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hi there folks , i am building a lamp for a uni project and am trying to get some help from u guys. the project is a lamp which is able to change from direct into indirect lighting(mood lighting) which must contain and on and off switch. the lamp must be powered by aa batteries with a condition{ that for every 4 led's there must be 2 aa batteries. ie a set => 2aa's=4led's} the lamp must only consist of led's. i have an idea of a simple circuit. what i am asking is for a combination which will give me the brightess light. note i already have a lamp design, it consist of using blue led's for mood lighting and white led's for direct. any help would be great.thanx

just asking how do i make 8 led's running on 4 aa's, be the brightest they can be
[no restrictions on parts]

{sorry bout posting in other section, this is the right place)

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Hi Iceman,
The blue and white LEDs that I have require more voltage than a pair of AA cells, and much more voltage than when the 3V from the cells reduces to only 2V over their life. They need around 3.3V. Are you using a voltage stepup converter circuit?

In my "6V Ultra-bright Chaser" project I use four AA cells so that the blue and white LEDs still light when the 6V battery voltage sags to 4V over its life.
Also in my project the LEDs are lighted only for a very short time, they blink, to increase battery life. How long will your batteries last when powering 4 LEDs at 20mA each?
I figure that four AA alkaline cells in series will provide the 80mA for only about 28 hours and the brightness will get dimmer as the battery's voltage drops to 4V.

I have "mood light" circuits that operate from a wall-wart transformer and gradually fade and increase the brightness of red, green and blue LEDs. The fading rate for each colour is slightly different, so they produce combinations of all colours of the rainbow and all brightnesses on the ceiling. ;D

post-1706-14279142427297_thumb.png

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ok, i had a feeling that something like that was going to be the case. thanx for the heads up audioguru. uhm i haven't put the circuit together yet so i really have no clue to stepping any voltage up yet. but the light must last for as long as possible. but there is also an on/off switch. and there is alos a switch to change from direct into mood lighting.

so in that case do i need to up the voltage or current across them? and how do i go about doing that, so that the led's don't blow because i have done it to a few already.

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do i need to up the voltage or current across them? how do i go about doing that, so that the led's don't blow?

LEDs blow if you allow too much current through them, or connect the LED or the battery backwards. The current is limited by wiring a resistor in series with each LED. The resistor is determined by Ohm's Law, the battery voltage, the LED's voltage and the current needed that is probably 20mA.

Is your project an exersise where a new 3V battery of two AA cells is used only for a moment?
Or is your project an actual requirement and its two AA cells must power the LEDs until the battery is used-up and its voltage drops to only 2V?

Measure the voltage of all your LEDs one-at-a-time by connecting each one in series with a 270 ohm resistor and powered from a new 9V battery. Their voltage will determine whether you need a voltage stepup circuit for your project. ;D

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yeah the project states that for every 4 led's there must be 2 aa batteries. [where a set is 4 led's to 2 aa's] this is a requirement that must be meet.

note[ there is no limit to the amount of sets u can have]i.e if i want 16 led's i need 8 aa batteries or if i wanted 24 led's then i need 12 batteries

with regards to the time it must be on, well i need to do a five minute presentation and it must be able to be keeped on for about say 4 hours when the marker marks the project. so it doesn't have to last long. it just needs to be as broght as possible.

if its not to much of a problem could u give me a parts list i require because i don't really have access to an electronic workshop. thanx

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Hi Iceman,
I couldn't find white or blue LEDs with a voltage less than about 3.3V so you are going to need a voltage stepup circuit to power them with only two AA battery cells.
The LMC7660IN is a "charge-pump" IC that makes a negative voltage. It has an output resistance of about 90 ohms and costs $1.31US at Digikey.
Three of them can be paralleled to drive four paralleled voltage-matched 3.3V white or blue LEDs very brightly when the battery is new. http://www.national.com/ds.cgi/LM/LMC7660.pdf

In my circuit the LEDs must be voltage-matched because they are directly in parallel.
Take a new 9V alkaline battery and connect an LED in series with a 270 ohm resistor to it. Measure the voltage across the LED and record it. In the circuit, connect LEDs in parallel that have voltages nearly the same. Get some extra LEDs in case one or two have a voltage very different from the rest. ;D

post-1706-14279142430624_thumb.png

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Hi Alun,
I looked at a Cmos 555 IC as a voltage doubler, but the loss of two rectifier voltage drops is much too high.
Also, Cmos 555 ICs like the one you posted, LMC555 and TLC555 don't have much output current at a supply voltage of only 3V and 2V.
The LMC555 and TLC555 are guaranteed to source only 10mA with a 5V supply. Using only 3V the current is much less, 0.3mA for 0.5V loss. At only 2V, forget about having any output current.
The LEDs would be very dim if one was used as a voltage doubler. The Cmos LMC7660 charge-pump circuit has the same problem, but supplies much more current than a Cmos 555. Even with its much higher output current, three of them were paralleled for more.

