multiplexor

Basically i will be providing a capacitor with 3 volts of electricity
how can i figure out how long it will take to lose the voltage once the voltage to the capacitator is stopped?

I'm not sure how many "farads" the capcitator will be since i'm still learning this right now

what should i be looking out for?

My goal is to have a light fade out. I'm hoping it would be progressive over maybe 2-3 seconds...

any insight, help, urls, anything would be great
Thanks

Dustyroads

Wow, damn this a good post. Multiplexor, put this on clubmax, or let UMD know about it, I'm sure he could figure it out and I'll look through some of my old linear circuit analysis books and try to get back to you.

multiplexor

hehe consider it done

problem is that i'm that the greatest in electronics... i can get around... but not nearly as easily as alot of you here, i'm sure.

If we could figure this out then we could probably have some nice dome lights that fade out when turned off...

or instrument clusters fading out, whatever we'd like hehe

actually something kinda neat woult be the red alarm led fading in and out... instead of just flashing.

Dusty, any help would be awesome

Matt93SE

iTrader: (44)

first thing you have to do is figure out how much current your load is using.. that, and the "fade-out" time will determine the size of the capacitor.

throw some numbers back and we'll see what we can do. shouldn't be too hard, just a couple of differential equations.

jiaxima96

I think dome light bulb is 10W. It'll be nearly 1A current. With such a large current, a really big size cap is needed to hold a few seconds. Besides, at the moment the light is turned on, the already discharged big cap acts like a short circuit I'm not sure if fuse can withstand that moment.

I was thinking using a CMOS power transistor as a variable resistor and the on-and off 12V as the signal to control the transistor. Never got time to do it.

Matt93SE

iTrader: (44)

hmmm, if I remember my equation right (which I just pulled out of my head at 1:30am after wathing Monty Python movies, so I doubt if it's right!)

but for the time constant you want and the current (12v/10ohms), the cap won't have to be too big.. about a 3500uF or so.. can pick 'em up at any rat shack for about $1.

I'll get around to it sometime this weekend and actually give you a real number, or the equations you'll need to figure it yourself.. not hard at all.

jiaxima96

Let's see. 10 Ohm X 3500 uF = 10 X 3500e-6 = 0.035 second. So if 3.5 second is needed then cap should be 350000 uF.

multiplexor

A friend of mine came up with:

C=di/dt

dt= i = amperes

dt= t = time

anywho came down needing a 5uf capacitor... for a led to dim over a period of 4 seconds.

so figured i would go out and try getting a 5uf capacitior, one at 3 volts and test it out. turns out the minimum i could get was 16V
so i bought one, tried it out. For some reason, maybe it could be explained here, when i attach the capacitor to the source, and to the led. The led brightens for a half second, then drops and you can barely see the light, but it is on...

i'm not sure whats hapenning here

multiplexor

jiaxima96

I think C=di/dt is wrong. I know for sure that Ic=C*dVc/dt. I am not sure if I understand rest of what you described. Did you try on the dome light or on the door step light?

multiplexor

oh my plan was to use it on the dome light.

but for my test i used a 6volt source in my home. At 800mA.

i bought a 16V 4.7uF capacitor and figured i'd try that out.

the led is a 5 volt 20mA led. The capacitor should be plugged into the negative wire correct? and you would follow the arrow on the capacitor, as if it were the flow of electricity from the source to the led?


I mainly wanted to test it inside my place before i goto the car and try it out there.

The main problem i've been experienceing is that when i plug in the led with the capacitor, the led just dims and when i cut the power source the led just turns off.. no fading

[FONT=courier new]
Capacitor
|--(-)------{<--16V 4.7uF}-----[----------]
{}6Volt 20ma led---------------|6volts out|
|--(+)-------------------------[----------]

the "............" above are just used for spacing since html will
probably squish everything together...

jiaxima96

Good idea trying at home, much easier than working in the car.
A cap with polarities should be used with + sign connected to + Volt with respect to the other terminal of the cap. "when i plug in the led with the capacitor", did you put the cap in series with the LED then connectd them to the 6V source? If yes then it is not the correct way.

multiplexor

i attached an image to show what it looks like

basically power goes through the capacitor , reached the led, then goes back to the positive side of the battery.

