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Mad Ax

Talk to me about LEDs (again)

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OK, so now I'm offiicially too poor to go around buying stuff that just works, I'm into the muddy waters of having to make things.

I have an SCX-10 with space for 5 LEDs in the roof bar, two in the headlamps and I'll have to make something fit out back because there's no bucket or molding for tail lights.  That's no big deal.

It's a heavy-use Class 2 scale rig so I don't want to go overboard with indicators, break lights, reverse lights etc - I just want two headlights, five spots on the roof and two tail lights out back so I can crawl in the dark.  I've got a selection of 5mm LEDs in the right colours from various other light kits, pretty much enough lights to make everything work.

I just took delivery of a KillerBody light kit with a whole bunch of lights.  I thought this would be perfect for fitting up just what I want but it seems it's more geared towards a specific body than it is a generic light kit - the rooflights are a string of 6, the headlights are a string of 4, and there's all sorts of features I don't care for (like the headlights brightening at full throttle) which I don't think I can change.  I had at least expected individual lights and individual sockets, not strings.  I'll either have to cut off the lights I don't want or tape over them and stick them out of the way.

Or, indeed, it might just be easier to rig up the old lights I've got in a single string and stick them where I need them, and power them off a spare receiver port.  The problem is I don't know the ratings on the LEDs (not sure if there's a way to tell just by looking at an LED?) and I don't know what value resistor I might or might not need to get 9 5mm LEDs (7 white and 2 red) to cooperate on a single series circuit from a receiver channel.

Anyone have any ideas?  I tried to rig up a single LED to a 2S LiPo and it fried, I don't want to try wiring up any strings without knowing for sure they won't burn out.

In theory I could hook all the lights I have up to a spare TLU-01 module that I have lying around but this rig tends to get submerged quite often and I don't think the TLU will survive for long.

The UK Scaler Nationals is just over a week away so I don't have time to order any crazy LED kits from Banggood...

 

Any advice on stringing up different LEDs in series, calculating resistor values, or anything else that might help?

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I think I just bought the same thing - mine has inputs for throttle and steering servo Y leads, so light controller gets the turn signal when the servo does etc.

All I was going to do was run all the 'permanently on' lights from the third channel on my radio so I can turn lights on/off as required

 

If you're considering a TLU-01, you could always take the PCB out of its casing and conformal coat the board, that'll give it a fair bit of protection

 

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Are u any good solderen things, you can do the setup you want with LED, wire, heat shrink and some resistors. Connect these to a BEC or your ESC/receiver.

Also works better if you want to submerge it ;)

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7 minutes ago, TWINSET said:

I think I just bought the same thing - mine has inputs for throttle and steering servo Y leads, so light controller gets the turn signal when the servo does etc.

That sounds like it.  I thought it was going to be the same as another cheap setup I bought a few years ago from a seller who trades at the local lorry meet.  It had a bunch of different sockets for different lights.  I don't think any of them were strings (besides a double for the headlights, double for the tail lights and two doubles for the indicators).  That too had inputs for steering and throttle for light operation, I was just going to not use the indicator LEDs and like you, plug the headlight inputs into another channel.  But this setup is different, it's more fixed towards a specific arrangement and the strings have got too many lights on.  I'm not sure what'll happen if I cut a light off.

Actually I took a special trip to the local lorry meet this weekend to get another one of those original light setups (or maybe even just some bare LEDs and some advice on how to wire them up - he does a lot of wiring for lorry setups).  But he wasn't there - he was on holiday.  Doubly annoying as, my wife being ill, I had my baby daughter with me so couldn't even run my rigs on the layout.  All-in-all an hour's drive to look at some toy lorries for five minutes then come home again.  Gah!

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7 minutes ago, waterbok said:

Are u any good solderen things, you can do the setup you want with LED, wire, heat shrink and some resistors. Connect these to a BEC or your ESC/receiver.

Soldering I can manage - it's the resistor values I don't know.  I have the LEDs I need but I don't know that value resistor I'll need when they're all connected in series together...  I can find resistor value calculators but only for a single LED; I'm assuming that the LEDs will each have some small resistance which might affect the resistor value I need?

It's a shame Maplins went, there was one not far from work and they kept all this stuff in stock :(

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OK, thanks, I think the only way to reliably do this is to buy discreet LEDs since I don't know what the current / forward voltage is of any of the LEDs I have.  Now I'm trawling through RS Components' website and trying to get my head around all the different brands and luminosities and all the other crazy things that I don't really understand :o 

I'll get the hang of this eventually :o 

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The calculator I linked to is pretty accurate for the different colour voltage drop of 5mm leds.

I can link to some on ebay or rs if you like.

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@IoWBasher that would be awesome if you can - there's hundreds of red 5mm LEDs on RS and I have no idea which ones I want.

I just need a couple of 5mm reds and 7 5mm whites.  I don't mind buying packs of 10 or 50 because I'll probably do something fancy with them at some point.

 

PS apologies for shouting you with no other text - Chrome seemed to develop a mind of its own and posted the reply without my consent :o 

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If I'm doing headlights I'll wire 2 white 5mm in series with a 1 ohm resistor to act as a current limiter/fuse, and connect direct to the Rx.

I'm about to do some over the next few days and could post some pics if it helps.

