with the idea, though. I would assume a light bulb would drain the battery very slowly, and it might take hours (if not days) to actually drain it. You might have to stay up all night just to see the "end" of the battery lifeYou may want something that draws a little more current, like a motor, for example.
I hope this helps, and good luck with the science fair project!
Der Strom
That sounds like you seem to have a handle on what I am trying to accomplish. Your initial assumption is correct....only problem is, you could have typed all of that in Ancient Latin and I would have understood just about the same amount. I will have to research some of the components you listed to see what the heck they even are.
Yes, I apologize. I thought of that when I was re-reading your post after I wrote that. You mentioned that you "naturally know nothing about electronics," so I apologize for the confusion.
A 555 timer sends out signal pulses at specified intervals (the intervals are determined by the values of resistors and capacitors connected to it). BCD stands for binary-coded decimal. It is basically a binary counter. The output from the 555 is connected to the BCD chip and the signal pulses tell it when to change its output. A 7447 chip is a BCD-to-7-segment display driver. In case you do not know, many digital clocks use 7-segment LEDs. These 7 segments make up a figure 8 and different segments can be lit up to make different numbers. Anyway, this is your basic clock setup. A voltage comparator, depending on how it is connected, COMPARES two different voltages--one as a reference and one as a variable. An example of how it operates is this: When the variable voltage drops below the reference voltage, it turns OFF the output of the chip. You could have, say, .3 volts going into the comparator as the reference and the battery under test as the variable voltage. When the battery drops below .3 volts, it is practically useless (drained). This would switch OFF the output of the comparator. If the output is connecting power to the 555, when the output turns off, so will the timer. This will turn off the clock for your circuit.
As for keeping the final display lit up, I believe some BCD chips have what is called a "latch," meaning it "holds on to" or "latches" the last displayed number. So, for example, if your final clock pulse (before the timer turns off) leaves a 5 on the display, the 5 will remain there even when the timer stops.
This is just a theoretical idea for the timing circuit, anyway. I may be able to design a basic schematic diagram for you if you think it will help.
Best regards,
Der Strom
Are you still willing to use a couple of small, simple chips? you could easily modify one of those step-counters to be triggered by a 555 (I have done this before) and still use the comparator to measure the voltage of the battery. The output of the comparator would switch power to the 555. This way, every time the 555 pulses, the counter will go up one. If you adjust the resistor and capacitor values for the 555, you could get it to send a pulse every second. This idea would eliminate two of the larger chips (The BCD counter and the 7447 decoder). I also believe that both the 555 and the 393 are sold at Radio Shack, which would make this much more convenient
A 555 timer and a 393 comparator are both 8-pin DIPs (dual-inline-package), so they are each just about a centimeter long. This would be significantly simpler than my original idea
Der Strom
Why not keep it simple and use a multimeter and a light globe, set your timer and discharge the battery for a given time period, then read the battery voltage with the multimeter.
Next battery repeat the test.
Pete.
My thoughts are the OP has very limited electronic skills and is limited to globes and wire, with off the shelf items.
I dont get the impresion he would be confident to tackle a circuit construction of component level, as simple as it might be for some of us, its a big deep end step to others.
Thats why i said globe, meter, and timer, a simple project acheived is far better than a automated project failure.
I could well be guessing wrong here, but do see the automated system not a simple as the OP would like it to be.
Pete.
What I had in mind was a digital timer from the grocery store. I have one that is a clock or a timer and keeps track of the time elasped when you have in timer mode and press the start button the time elasped is shown on the dial.I am a mechanical engineer by trade....so naturally I know nothing about electronics and circuits, and I need some guidance. My son has a science fair coming up, and I helped him come up with a project to enter. The idea is to test AA batteries for battery life. To do this, I was wanting to make a simple circuit that consisted of a battery (pretty sure Radio Shack has one of those battery packs with wires coming from the terminal ends), a light bulb to create a load to drain the battery, and a digital stop watch/timer that would count minutes and seconds. All of those wired in series so when the battery drains, the light and stopwatch/timer stops but retains the numbers. Actually, now that I think about it, my initial idea might not work because when the battery drains, the timer will shut off and lose the numbers.
So....I was going to ask where to find a timer/stopwatch that would work...but now I have to ask, not only that, but maybe a schematic or word description of a simple way to do this.
Can anyone point me in the right direction?
Thanks.
An easier thing to do, would be to rip the button off of a stop-watch/kitchen timer that has a "stop" button.
Using a Dark-on circuit, you could start the light and timer at the same time.
When the CdS light cell no longer detects enough light, it will "press" the stop button for you.
That will make a "neat" thing to show at the fair.
When showing the equipment, your kid can start the timer, then sheild the sensor from the light showing how it works.
That will impress everyone and should instill the possibility of "hacking" off the shelf components to the young kids.
You only need a simple dark-detecting circuit to "push" the stop button.
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