Electronics help :)

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Hi all,

I have replaced my internal car lights with blue LEDs. They look great, and as so much brighter than the original lights, they are actually useful.

But I have decided I want the lights to not switch off the moment I shut the door, but instead fade down once the power is cut.

This is an easy mod, done by simply putting a capacitor in parallel with LED unit... but it has been years since I got my hands dirty with analogue electronics... can someone help me spec out what capacitor I would require?

Cheers!

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OMG, you put wanker lights inside your car?

Um... yes... the original bulbs were the horrid festoon types... that got very hot and didn't really light up the foot wells... so 4 3volt superbright leds were installed into each socket in series, to take the 12volt supply...

Anyway, what size capacitor do I need?

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Poor MX5, you've totally killed any sense of class it had.

It's an MX5!!! It is hardly classy -Edit- but it is fun!

Are the LED's pre-resistored up for 12VDC?

Yup, they can't be overcurrented!

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-edit-

Never mind. Series wiring. Just keep sticking 1500uF 16V capacitors in parallel to the array of three diodes until you get the delay you want. Note that you'll probably have a fade on too since the capacitor charging will divert a lot of the forward current initially as they charge.

Cheers Karlos! I now know what spec capacitor I want, so I can experiment and get it right!

The harsh On/Off nature of LED is quite distracting when compared to the more organic fade up/down of incandescent bulbs

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I'm not sure a simple capacitor based mechanism is your ideal solution though. Charging and draining capacitors with a load like that isn't good for them in the long run. You'd probably want to limit the maximum current into the caps.

Hmmm... I can't find any caps with 1.5kuF caps with a 16v spec either

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Higher voltage ratings would be OK, if you can find those. Just not less than the supply voltage.

Ok, I've not played with analogue electronics for 10 years

I think given the amount of time the lights are on, this simple capacitor system should be fine... how long does it have to last? I think I can afford a new 20p capacitor every year or so

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Just found a nice simple tool for this kind of thing;

A Java based circuit simulator, brilliant piece of kit with oscopes and everything

http://www.falstad.com/circuit/

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Hi,

Late reply, but I would actually recommend higher voltage caps. Crank conditions can produce rather high voltage spikes. You can actually get caps recommended for automotive applications, but they tend to cost a fair bit more.

The Farad capacity I want to use to get the desired fade down rate, only tend to come large voltages anyway

Unexpected expenditure (in the form of a new bed) has resulted in me pushing back any hardware projects for a while

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Nah... Too easy :lol:

No socks were harmed... Etc...

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Wired in a 4.7mF (24v) capacitor, looks magic (also added a 1ohm metal film resistor to give some current limiting to the capacitor).