fitting LEDs to turn on when side lites are on!

justjack

Newbie
Points
61
From
london uk
im trying to fit some leds to the speaker mesh in my 106 , i want them to come on when i flick the side lights on but cant find any tutorials.. anyone know how or have a tutorial.. thanks :?:
 

loxx101

Wrench Pro
Points
66
id say quite simply....

take a feed off the sidelights live wire and run it to your new lights.... and then run a negative wire off the negative one etc.

i dont really get the question :p
 

wolfmankurd

Track Warrior
Points
92
From
London
Car
Renault Clio 1.2
Make sure you don't put too much current through your LEDs.
If you know the Vsup from the wires you're tapping and the Vdrop across the resistors and the current you want (I) (Check datasheet for last two).

Then it's just (Vsup-Vdrop)/I gives you the resistor value. They don't come in all sizes so find one near what you want (ideally bigger ).

This looks like a good link for calkcualting resistor values.
http://www.quickar.com/noqbestledcalc.htm

edit: wow old thread sorry
 
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HDi fun

TC ModFather
Points
637
From
Buckinghamshire UK
Car
Passat 2.0 TDi
Quite a lot of automotive LEDs have series resistors as part of the lamp assembly.

Not that you cannot parallel multiple LEDs and use a single series resistor.
 

pgarner

TC ModFather
Points
417
From
Lockerbie, SW Scotland
Car
Octy smoke machine
think this is the oldest thread resurrected nearly 4 year olds

as Hdi has said you can buy 12v leds just as cheap that have the resistors built in saves you working out the resistance needed
 

wolfmankurd

Track Warrior
Points
92
From
London
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Renault Clio 1.2
Quite a lot of automotive LEDs have series resistors as part of the lamp assembly.

Not that you cannot parallel multiple LEDs and use a single series resistor.
You could but it might not last long :) Infact you might be better using parallel resistors for a single LED.
 

HDi fun

TC ModFather
Points
637
From
Buckinghamshire UK
Car
Passat 2.0 TDi
You could but it might not last long :) Infact you might be better using parallel resistors for a single LED.
LEDs are semiconductors and as such do not obey Ohm's law. Wired in parallel what will happen is that ONE of the devices will 'tip' over into forward conduction. LED's have very very low forward resistance once conducting. This has the effect of leaving all the rest of the parallel devices unlit.

Parallel resistors are only necessary if you cannot find resistors with a high enough current rating and have to make up your own 'network'.
 

wolfmankurd

Track Warrior
Points
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From
London
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Renault Clio 1.2
LEDs are semiconductors and as such do not obey Ohm's law. Wired in parallel what will happen is that ONE of the devices will 'tip' over into forward conduction. LED's have very very low forward resistance once conducting. This has the effect of leaving all the rest of the parallel devices unlit.

Parallel resistors are only necessary if you cannot find resistors with a high enough current rating and have to make up your own 'network'.
Sure you can.

That picture didn't come out as well as I'd hoped shaky hands. But it's 3 LEDs in parallel being run by a 5v supply.

They are funky coloured cause the are UV LED's. I've made LED lamps (using parrallel banks of serial LED's) that have been running fine nearly a year now.
 
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HDi fun

TC ModFather
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Passat 2.0 TDi
Are you sure they don't have inline resistors as part of the components themselves?

LEDs traditionally have very very low DC resistance when forward biased.

Or perhaps there's a new technology / new chemistry LED now which doesn't exhibit the trait I've described above? Most of my experience is based on the gallium-arsenide junction type LEDs. Could well be outdated now.
 

wolfmankurd

Track Warrior
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Renault Clio 1.2
No, I mean I don't think so if the resisitance didn't drop in forward bias they'd stop being diodes.

I've always been told LEDs don't work in parallel I've just never seen it in practice, with LEDs from the same batch. I guess modern LEDs are just made with tighter tolerences (or the process has been perfected so they come out very similar).

There are still loads of reasons to use individual resistors mainly to stop cascade failiures.

edit: just a thought. if you have parallel banks of serial led's then the difference average out to make it more likely to work?
 
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HDi fun

TC ModFather
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I doubt they'd work in series for the simple reason that you'd have to force every single one into forward bias before any current would flow.

As you say, 1 resistor per LED is good (and commonly acknowledged) practice to avoid cascade failures.

It is possible that the semiconductor mkrs have started incorporating series connected current limiting devices (resistors) within the LED package itself. WHich makes things a whole lot easier if you just want drop in replacement lighting.
 

wolfmankurd

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Renault Clio 1.2
I doubt they'd work in series for the simple reason that you'd have to force every single one into forward bias before any current would flow.

As you say, 1 resistor per LED is good (and commonly acknowledged) practice to avoid cascade failures.

It is possible that the semiconductor mkrs have started incorporating series connected current limiting devices (resistors) within the LED package itself. WHich makes things a whole lot easier if you just want drop in replacement lighting.
I doubt they've included reistors in the package because they'd foul up the current if you had to add your own if you had to work with different voltages.
 

HDi fun

TC ModFather
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I doubt they've included reistors in the package because they'd foul up the current if you had to add your own if you had to work with different voltages.
But would it mess with the current? If a forward biased LED has virtually zero resistance (which it does) then anything in series will appear as a current limiting device. The applied voltage is of no consequence. The LED will exhibit a anode/cathode voltage differential between 0.7 and 1.0 volts.
 

wolfmankurd

Track Warrior
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Renault Clio 1.2
But would it mess with the current? If a forward biased LED has virtually zero resistance (which it does) then anything in series will appear as a current limiting device. The applied voltage is of no consequence. The LED will exhibit a anode/cathode voltage differential between 0.7 and 1.0 volts.
Eh? The voltage between an LED leads is 2-5V usually depending on type(and roughly colour).

What I mean is. If they had a built in current limiting resistor that would only limit the current to say 25mA with a given voltage. The fact that it has nearly zero resistance in forward bias is qhy you can use resisitors to regulate the current so accurately, but in that case If I wanted to get 20mA at 12v then I would need to know the value of the resistor built into the LED otherwise it'd screw up my calculation. Also some of the heat would be dissapted inside the LED ruining it( by discolouring the plastic).

They do build some circuits into LEDs though like flasher LEDs.

When I hooked up the LEDs above it was limited by the internal resistance of the PSU. Which nearly happened to be the right value for 3 of those LEDS in parallel.
 

wolfmankurd

Track Warrior
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Renault Clio 1.2
SO if the PSU could deliver unlimited current and presented zero internal resistance what would have happened?
It'd burn out instantly. I just hooked up one of those LEDS to the 12v supply and there was a quick flash then it died.
 
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wolfmankurd

Track Warrior
Points
92
From
London
Car
Renault Clio 1.2
We had a play with Tesla coils in physics and it was extremely f***in' dangerous. :)

Health and Safety wasn't so prevalent in the early 1980s
Ah so lucky! we got a van der graff generator which we were assured would make our hair stand on end if anyone ever got it working!

Making a tesla coil is on my life long todo list :D
 
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