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I've used the LED calculator to calculate for 7 white LEDs in series and it seems to think I'd need a 20V battery to power that.  I've just checked the Killerbody set that I have and it has 6 white LEDs in parallel, so I think I'll need to wire the whites in parallel at least for this application.  I'm sure I can work that out - just need to know what type of LED to order.  I'm guessing 90% of the ones on RS are pretty much identical but knowing me I'll guess and buy the wrong ones :D

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Thanks @IoWBasher that's royally awesome, everything I need in one shop!  Got what I (hopefully) need on order, will get everything wired up next week when it arrives and should be able to hit the trails in the dark and see where I'm going :D

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No worries, my pleasure. ;)

Did an order myself, I'm going to try out the Flat LED's. They supposedly have a wider spread of light.

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Mad Ax,

Red and green LEDs usually have a forward voltage across them of about 1.7 V with 20 mA current passing through them.  White and blue LEDs usually have a voltage drop of about 3.5 V with 20 mA current passing through them.

If you're going to power all of the LEDs from a receiver, then you can only wire the two red ones in series.  So for that circuit you'll need (5V - (2*1.7)) / 0.02 A = 80 ohms.

Then for the white LEDs, each one will need to be powered through its own resistor.  So you end up with 7 white LEDs and 7 resistors, essentially 7 copies of a LED/resistor pairing.  Then the 7 pairs of white LEDs/resistors and the one trio of red LEDs/resistor are all connected in parallel with each other to the receiver.  The resistor for the white LEDs is (5V - 3.5V) / 20 mA = 75 ohms.

Now, honestly, 20 mA will give you some fairly bright LEDs.  You'll also be pulling 160 mA from the receiver continuously since there are 8 LED circuits in parallel with each other.  I might consider doubling the resistor value and just use 150 ohms for each circuit so each circuit draws only ~10 mA for a total of ~80 mA coming from the receiver.

The issue with the white LEDs is if you connect them in series then the forward voltage needed to light them up is higher than the source voltage coming from the receiver (5*3.5 = 17.5V, which is way higher than 5V from the receiver).  You won't see any light or barely any light at all.  Even two white LEDs in series needs 7V, still higher than the receiver voltage.

LED controllers like Tamiya's TLU-01 or some of the third-party controllers include a voltage booster circuit that closes the loop on current.  In other words, the LED controller might take in battery voltage or receiver voltage, and then generate as much voltage as is necessary to make sure the LEDs are getting 10-20mA of current each.  One of the GT Power LED controllers I have for tractor trailer trucks actually has a programmable parameter for setting LED brightness; all it does is change the target amount of current to flow through the LEDs.  The voltage goes to whatever level is needed to make it happen.  So, the reason why some of these LED controllers cost some money is they do contain active electronics and a boost power supply; it's not a bank of resistors and some simple switches to turn things on and off.

Long story short, 8 circuits -- 1 circuit with 2 red LEDs and a single 150 ohm resistor, 7 more circuits each with 1 white LED and a single 150 ohm resistor, all circuits wired in parallel and connected to the 5V receiver.

If you're interested in learning how to read a LED datasheet with the voltage/current/brightness curves and how to use Ohm's law to calculate resistors, I can lay it out in more detail in the "Build Tricks and Tips" section of the forum.

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21 minutes ago, speedy_w_beans said:

If you're going to power all of the LEDs from a receiver, then you can only wire the two red ones in series.

That depends on ESC/Receiver setup, the voltage output of modern ESC varies some have 5 other 6 volts, some can even be adjusteted up to 7. 

A quicrun 1060 for example has 6V/3A, the quicrun crawler esc even has 6v or 7.4v

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1 hour ago, waterbok said:

That depends on ESC/Receiver setup, the voltage output of modern ESC varies some have 5 other 6 volts, some can even be adjusteted up to 7. 

A quicrun 1060 for example has 6V/3A, the quicrun crawler esc even has 6v or 7.4v

Good point on the different BEC voltages.  The key thing is the source voltage needs to be the same or higher than the sum of the LEDs you want to string together in a basic LED/resistor setup.

So, at 6V you would still struggle to make a pair of white LEDs in series light up.  But at 7V (and above) a pair of white LEDs could be powered in series no problem.  So you could cut the number of circuits in half with a higher source voltage.

6V should support three red/green LEDs; 7V could support four red/green LEDs in series.

For some reason I assumed 5V for the BEC voltage, but you're right, other voltages are possible.

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Thanks for the additional advice :)

I remember the basic stuff like VIR and PVI from college but I get a bit confused once things get a bit more complicated.

So for now I'm planning to run 7 white LEDs in parallel with a single resistor in series.  I've calculated based on nominal 7.4V from the Quicrun BEC.  The red LEDs will be in series with their own resistor.

I might end up adding an additional resistor to the white circuit or even to the whole circuit if it's too bright.  I don't need to overdo it.  It'll also be on a manual switch under the body so I'll only run the lights when I need them.  My Budget Bruiser has a 4ch receiver and I run the lights via a transmitter switch and a powered switch module mounted on the 4th channel; I run the SCX10 on a 3ch transmitter and ch3 is reserved for the winch, so until I can afford to buy a new radio system I'll be switching manually.

I'll try to document my progress so if anything goes hideously wrong someone can hopefully point me in the right place.  If it all goes well then I'll have enough spare LEDs lying around to fit out most of my other cars :o 

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