LED Issues

I bought two LEDs from Home Depot, one was a 3watt 3.7v LED and another was a 'garden' 12v LED light.

I noticed with a PWM enabled on the Arduino, I was able to get the 3watt 3.7v LED to work just fine. The garden light had no chance, it did not want to light up at all, not even on the Vout of the arduino (5v).

it was not until I plugged in a 6v lantern battery, where it lit up completely. So to me, it sounds like some LEDs don't have the ability to dim? (I'm new to the whole LED thing). I tried doing the whole DAC thing as well using a low pass filter to ensure that it wasn't a PWM issue with the garden light.

What I'm trying to do is enable these LEDs to dim but I need some nice lumen output, can anyone make any recommednations for me, for dimmable LEDs if not all LEDs are in fact dimmable? Something preferably I can nab at Lowes or Home Depot...

Without knowing anything at all about the LEDs, how you connected them, etc. there’s not much people can do to help.

RET80:
I bought two LEDs from Home Depot, one was a 3watt 3.7v LED and another was a ‘garden’ 12v LED light.

The garden light had no chance, it did not want to light up at all, not even on the Vout of the arduino (5v).

The clue might be in the “12V” part of “12v LED light”…

fungus: Without knowing anything at all about the LEDs, how you connected them, etc. there's not much people can do to help.

RET80: I bought two LEDs from Home Depot, one was a 3watt 3.7v LED and another was a 'garden' 12v LED light.

The garden light had no chance, it did not want to light up at all, not even on the Vout of the arduino (5v).

The clue might be in the "12V" part of "12v LED light"...

Well if that's the case, why did it work just fine with a 6v lantern battery, but not a 5v arduino. Unless it has a cutoff voltage, either completely on or completely off.

RET80:

fungus: The clue might be in the "12V" part of "12v LED light"...

Well if that's the case, why did it work just fine with a 6v lantern battery, but not a 5v arduino. Unless it has a cutoff voltage, either completely on or completely off.

Did the Arduino's power LED go off when you connected it? That's a clue...

fungus:

RET80:

fungus: The clue might be in the "12V" part of "12v LED light"...

Well if that's the case, why did it work just fine with a 6v lantern battery, but not a 5v arduino. Unless it has a cutoff voltage, either completely on or completely off.

Did the Arduino's power LED go off when you connected it? That's a clue...

Yes, the Arduino's power LED remained on.

For those of you wondering what light I purchased, this is it:
http://www.homedepot.com/p/Malibu-Low-Voltage-Mini-LED-Flood-Light-8406-2606-01/203206443

It’s a fantastic light, but I want to figure out (if any) a way to make it dim! especially using a PWM.
But by the looks of it all 6v will turn it on really brightly, but if I plug it into a 5v of the arduino, nothing happens, it’s not slightly dimmer, it’s just completely off.

I thought LEDs normally fade as less voltage is applied to them. Is this necessarily true? Or are some LEDs just designed to be cut off at a certain voltage?

In general, you can't connect an LED directly to the Arduino. At a minimum you need a current limiting resistor, and for "high-power" LEDs (and 1 or 3W is considered high power) you need an external power supply and a transistor or MOSFET to "boost" the voltage & current.

If you connect a 3W LED directly to an Arduino, you risk frying your Arduino. If you hook-up a regular LED (like the one on pin 13 of your Arduino board) directly without a current limiting resistor you may blow the Arduino and/or the LED.

The Arduino puts-out 5V at 40milliamps or less or less (the actual current and power depend on the load per Ohm's Law). That's a maximum of 0.2 Watts. A regular LED is usually about 0.04 Watts (40 milliwatts) with some additional power also being consumed in the series resistor.

LEDs from the hardware store sometimes have some additional circuitry in them, so I don't know what you have. You can't connect an LED directly to a constant voltage supply, it has to be current-controlled/limited. The circuit in the hardware-store may be a current controlled power supply, but that does NOT mean you can connect it directly to an Arduino. And, this circuit may make the LED non-dimmable.

I thought LEDs normally fade as less voltage is applied to them. Is this necessarily true? Or are some LEDs just designed to be cut off at a certain voltage?

Yes and no… Mostly no… :wink: Like all diodes, LEDs are extremely non-linear… You have to control the current, not the voltage. An LED rated at 2V LED might not come on at all at 1V, and it might get tons of current and fry at 3V. You power it with a current-controlled source and the voltage “falls into place”. The actual operating voltage may vary from part-to-part and with temperature, so even if you could very precisely control the voltage and dim the LED that way, it’s not practical and it’s never done that way.

DVDdoug: In general, you can't connect an LED directly to the Arduino. At a minimum you need a current limiting resistor, and for "high-power" LEDs (and 1 or 3W is considered high power) you need an external power supply and a transistor or MOSFET to "boost" the voltage & current.

If you connect a 3W LED directly to an Arduino, you risk frying your Arduino. If you hook-up a regular LED (like the one on pin 13 of your Arduino board) directly without a current limiting resistor you may blow the Arduino and/or the LED.

Strange, I have a 3watt LED device that worked perfectly wiith PWM and it ran for 3 weeks straight and it didn't seem to harm the arduino at all.

