Noob help, science fair project for my kiddo

[mfulls]

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.

 

[k7elp60]

mfulls, welcome to the forum. I try to help you. I think your project has some merit. For a timer I would go the the grocery store and look in the utensils section. You should be able to find a digital timer that will keep track of the time with a start /stop buttons. So you push the start button when you connect the light bulb to the batteries. The trick is to know when the batteries are discharged as the light bulb will dim as the batteries discharge. If you want to get all the parts at Radio Shack I can help you there as I have a little knowledge of what they have and I can lead you to the way to make sure the parts are available locally. Before I do that I need some help from you. What I need to know is a time limit for the battery test, as different bulbs will discharge the batteries at different times. So if you can give me an approximate time frame lets get started.
Ned

 

[DerStrom8]

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 life You 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

 

[mfulls]

Thanks for the reply Ned. You kind of touched on my conundrum. How to get the timer to stop when the light goes out. That is why my initial thought was to wire it in series so when the battery dies, the timer stops......but how do you keep the numbers in memory or to remain displayed? It reminds me of the clock in your car. When the ignition is turned (battery and light and timer all on), you get power and when the ignition is off (the bulb burns out), it still keeps the memory of the clock. Since I have no idea about circuits, I am wondering if there is some sort of circuit board or something that can do that. As far as the time limit for the batteries...well, that is another unknown. I know someone that did something similar using some stuff we have at work (which I can do as a last resort, but I want this to be a little more interactive for my son, since it is his project), and he said that a few D size batteries lasted around 24 hours. I would think a timer that counts up to 999 minutes and 99 seconds would be more than adequate, but again, I am not sure if that is possible with the type of components I am looking for.

 

[mfulls]

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 life You 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.

You could be right about the light bulb not drawing enough current. I just thought it was a very simple thing. I have a week or so, so if I could come up with the circuit, a test run would tell me if the bulb is drawing enough power, or if I need to step it up to something with a larger load.

 

[DerStrom8]

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

 

[ronv]

What you might be able to do is use 9 volt batteries to "pick" a relay as well as your load. If you use a 5 volt relay it would probably drop out in the "dead" part of the curve and could stop the timer. See the link.
Discharge tests and capacity measurement of 9 volt transistor radio batteries

 

[mfulls]

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

Ok, some of that makes a little sense. I did take one circuits class in college...many years ago, but it certainly didn't touch on integrated circuits or how to build something. Perfectly understandable to get "too technical" when trying to describe something you are knowledgeable about. We all do it.

Let me back up a second and ask this. We are now talking about building something to do this function. I was just envisioning something I could buy at Radio Shack, or somewhere else that would already do this. Is there not some product already out there that would accomplish the timing function? Then all I would have to do is figure out a way to wire it up so it shuts off when the battery dies. That would be more my style and my knowledge base.....not only that, as I am sitting here and we are getting into the nuts and bolts of it, I realize I am going to have to build this thing. I probably could...but I am not sure I want to! I would like to start off with a simple idea and go into some complex series of IC's and things only if necessary. Any help on going that route?

 

[DerStrom8]

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

 

[mfulls]

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

now that sounds more like it. I will have to do some research into them so I know what is going on and I might even try and come up with a schematic and scan it in. I work better with drawings. Then you can tell me if I am close or not.

 

[DerStrom8]

That sounds like a good plan to me
Good luck!
Der Strom

 

[SABorn]

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.

 

[DerStrom8]

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.

I believe the OP was hoping for a completely automated system that would do everything by itself. You do have a good point, though, Pete. The battery would not necessarily need to be entirely drained. You could set the reference voltage on the comparator in my previous idea to HALF the battery voltage (~.75 volts) and measure the half-life instead. This wouldn't take as long and would be easier to monitor. What do you think?
Der Strom

 

[SABorn]

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.

 

[DerStrom8]

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.

I understand your thinking, and your thoughts may very well have merit. However, the OP has mentioned several times that he would prefer the automatic timer which we have been suggesting. I, personally, get the impression that he may easily be able to build an automated system as long as we assist him in the process. That is what I have been trying to do. If the OP decides he does not have the necessary skills or knowledge to build this by the time his kid's project is due, I have no doubt he will definitely consider your ideas.
Best regards,
Der Strom

 

[k7elp60]

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.

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.
If you go to RadioShack - mobile phones, MP3 players, laptops, and more and type in these part numbers you get an idea what I suggest. A lamp p/n 272-1124, and socket for the lamp 272-357, and the battery holder 270-408 and the batteries 23-849. If you wire the battery holder to the lamp socket, screw the lamp in part way, put batteries in the holder, screw the lamp in utill it lights and then press the start button the timer, watch the lamp until it goes out the time will be about 1.5 hours. That is will AA cells if you use AAA cells the time will be less.
Ned

 

[RMMM]

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.

 

[SABorn]

Ok, if this was myself doing this for my kid, i would take a different approach.

I would use a chip that is somewhat frowned upon on this forum, but is a wonderful little chip........The picaxe chip,....... very easy to program and reasonably vertisile.

I would do the light globe bleed down test as surgested and simply log the voltage drop of the battery to a laptop computer.
It could be shown in a graph form on screen or in a digital number display.
The timing is provider by the computer.

All very simple with only one little 8 pin chip needed (picaxe 08m) and a few resistors.
The chip itself can be powered by 3xAA batteries.
There is no risk of damage to the computer and these chips are designed to interface with a computer, or as a stand alone circuit.

There is no special porgramming board needed to program the picaxe and it will program direct from a serial port on a computer. (or via a usb to serial cable)

I have the software for the computer to display the data, the circuit needed to interface with the computer, and the program needed to be loaded into the picaxe.

Should you be interested in this method i would be happy to help.

Pete.

Ps:- It could be even possiable to test and record several batteries at the same time to show a comparison graph of different batteries, like a carbon battery, a alkaline battery, even a lithum battery, it would all come down to the choice of picaxe to handle the inputs (08m can handle 3 batteries) and the computer software adapted to display the number of batteries.

Premade kit circuit boards are avaliable to suit this type of project but would need to be assembled.

 

[DerStrom8]

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.

I like this idea. I have a simple dark-detecting circuit that might work. I've posted it below:



The output of the 393 could be connected to a transistor across the "stop" terminals on the watch. The output would normally be low, but when it gets dark enough (adjusted by the variable resistor) it turns on the output, switching the transistor, and stopping the watch.
Der Strom

 

[MrDEB]

thinking along the same line

buy the stuff from rat shack but detoure to a hardware store and obtain a timer/kitchen thermoeter. if they have just a timer then great.
as an addition, hopefully have a digital volt ohm meter that will read current.
do a comparision of different battery brands.
say a AAA has X number of amp hours. using your test setup measure the amperage draw and determine how long the light will stay on. If you have more light bulbs in series the amount of time the bulbs stay on will be shorter making the test shorter.
this demo would expand the information within the display.
using a kitchen thermoeter you could replace the probe input with your bulbs (in theroy the temp probe is basically a temperature dependent resistor. never did before but would need futher investigation. might work?? just a thought. when the temperature readout reaches a certain point yourbattery is useless.
an electronic stop watch (wrist watch) could be altered so the button that starts the stop watch function has a relay(simple transistor switch that acts as the function button).
lots of hacks can be built but remember, the more info your display has the better chance of winning a prize. I personally like the battery comparrision display with charts showing graphs of battery performance coparision.
Make that bunny show its stuff