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PostPosted: 19.09.2004 07:42 
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Hi, i know nothing at all about transistors and that chip. the only electrical education i have is basic physics, so i only know how to calculate basic stuff, and build basic led circuits, so im very confused about how this fader circuit works.
I'm a 2nd year architecture student in chicago, and after reading this site about making led circuits, i decided to light one of my projects with leds (1st year 2nd semester. but i only made a very simple circuit, which anyone with a little knowledge can make.
here are some pictures.
Image
theres an on off switch on the lower right. the current flows from the batteries to the switch, and then to resistors on the lower left, and up to a parallel circuit of 5 red LEDs, and then back to the batteries. there are mirrors underneath to help reflect the lights.
Image
Image


could someone help me understand the fader circuit diagram? how does it work? what does the chip do? i tried looking at that explanation link but it makes no sense to me. what does the transistor do? which part of the circuit actually controls fading? what causes electricity to stop flowing to the capacitor and make it drain to cause the fading? what makes it charge again?

i have this idea for a birthday present. this person likes the color purple, but there is no purple led, i think, except uv, but thats harmful to the eyes.
so im thinking of using red and blue leds together to make purple.
i want to make a fuzzy, sand blasted acrylic material to create a hollow softball sized sphere in which the LED circuit goes.
in it, i would like to use maybe 8 red and 8 blue LEDs, in which the blue is constantly on all the time, while the red fades in and out, causing the ball to glow blue and purple in intervals.
i would also like to have the option of making the ball portable or plugged in, so maybe a battery compartment in the bottom of the sphere, and also a molex connector with a switch to change between battery power or wall outlet power with a 12v ac adapter.

have you seen the apple powerbook power buttons? during sleep mode or whatever, the button dims and goes to full brightness in intervals. does it use 2 leds, one on constantly, and the other fading, or does it just use 1 led that fades halfway and then goes back to full?
if i can get both red and blue to stay on all the time but just fade halfway, then that would be good too.

the circuit diagram was so confusing to me. i have trouble understanding where the power source is, and the direction of the current. i have very little experience with circuits.

so...........how do i make a circuit for this glowing ball?
could someone show me the circuit i would have to create and explain how it works?
any help with this birthday present idea would be much appreciated!!!
and thanks for putting up with my lack of knowledge.

azntwboy@yahoo.com


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PostPosted: 21.09.2004 03:43 
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Im surprised Japala didnt respond to this one personally. I wish I could help you but I dont know much about that circuit myself. :? Sorry, just had to answer because such a long post didnt deserve to go unresponded to.

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PostPosted: 21.09.2004 08:30 
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Image

On the top left is a black dot that has +5v written next to it. That is where the positive wire coming from the power supply gets connected. Now look bit to the right and you see gnd(-) written next to a similar black dot. This is for the negative wire.

Fading is controlled by the LM324 and those transistors are there to drive the leds. Leds take a lot of power in terms of what TTL circuits can provide and those ordinary transistors can quite easily handle ~100mA or so of current. This way you can connect several leds on to one fading "channel".

PNP and NPN? Think of these as flood gates. When other opens, the other closes. That causes the leds on the other transistor to get brighter when leds with the other transistor gets dimmer.

Fade only half way? You could try to connect a resistor to the pin 1 coming from the circuit and from there to the positive wire coming from the power source. With proper value, it should keep enough current flowing to the transistors even when the current coming from the driving chip itself has gone low. But this may only work with one led in NPN channel because PNP transistor requires that the voltage goes down for the leds to light up.

Hopefully this helped a bit. Ask more if needed. I don't think there there is any point on going any further explaining why the LM324 does what it does. At least I'm more interested on getting the leds to fade. :)

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PostPosted: 21.09.2004 09:15 
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Junior Modder
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hmm.....im starting to get it, but the jumble of connections with the several resistors and capacitor connecting to the chip is confusing. how does that part of the circuit work? what does the 2 resistors on top between the 2 nodes do? does it limit the votage going to the rest of the circuit? maybe, could you draw arrows for which direction the current is going?
the circuit i would like to build only has 1 set of fading LEDs (or as you probably call, 1 channel), so which part of the circuit would i remove to only have just 1 set of LEDs fading?
so, if i wanted the blue leds to stay on constant, and the red ones to fade, would i connect the blues in parallel to the rest of the circuit, and then connect just the reds to the fading part of the circuit?
wouldnt i have to make sure that the reds and blues require the same amount of voltage?
thanks a lot for your help.