what would be the correct way to plug in the cap?

jiaxima96

Your drwaing says it's a series connection. A cap does not allow constant current(DC) to go through. The dim light you said you got is from the cap's leakage only, I think. If you want to hold the LED voltage for a certain time period you can put cap in parallel with LED, with the correct polarity, then connect them to the power supply. When power is turned off, the cap and the load(LED in this case) form a time constant that determines dim speed.

jiaxima96

Let me make it clearer. If power is turned off by disconnecting the wire in your experiment then it's just what I said above; if power is turned off by voltage of 6V down to zero but still connected then the 6V internal resistance will also contribute in time constant(smaller because 6V internal resistance in parallel with LED makes overall resistance smaller ---> faster dim).

multiplexor

so your saying basically connect it like i have it in the attachment below...

Connect to the input source to make a full circuit. Just attach the led onto the capacitor wires, so when the power is turned off then the capacitor will just drain out into the led.

B.T.W Thanks very much for helping me out with this

jiaxima96

Yes in general.

multiplexor

when people keep mentionning having a big cap, i'm assuming the cap literally be pretty big in size? Not the it's for example 900uf instead of a smaller 2uf correct?

with some calculations help from a friend he figured i should use a 4.7uf cap. but so far it's not going as planned. I'm going to try a bigger cap of 330uf, which is also ALOT bigger in size.

ehh it's fun to experiment...

jiaxima96

In industry "big cap" is often used for both size and value, depending on situation. Usually big value cap is also big in size. With the same value, higher voltage rated cap is also big in size.

Yes when it comes to the real dome light current of about 1A, a really big value cap is needed. However the same time constant will "probably" work for both dim and trun-on, meaning it'll also take a few seconds to turn the dome completely on. I said "probably" because I don't know what is inside the light control unit, what gives +12V or 0V, linear or non-linear... I have handbook not with me now(sorry forgot the spelling, "Hayne" something) but just can't find in the schmatic where dome light is.

multiplexor

hmm and whenyou say it'll probably take a few seconds to turn completly on. Will that be shown as a fade in type of effect?
or just a few seconds and then it just flips on?

ugh i wish i hd taken EE courses now... dang! i hate bothering other people about this...

so far i've tried the series connection method... then tried the parallel method. (basically completeing the circuit with the capactior, and attacing the led around the cap.

Then... i tried removing the led and just having the power flow through the cap... then i'd turn off the source, and touch both led wires to both ends of the cap. This would then discharge the cap and make the led fade, but extremly extremely quickly. Like less then half a second kinda deal.

the cap i'm using is probably the height of your mouse curor... at 4.7 uf

Dustyroads

Here's something for you to consider.

I=C*dV/dt

I = 20mA (LED's current draw)
C = the unknown
dV = 6V
dt = 4sec

That will yield a cap that is 13.3mF.

I'm not 100% sure if the current is correct but that makes a bit of sense to me so if you can find that size of cap give it a whirl. If you want the fade to only be 3 seconds, then the cap value is 10mF. I have a feeling that these guys are gonna be pretty good sized though, so it may not be possible. Good luck!!

multiplexor

hehe so i wouldn't be dealing with uF but mF instead... hmm k i'll try and find one for the hell of it... 13.3mF... yikes!

Thanks.
I'm willing to give anything a try to just see if i'm doing it correctly first, and then work from that.

jiaxima96

Assuming a resistor (about 300 Ohm or less, with 6V power source) is in series with the LED, otherwise the LED current will be too large.

multiplexor

Just a little update. I was messing around with some caps i had lying around. I found a 0.00467 farad cap rated at 6.3 volts. I plugged it in... and to my surprise actually got it to work.

though the led took over 2 minutes to fade.

i have to go down to radio shack or somewhere gotta pick up one of those 13mf caps and try that. I'm hoping it won't be too large.

the fard cap i used was probably the size of a penny and as thick as an index finger. I found it on a VCR circuit board that showed the time, and other functions.

oh and plugging 6volts into a 3 volt led for anything longer than 10 seconds will make it get VERY HOT! heh. Though that's something i knew, but forget when i put my finger on the led... ouch.

multiplexor

curious.... do you have any experience with oscilators?
Do you think it might be easier to reproduce a fade in/fade out effect with an oscilator?

