# PWM fade effect with two 1-watt LEDs

I've got a project that involves getting two LEDs to fade in and out using a PWM signal generated by an Arduino. The thing is, I need to use 1-watt LEDs to achieve the brightness I'm looking for (the LEDs are illuminating a 2"-diameter acrylic rod from both ends). I know how to fade lower-power LEDs directly and I how to use an Arduino to control independently-powered LEDs, but I have no idea how to combine the two. Any suggestions?

KalebPSpector: Any suggestions?

Yep. 1W LEDs run at 350 mA, so you want a constant-current supply. For 350 mA, R in the above circuit is 2.2 Ohms. If your LED supply is greater than about 7V, you can put your LEDs in series. You may however require a beefier transistor than a BC547.

I was going to use a 3.3V supply on the LEDs. I've used a circuit like this before to control power to a servomotor but it only had one transistor; what does the second one (the lower one) do here? Also, is the 2.2-ohm resistor actually necessary or could I short it? I know sometimes you just need some kind of load for things to work right so I figured I'd check.

No, you should follow the circuit. The pair of transistors and 2.2R resistor together form the constant-current circuit. Without this, the current would increase as the led warms up and burn it out. For supply voltage, you will need at least 1.5V more than the led forward voltage. The 2.2R should be at least a 1/2 Watt rated.

PaulRB

The only reason I can imagine to use a 3.3V supply is to operate it from a single Li-Po cell, in which case you would need to use a different circuit for white LEDs as they only just function at 3.3V (and two separate control circuits, one for each LED).

The preferred way to use that circuit is to put the LEDs in series so you only need one control circuit.

It really is quite fascinating to have someone ask for a circuit, then when the correct circuit is given ask “But can I just remove half of the components?”

Sort of like asking “but the Intel I5 CPU is too expensive, can’t we just use a NE555?”

Not really a surprise. We get that in medicine too: “Couldn’t I just take this herbal remedy or some vitamins?”

Paul__B: It really is quite fascinating to have someone ask for a circuit, then when the correct circuit is given ask "But can I just remove half of the components?"

I'm a young digital electronics engineer so my questions were honestly an attempt to understand how the circuit works more than anything else. I don't like black box circuits because I can't use or adapt them to situations other than the one it's originally presented in (that formula in your image is really helpful). Thanks for all your help!

Also, the BC337 is rated for 800mA so I'm guessing that should work for 2 LEDs in series?

Of couse, even for 3, 4, 5... If you place something in series the current stays the same... (350mA here...) You only need enough volts because you have to add them in series. As a electronics engineer you should know, it's electronics 101.

And how it works, it the arduino is on the base of the upper NPN is pulled high, turning it on. With a BJT (transistor) the voltage across the base and emitter (Vbe) is around 0,7V if the BJT is on. But turning on the lower NPN would pull the base of the upper to GND and turn of the upper NPN. So by calculating the R in a way there is 0,7V across it when the desired current is flowing an equilibrium is created in a way the upper NPN is on and passing the desired voltage.

Because if the current is to high the voltage across the resistor will increat, turning the lower NPN more on and thus turning the upper off.

The other way around, if the current is to low the voltage across the resistor drops, turning the lower NPN more off thus turning the upper NPN more on.

septillion: Of couse, even for 3, 4, 5... If you place something in series the current stays the same... (350mA here...) You only need enough volts because you have to add them in series. As a electronics engineer you should know, it's electronics 101.

Haha you're right. My mind must be frazzled trying to figure out my graduate course schedule :P Thanks for the explanation!

KalebPSpector: I've got a project that involves getting two LEDs to fade in and out using a PWM signal generated by an Arduino. The thing is, I need to use 1-watt LEDs to achieve the brightness I'm looking for (the LEDs are illuminating a 2"-diameter acrylic rod from both ends). I know how to fade lower-power LEDs directly and I how to use an Arduino to control independently-powered LEDs, but I have no idea how to combine the two. Any suggestions?

Bro , can you give me circuit diagram that works without arduino , i want to make this do breathing effect / Fade effect with two 1w leds . Can you please help , because stuff i found until now only works for normal 3v led .

YKM999: Bro , can you give me circuit diagram that works without arduino , i want to make this do breathing effect / Fade effect with two 1w leds . Can you please help , because stuff i found until now only works for normal 3v led .

OK, you seriously need to explain what microprocessor you imagine using instead of an Arduino? :cold_sweat: The circuit has already been given in my first reply.