Newbie Fading lamp project

[Halford]

Hi everyone,

I'm going to build a small fading color lamp for my kids and as a good newbie I'ld like to see if the idea I got could work or not.

I know there are several ways to do it, but this is what I came up with and I'ld like to know if it will work.

I'll be using 5050 rgb smd LED that have 6 pin (one package 3 colors all independent), each color use a 20mA. I would like to drive them with a Attiny85. My thinking is that I will put the leds in serial by color and let the Attiny drive 3 NPN transistors  to fade the colors. In my mind I should use only 3 pins from the Attiny (one of each color/transistor).

There will be not many leds, under 10 and the Transistors I have they are rated up to 200mA, so I should be good on that side. (using 7 leds I should be at maximum 140mA per color, and I don't plan to use them full brightness)

Am I on a working path or is it going to be a failure ?

Thanks for your input

Hal.

[Simon]

How will you fade the LED's ? PWM ? make sure you have enough channels or be prepared to do software PWM. 140mA is ok in principle but what is more important is power dissipation. Work out how much power the transistors will dissipate and check the datasheet. If your switching at a speed this in itself will cause losses in the transistors

[Codemonkey]

If your LED's are rated at 20mA max and you're connecting them in series, then they will still only require 20mA per string.

Personally I'd use some small logic level MOSFET's for driving them rather than NPN bipolar transistors.

[Halford]

thanks guys,

the fading will be slow... kind of a night lamp.

I will check power dissipation, I did not think of it at all, thanks for pointing that out.

low level mosfet, I'll have to read about it, I don't use mosfet often but to power protect ... I know, I'm a newbie I'll do some researches

Thanks

Hal.

[Halford]

so a N-channel Mosfet would be more efficient and will be used exactly as I was planning for the transistors right? I see there are even dual n channel mosfet to save space. Moste of them have higher A values then the transistors, this is always welcome.....

[Simon]

if your connecting the LED's in series then only one transistor is needed. Yes the mosfet will be more efficient, for starters you don't need to supply it current to turn it on. It just needs to see a voltage and with it's very high input resistance you can turn it on with static!