I'm stil going to try with the capacitor first, see how it goes.
At lunch time I drove down to an electronics store and bought
a 6800uf capacitor and a 10,000uf capacitor both rated at 16V.
they are pretty damn big.

After work i'll plug those in, see what effect i get.

Once all is said and done, and everything works properly, i can post a thread showing how to do this, what's needed, etc...
This way everyone can have a fading dome light ;-) hehehe

jiaxima96

I have some experience with oscillators.
Glad to know your experiment basically works.
Yes please post once it's done, and it'll be a birthday gift to yourself just in time (I saw it in your profile).

multiplexor

hehe yes you are correct. If i can finish this project before 12 tonight, i can wrap it up and give it to myself tomorrow morning

hehe

The reason why i mention oscilators is that a friend of mine has a probe gt, and his dome fades out, but uses an oscilator instead of a cap. Well time to learn more on oscilators

Though for this project i'm sure i'll be using a cap

jiaxima96

I'm thinking about applying similar thing to the headlight. Over the past five years I forgot to turn off headlight a few times, and one time battery totally died(OK after jump-started the car, and the battery is still the original one now). Either let it turn off right away or with a delay of say 20 seconds, after ignition is off.

multiplexor

Actually that would be a prett cool idea. I often forget to turn my lights off... Once drained the battery also, had to buy a new one. that sucks!

jiaxima96

I built a module and tested it. The delay is about 20 seconds. Not put into my car yet. It has one relay ($8) and one transistor ($2), both from Radio Shack, and two diodes, three caps and three resistors from my previous junk. The cap for time constant is 20uF. This module has total seven wires to be connected to the car. Schematic is attached in case someone is interested.

multiplexor

I took a look at it... but it's kinda hard to read. hard to make stuff out on the paper

get a better copy up here looks like a neat idea. heh

jiaxima96

I'll make it better today. Was a word file but realized it's not a valid format to attach in this forum. BTW I found a good place for my module. Took out the lower plastic panel under steering wheel, and behind the panel took out the metal piece for knee/leg protection. The left metal bracket (two pieces weld, one on top of the other)for the metal protection piece has a hole in the center(the top half of the bracket)that I can mount module on meanwhile it serves as the electrical ground. I checked with steering wheel moving all the way up and down and the module is not in the way. Module size is approximately 2 x 3 inches.

jiaxima96

Not sure if it's clear enough or not.

jiaxima96

Attached is the pic of my module.
Just bought a few photocells for the next project: Headlight auto turn-on when it's getting dark, like in the 5th Gen Maxima and many other cars. Since my 96 Max's light switch has only three stops, off, parking and lioght on, not like cars with a 4th stop for the "auto" light, I have to decide wether I want to put another switch to turn on and off this "auto", or let it always on.

multiplexor

cool thats much more readable.
can't wait to get the car back to try these things out...

hey what about using a capacitor so that after 20 seconds, your lights just fade out? ;-D hehehe

jiaxima96

I just finished installing the module. Perfectly working as I expected. 20 second delay after ignition key is out. The thing I did is that everything else is the same as before, such as parking light, high-beam light, ..., therefore after 20 seconds the parking light is still on. I wonder if I need to change that to turn off everything. Anyway the headlight takes much larger current. If my battery can last say about two hours with headlight on, now I guess it can last 3 X longer or more?

I don't think using a cap to fade the headlight out is practical, as its current is about 3.75A (huge in this case). If you want 2 seconds only then C = (t/V)*I = (2/12)*3.75 ~ 0.6 F. I don't have experience with such a BIG cap. BTW I don't know if there is a car with headlight fade-out feature.

OK next will be the auto headlight turn on/off based on environment's darkness. Who knows when it'll be done...

multiplexor

dang that would be a huge cap... and yeah i've never seen that effect on headlights. 0.6F yikes!


So the next project involves turning off the headlights if it bright out, and auto ON once it's dark enough? that would be pretty cool. With the whole setup working, you can have your headlights literally always on the ON position. If it's bright out, lights are off, when it gets semi dark, it'll turn on your fogs and inner instrument panel.
Once it gets dark, your lights turn on.

That would be cool