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Making a LED act like a filament lamp

Circuit ideas?

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Stevo31/03/2015 17:35:36
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Probably a simple one!

I'm fitting wing LEDs to a Chippie at some point with remote switching, - around 250mA per LED. No problem so far!

Now, being pedantic, I want them to react like filament lamps, so when I switch on they ramp up in brightness instead of just instantly going 'ON!', and obviously when I switch off I want them to ramp down in brightness until they are off. The time from nothing to full brightness is just a fraction of a second of course ... but as I say, It's me !!

So, anyone done it before...

Yes I have the idea of a series transistor... capacitor charging between base & emitter... but if anyone wants to share it it may save me some experimentation.

Edited By Stevo on 31/03/2015 17:36:30

 

OH GAWD!! MODS - Change the title! "making a LED act like a filament lamp" Dodgy keyboard!!

Edited By Stevo on 31/03/2015 17:38:53

Don Fry31/03/2015 19:09:04
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NA, you are now an Italian.

Stevo31/03/2015 19:11:58
2699 forum posts
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Ha! Thanks Donald

Edited By Stevo on 31/03/2015 19:12:12

Vaughan Wilson31/03/2015 20:36:54
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This could be adapted to serve your purpose:

**LINK**

Stevo31/03/2015 20:48:57
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419 photos
Thats pretty much the circuit I had in mind Vaughn...

It now saves me fiddling round too much with valuess - thanks!!
Allan Bennett31/03/2015 21:17:05
1686 forum posts
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Posted by Vaughan Wilson on 31/03/2015 20:36:54:

This could be adapted to serve your purpose:

**LINK**

That is very interesting. I believed that LEDs by their very nature were either on or off because they are diodes. I've made them "fade" on and off in the past by using a PICAXE to create a variable-frequency but constant-voltage supply, which creates the fading effect with the change in the on/off ratio.

I'm going to try out that circuit when I get a chance, hopefully tomorrow.

Stevo31/03/2015 21:46:14
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Picaxe? Now yer talkin'.
Ive got experience with those! Would you mean a variable pulse width output, as opposed to variable frequency?
Martin Whybrow31/03/2015 21:52:54
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A much simpler way would be to put a capacitor across the LED, say 100uf and a series resistor, say 300R for a 5V supply, between the supply and the LED / capacitor; that should get close to what you want.

Stevo31/03/2015 21:58:11
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419 photos
Good call Martin. Alas my led and series resistor are "all in one"... i.e. a Braincube 1.2W LED assembly.
Allan Bennett01/04/2015 08:36:47
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Posted by Stevo on 31/03/2015 21:46:14:
Picaxe? Now yer talkin'.
Ive got experience with those! Would you mean a variable pulse width output, as opposed to variable frequency?

Yes, a variable pulse width. I used it to simulate a pulsing beacon light, rather than the usual flashing strobes.

As I said, I'm going to try out the referenced circuit this afternoon, but the more I think about it the more I think it won't work unless he's got some special dimmable LEDs. All the circuit seems to be doing is applying a varying voltage to the base of the transistor, which in turn allows a varying current to flow through the LED. Isn't that the same as simply applying a varying voltage direct to the LED which, with the LEDs I have, simply causes them to switch on to almost full brilliance as soon as the threshold voltage is passed?

Stevo01/04/2015 08:57:52
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Once the braincube device arrives ill experiment and report back...
Martin Whybrow01/04/2015 09:39:22
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Allan, LEDs are not digital devices, they have a direct, albeit non-linear, relationship between the current and the light output, so varying the current will do exactly what Stevo's after.

Stevo01/04/2015 09:43:58
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Ok... non-linear under 0.6v. Or until it Zeners...

Ish......
Allan Bennett01/04/2015 17:08:05
1686 forum posts
49 photos

Posted by Martin Whybrow on 31/03/2015 21:52:54:

A much simpler way would be to put a capacitor across the LED, say 100uf and a series resistor, say 300R for a 5V supply, between the supply and the LED / capacitor; that should get close to what you want.

Well, we learn something every day. Thanks for starting this thread Stevo.

I've built the circuit in Vaughan Wilson's link using the closest equivalents I had in my box, and it works, though I'm still not sure why because in the past when I've gradually ramped up the voltage to an LED it suddenly switches from off to on, with no nice fade. I'm going to experiment with different capacitor and "drain" resistor values to get a quicker fade in/out, but I'm not sure it's going to be any good for a continuously pulsing light because it extinguishes well before the capacitor's fully discharged. That means that if I switch it on again immediately after it's faded, it comes on more quickly than the previous time.