I'd still like to use this other 3 watt LED device, would you suggest just putting a limiting resistor on it? I really don't want to bring in an external power supply...

The reason being is that there will eventually be 15 of these lights hooked up to it in the end and the arduino is going to manage these lights and their responses as people walk by using a sonic rangefinder...

DVDdoug:

I thought LEDs normally fade as less voltage is applied to them. Is this necessarily true? Or are some LEDs just designed to be cut off at a certain voltage?

Yes and no.... Mostly no... ;) Like all diodes, LEDs are extremely non-linear... You have to control the current, not the voltage. An LED rated at 2V LED might not come on at all at 1V, and it might get tons of current and fry at 3V. You power it with a current-controlled source and the voltage "falls into place". The actual operating voltage may vary from part-to-part and with temperature, so even if you could very precisely control the voltage and dim the LED that way, it's not practical and it's never done that way.

Great advice! Any way I could check to see the operating range of this 12v LED I have? stick it onto a scope and monitor the current?

I ran across the street once with my eyes closed, nothing happened.

A 3W LED needs a switching transistor or MOSFET to switch the current (best part of an amp usually). Normally a constant-current LED power supply would be used to power the thing, or you could use a large, high-power series resistor and run from 5V, but that's wasteful.

If you deal with large currents or large powers you need appropriately rated devices or they fail. Maybe not today, but at some point, as they are stressed too much.

Arduino pins can take 20mA comfortably, that's enough for a small LED. 40mA is the absolute limit - beyond that and the internal transistors are becoming too hot to stay reliable, permanent degradation or total failure may result. If your Arduino survives, that's luck, not judgement (devices vary due to manufacturing spread).

MarkT: A 3W LED needs a switching transistor or MOSFET to switch the current (best part of an amp usually). Normally a constant-current LED power supply would be used to power the thing, or you could use a large, high-power series resistor and run from 5V, but that's wasteful.

If you deal with large currents or large powers you need appropriately rated devices or they fail. Maybe not today, but at some point, as they are stressed too much.

Arduino pins can take 20mA comfortably, that's enough for a small LED. 40mA is the absolute limit - beyond that and the internal transistors are becoming too hot to stay reliable, permanent degradation or total failure may result. If your Arduino survives, that's luck, not judgement (devices vary due to manufacturing spread).

okay-okay then.

If that's the case, how would I be able to implement a proper 3W lighting system that demands something far beyond 40mA of power without the use of an external power supply.

Here's the "problem"

I am designing an interactive lighting system when folks walk near an art piece, it lights up. There are a total of 15 art pieces and I need to make this as cost effective as possible, so I was thinking of having a single unit (a single MEGA ADK) handle 15 sonic sensors and 15 lamps individually.

You're all probably laughing right now at the idea and how impossible it may seem.

so if that's the case, how could I implement it if it was just one sonic sound sensor and 3 watt light beyond 50mA on a PWM ?

You're all probably laughing right now at the idea and how impossible it may seem.

Yep.

and 3 watt light beyond 50mA on a PWM ?

You feed the PWM output of the Arduino to the gate of an FET through a 120R or greater resistor. Put the source to ground and the drain to the -ve of your LED. Then connect the +ve of your LED to the +ve of an external power supply and connect the -ve of that supply to the Arduino ground. You can use the same external supply for multiple LEDs as long as it can handle the current.

Grumpy_Mike:

You're all probably laughing right now at the idea and how impossible it may seem.

Yep.

and 3 watt light beyond 50mA on a PWM ?

You feed the PWM output of the Arduino to the gate of an FET through a 120R or greater resistor. Put the source to ground and the drain to the -ve of your LED. Then connect the +ve of your LED to the +ve of an external power supply and connect the -ve of that supply to the Arduino ground. You can use the same external supply for multiple LEDs as long as it can handle the current.

haha thanks! Mind doodling up a rough schematic to help me understand?

Make R1 120R or greater. If there is current limiting on the LED modules leave out R3.

1JtOP.png

RET80: how would I be able to implement a proper 3W lighting system that demands something far beyond 40mA of power without the use of an external power supply.

You can't.

(very obviously, if you stop 'hacking' for a moment and think about it....)

fungus:

RET80: how would I be able to implement a proper 3W lighting system that demands something far beyond 40mA of power without the use of an external power supply.

You can't.

(very obviously, if you stop 'hacking' for a moment and think about it....)

I know I can't, but I'm trying to figure out any way possible to do so, that's how innovation happens, man; just trying to figure out any configuration possible that would be cost effective and useful.

I know I'll have to take a different route with this but by having this dialogue, I'm able to guide myself more. I'm a software engineer by trade, not an electrical engineer so I'm acquiring this knowledge as I go along, so what may be easy and intuitive for you, may not be so much for myself, but I am learning and understanding.

So any and all suggestions are greatly appreciated for this project (I know this thread evolved more into project guidance) but I'm still trying to assess the best plan of action for these higher power LEDs in the most cost effective way possible, I have 15 of them I'd like to make and I'd rather not shovel $2,000 to make it happen....

Grumpy_Mike: Make R1 120R or greater. If there is current limiting on the LED modules leave out R3.

Thank you very much! I'll see if I can implement this tonight.