Hi Ante,
Those simple blocking oscillator switching converters are great! ;D
It is too bad that a special ferrite core and special transistor might be required.
It is also too bad that the regulated one doesn't have an amplifier to reduce the power wasted in its current-sense resistor.

I saw hundreds of white LED driver ICs that are very efficient. But they all use surface-mount packages and many are so small that it is hard to even see them. :o

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Hi Alun,
I looked at a Cmos 555 IC as a voltage doubler, but the loss of two rectifier voltage drops is much too high.

I've already thought of this, you use  schottky or even gernanium diodes, yes I should've mentioned this too also.

Also, Cmos 555 ICs like the one you posted, LMC555 and TLC555 don't have much output current at a supply voltage of only 3V and 2V.
The LMC555 and TLC555 are guaranteed to source only 10mA with a 5V supply. Using only 3V the current is much less, 0.3mA for 0.5V loss. At only 2V, forget about having any output current.
The LEDs would be very dim if one was used as a voltage doubler. The Cmos LMC7660 charge-pump circuit has the same problem, but supplies much more current than a Cmos 555. Even with its much higher output current, three of them were paralleled for more.



I don't know about the TS555 as the maximum output current wasn't even specified I just made the assumption it's about 100mA, I do take your point about CMOS ICs in general though, I think the problem is due to the fact that 3V is too low for the MOSFETs on the output to saturate fully, a 74HC04 inverter with all of the inputs and outputs paralleled on the output my help but I think we might still be in the same situation. :(
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hi its iceman here,
uhm i'll see if i can get hold of the parts, i don't know about the digikey chain of stores because i live in australia, but i'll see if i can find those parts.
another thing is, in regards to audioguru's diagram so do i connect all those little arrows to the negative end of the battery pack.

ok thanx guys i'll give it a try and see wat happens and report back.

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i'll see if i can find those parts.

Other companies make the 7660 IC. They have their own letters as a prefix like the ICL7660.
Look on the web at www.farnell.com who have an Australian site. G'day mate! ;D

in regards to audioguru's diagram so do i connect all those little arrows to the negative end of the battery pack?

Yes.
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audioguru,
oh yeah also forgot to ask i f i was going to use 8 led's and 4 batteries do i just connect the circuit together.

and i regard with the switching part i'm a bit hazey. uhm i need to have one circuit[the one u gave me(clear led's)] switch to another[same type of circuit(blue led's)]. where it switches from mood to direct. and also i need an on off switch so where does that go?

uhm if u r unclear i'll try to post wat i think the circuit should look like.

god i'm really in too deep.

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ok heres what i thought at first the circuit would be like, but found that it was too dim and sometimes just didn't work.
uhm i also noticed a loophole in my project brief. although i have 16 led's, i only use 8 at a time so i'm sure that i could use the full 12volts across 8 led's.
my question is will this work? and will it be brighter?

post-11696-14279142431961_thumb.jpg

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Hi Iceman,
You can't do that!
An LED or a bunch in parallel have a 3.3V voltage drop. If you put a 12V battery across them like that then the current will be enormous and either the LEDs will burn out or the battery will be killed.

The circuit I posted steps-up the voltage of two AA cells high enough to light four LEDs and limits the current to a safe amount for the LEDs and for the battery.
You said you could use groups of four LEDs together so I designed it like that.
If you want to turn on two of those circuits then use a two-pole switch.
If you want to switch from two of those circuits to two other circuits (different color?) then use a two-pole, two-way switch called DPDT (double-pole double-throw).

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for better life of battery is it necessary to blink the led instead of constant glow?

Of course. The LED uses battery current for the duration it is lighted. When it blinks then it is off for much of the time.
In my LED projects I blink the LED for a very short time so the battery lasts for a very long time:
http://www.electronics-lab.com/projects/games/003/index.html
http://www.electronics-lab.com/projects/games/004/index.html
http://www.electronics-lab.com/projects/science/018/index.html

at what frequency?

You can blink at a low rate to save battery current. Your vision can't see blinks less than about 10ms or less in duration so the percentage of the time that an LED is lighted is less with very low blink rates.

If led is blinked at high frequency so that it will appear constant ; will the battery life affect? how much compared to low frequency?

It will appear dim and the battery will last longer because the current isn't constant. In my projects I modulate the LED current using Pulse-Width-Modulation for variable dimming.
If the LED is off for 50% of the time then the battery will last twice as long. If the LED is off for 90% of the time then the battery will last 10 times as long as if it was lighted continuously. The brightness also is reduced by the same amount. ;D
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