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PostPosted: 21.09.2004 10:24 
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While the title states that single led, you can actually connect many leds after that transistor but the catch is that they all fade the same way.

The two resistors are used as a voltage divider so that the pin 3. of the circtuit gets half of the supply voltage. Circuit is still powered by pin 4. that receives the full supply voltage.

http://www.national.com/ds/LM/LM124.pdf

From here you can see that we use two opamp of the four available. Other side of the chip is not used so it can be used to make even more "fading channels".

Leds that will be lit with constant brightness need to have a proper current limiting resistor that you can calculate with our resistor calculator.
http://metku.net/index.html?sect=view&n ... /index_eng
This leds or leds are totally independent from the fading circtuit so just connect the to the power supply as you would if you didn't have this fading circtuit at all. In parallel with everything.

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PostPosted: 21.09.2004 11:33 
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oh. ok. that diagram simplifies things. even though i still dont understand how the chip controls the switching, i can see now where to add the LEDs and how to calculate what resistors to use with them.
so, how much "on" time does that blue 100kohm resistor give to the led?
would increasing the resistance give you more time or less?
i would like it to stay on for 5 sec, dim, and then come back on right away.

i will be using a 12v ac adapter rated at 1000mA max. is that safe to use if accidentally a wire were exposed? is it powerful enough to hurt?
i think i will be building a switch between the AC adapter and a battery pack, so i can unplug it and carry it around while using battery power.
i just dont want the person im giving this to to get shocked when using the switch. are switch casings supposed to not contact the connectors?
i think i have a switch like this except its got 3 connectors instead of 2, and the left and middle connectors are closed when the switch is on the left, and the middle and right connectors are closed when the switch is on the right.
Image
is it safe to use this switch with the ac adapter?
thanks for all the help.


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PostPosted: 21.09.2004 12:36 
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You need DC power for the circuit so you need to rectify the current. The switch you posted will do just fine and the casing should not be connected to the terminals. Timing etc. depends on many things and I don't remember which way the timing went when you altered the resistor up and down. Just experiment a bit with different values. It is basically a regular RC timing circuit so the when the resistance goes up, the time period goes up as well. By using a bigger capacitor in terms of farads will alter the timebase too. Higher value should result longer fade. Just experiment, components are cheap. :)

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PostPosted: 27.12.2004 01:23 
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im finally on winter break, so i have time to work on this project finally.
so heres what my circuit is going to look like. the blues will remain on, while the reds will fade in and out. when on together, they will look purple. does it look right?
i will be using a 4.8v, 350mA ac/dc adapter. is that good? or should i go and get a 12v adapter? the current rating is just the maximum draw, right?
each led on average draws 20mA, so if i use 10 leds, it will be a total of 200mA. i will probably use 6 or 7 blues because the reds are brighter than the blues. is it ok to have that many leds in parallel? arent the leds dimmer in parallel than series? how do i get them brighter when they're in parallel?
i might actually use 6 reds, but if i do, doesn't the bc547 npn only handle 100mA? which other transistor can handle more than 100mA?
the closest capacitor that the store im going to buy this stuff from to 30uF is 47uF, is that ok?
thanks a lot for your help.


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PostPosted: 28.12.2004 07:26 
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Junior Modder
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help anyone?
please?


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PostPosted: 04.01.2005 06:42 
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Junior Modder
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um....my winter break is almost over so i need to order the parts now if i want to finish before school starts again. sorry, cant wait more for a reply.
so heres what im going to buy:
BC547 NPN (Pack of 4) $0.60
LM324 Quad Op-Amp $0.50
Cap Electrolytic 47uf, 25V (Pack of 2) $0.50
Premium Wire Stripper $5.95
Carbon Film 1/4w 100K ohm, 5% (Pack of 20) $1.00
Carbon Film 1/4w 47K ohm, 5% (Pack of 20) $1.00
Hook-Up Wire 22ga Solid Black 25' $1.30
Product Total $10.85
USD
how does this sound?
im buying this from hobbyengineering.com which is a store in the USA. i hope i can trust them. they accept paypal, which is what im going to use.
i also bought some 12"x48" white translucent acrylic for $23 at a local plastics store for the laser cutting. i will post my pics when i finish with the project.


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PostPosted: 13.01.2005 10:43 
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Junior Modder
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i finished it!
im going to post a new topic with pictures.
i also have a link to the page with all the pictures and videos.
thanks


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