Need help solving problem on daughter's schience project.

[Fatherof2]

HELP for a 7th gd science project.  HYPOTHESIS: do disposable AA cells last longer than rechargeable cells in a portable CD player?  SET-UP: 2 AA cellS driving CD player.  In series with cd PLAYER is an ampmeter; a DVM measures cell voltage.  Added a digital clock to stop when the batteries discharge enough to stop the CD Player.  THE PROBLEM: the CD player stops playing within an hour but the clock still ticks.  The clock uses only 1 AA cell whereas the CD uses 2.  The clock uses only .16mA and the CD starts at 300mA then drops to 120mA after 30 sec.  Tried, among other configurations, to add 0.5 ohm resistor in series with CD player to drive the clock but this produces an IR drop of only 0.6V or so, not enough to drive the clock.  QUESTION: is there a timer with a sensing input that I can buy, or build, that I can connect to this lab set-up in order to sense when the CD player stops running?  It could be connected to sense the current because the current drops to zero when the CD player shuts off.  Would an inductor in series with CD player be the appropriate component to produce a pulse?  PLEASE HELP, DEADLINE IS LATE JANUARY.

[EdwardM]

Hi Wesley

seems to me that one solution is to buy or build a simple data logger, I've used PICOscope in the past which is essentially an analogue to digital converter(ADC) and will in conjuction with some PC software produce a graph of some quantity(current, voltage, pressure etc) against time.

You could also design your own based on a PIC or other microcontroller which could use its own display of time or be connected to a PC.

For myself, provided there are no constraints in using a PC for the project, I'd use a low cost PIC with built-in ADC and make an RS232 link to a PC - the PC can then provide, using Visual Basic or other programming language, real time displays of current and voltage as well as showing a graph of load current against time.

Hope this helps (Bearing in mind that in the UK, we know not of these 7th grades or otherwise or the age of the student  ;D)

Ed

[Fatherof2]

Thankyou so much for the quick reply.  To answer your question 7th gd. is middle school; age for 7th graders is 12-13y.o., 8th gd is 14-15, etc.  Your solution is a bit much for this 12 year olds science project.  I was looking for something much simpler.  Plus she needs to monitor the test and note the findings in her notebook.  Thank you anyway.

[EdwardM]

Hi

an even simpler way of doing it...

It requires the student to start a stop-watch at the beginning of the test and at every 5 minutes to record current and voltage in her notebook - no hi tech required ;D

Best of Luck

Ed

[DMAN]

Another way to look at the problem might be counting the revollutions of the disc.  You could set the CD player to play one song on repeate and find a way to count the revolutions on the disc with some sort of counter circuit.  You might be able to use the sensors from a old floppy drive, or even tap into the sensors on the cd player itself.  The experiment could then be the number of revolutions you could get out of different battery makes and types.

cheers

[Fatherof2]

Thanks all for the suggestions.  I may use the Stop Watch method, as suggested, for awhile to get get things started.  Meanwhile looking for a more automated way to time it.  The stop watch idea is good and basic, but poses potential problems like forgetting to check it before the cells run out of juice...then you don't know when it stopped.  And we have about 8 different cells to test.  But better to get on with it than to hold things up while Dad is trying to do something fancy, eh? ;) ;)

[EdwardM]

Absolutely  ;D

Happy New Year

Ed

[spat]

Just a thought! I don't know what type of set-up you have but maybe a couple cheap odometers from a bike shop or salvage yard could be conected to each player. Once the player stops so does the odometer. Most modern odometers in automotive are controled by electronics so you would want to look for older types if you want to do it based on mechanics.

[Sukhbinder]

why not try to power the digital clock from the power supply of the spindle motor.
However the clock must be able to save the time just before it shuts off. u can try to use the signal at the spindle motor to trigger the counting of the digital clock (not the clock signal). generally most CD players use a permanent magnet DC micromotor for the spindle. computer CDROMs however use a stepper motor. the spin speed in CD players is usually controlled by using a PWM signal for the motor, so u would need filter (simple RC type would do it) before applying it to the clock. i suggest u use a diode in series to protect the clock circuit. if the player runs from 3V and the clock from 1.5v use 2 silicon diodes in series and the clock and player ground or negative must be common. as the CD player starts running the clock starts counting and as the CD battery goes dead the spin motor stops and the clock stops counting. u might need to invert the signal (use a simple common emmiter inverter as digital inverter ic's may not function well ,may be CMOS may work but it adds to the parts needed, so try keeping things simple). tell me if it works. ;)