During the build I was wondering what's the purpose of the diode and transistor. Presumably the diode is simply for reverse-polarity protection, but if we omit the transistor too, and feed the LED directly from the supply voltage, that seems to be basically what you're suggesting.

Area 5101/04/2015 17:09:58
653 forum posts
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This looks interesting, but it a little lost on me.. (electronics).. Any chance of a how to guide on making your own LED set up similar to the ones sold at shows/braincube etc.

Or links to videos that do same/sell the parts to "have a go"

Many thanks

Stevo01/04/2015 17:36:38
2699 forum posts
419 photos

Allan,

I think the diode is in there to potentally stop the timing capacitor discharging via anything that feeds it, and not soley via the designated discharge resistor.

In this case, and charging of the capacitor over around 0.65V is not needed as the tranistor would have turned fully on by this time.

For a low current LED, say 30mA, you may get away with no transistor. In my case I have two high power LEDs consuming 250mA each, so no option but to control with a transistor !

As I said for my purposes a turn on/off time of around 200mS is probably what Im after. I would want to achieve this with a small PicAxe chip (08M) which takes the signal from the Rx, and, via PulsIn decides to make an output high or low. Inturn I would feed that via an RC combination to perhaps a Darlington pair, in turn driving the LEDs.

Area51 - Indeed some of these high power LEDs are available ascomponents, but by the time you've encapsulated them with a series resistor, a housing and a bezel, you may have well brought one from BrainCube.

Allan Bennett01/04/2015 18:14:47
1686 forum posts
49 photos
Posted by Area 51 on 01/04/2015 17:09:58:

This looks interesting, but it a little lost on me.. (electronics).. Any chance of a how to guide on making your own LED set up similar to the ones sold at shows/braincube etc.

Or links to videos that do same/sell the parts to "have a go"

Many thanks

I've used CREE high-power LEDs as strobe lights on some of my models. One issue with them is that they (or any other brand of high power LED) can't be switched on continuously unless they're mounted on a substantial heatsink, which doesn't lend itself to wingtip lights. As strobes (driven by a PICAXE program) they don't need the heatsink because they're only on for a fraction of a second every second or so. I believe the metal tube housing the Braincube lights is the heatsink -- an elegant solution. The CREE LEDs are surface mounted devices, so I wouldn't recommend them unless you've had experience of soldering very small delicate electronics.

Stevo, for my lights I use the 08M driving a Darlington pair (an 8 in 1 module) to give me four separate independently controlled outputs. One is for switched landing lights, the other three for strobes which flash independently of each other. On my helis I've also programmed the navigation lights to flash when my time is up, since I usually can't hear the beeper on my transmitter!

Area 5102/04/2015 11:08:14
653 forum posts
1 photos

Hi guys, thanks for response and input. Not a straight forward as I thought!

I will follow your thread though and learn some more!

Cheers

Allan Bennett02/04/2015 11:36:51
1686 forum posts
49 photos

Area 51, the simplest form of lighting is using LEDs as navigation lights, which don't require any switching or flashing. If you start with them, you can venture into switching and flashing later. You can connect them to your receiver's power supply, or direct to your motor battery if it's an electric model. Either way, you get an traditional 5mm or 3mm LED with wire "legs" on it, check out its spec sheet to find its volt and miliamps rating (note that different colours will have different specs), then use this simple calculator to figure out what resistor you need to connect in series with it so that the voltage won't blow it.

Martin Whybrow02/04/2015 12:28:10
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Posted by Allan Bennett on 01/04/2015 17:08:05:

I've built the circuit in Vaughan Wilson's link using the closest equivalents I had in my box, and it works, though I'm still not sure why because in the past when I've gradually ramped up the voltage to an LED it suddenly switches from off to on, with no nice fade.

I can explain! The LED is a current driven device (as are most transducers); they have a threshold volatge below which they will not illuminate, this is shown on data sheets as the Vf (forward voltage drop); as soon as the voltage exceeds this value, the LED will try to draw lots of current and will light up at full brightness (and possibly for a short period if you have a high current capacity supply); for this reason, LEDs must be run from a current-limited supply, commonly just a series resistor. Note that the LEDs sold as 12V, 24V, etc. already have the series resistor built in.

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