ledmicrocontrollerpwmrgb
Sorry if this is too basic but I don't understand this issue.
Right now, using an MCU, I change the RGB colours of an LED strip with 3 PWM signals.
All colours works great .
The thing is, when I am trying to set the intensity of the light, I don't know how to do it.
So if, for example, I have RGB(15,170,230), then I want to slowly reduce the intensity of the light, keeping the same colour, how would I change the RGB to get that?
The problem ended up being a PWM issue. (It was solved in the comments, however I've posted the following to help future readers of this.)
First switch the Red or Blue with the Green. This will determine where the problem lies. If the Red or Blue still doesn't work then the problem is with the LED (very unlikely) or somewhere in the connection. Otherwise it is a software problem, like incorrect use of PWM.
Helpful links:
What commenters are saying is that your current design without something to boost the voltage will never work. Take a look a this page from the datasheet . If you draw 15mA from a pin there will be a .6V drop from Vcc across that pin. So now you starting at 3V trying to drive a 3.1V LED.
As for the software side you do not need very fast PWM. People stop seeing the flicker at a pretty low frequency. So say you want 120hz for your PWM. Have an interrupt fire at what ever number of brightness steps you want times 120hz. If you want 20 brightness levels then interrupt every 20*120hz or (1/2.4kHz) ~= 400us. Each time the interrupt fires increment a counter and compare it to your brightness value(duty cycle) for each of your LED. If its greater then toggle the LED. You can adjust the frequency and brightness steps depending on your timing requirement and number of steps you need. If you need the ISR to be really fast you may need to write it in assembly.
Or you could use something like this http://www.ti.com/product/tlc5971 with a 5V boost converter.
Best Answer
Start by scaling the R,G and B by the intensity value, for your example at 50% intensity you would set 15*0.5, 170*0.5, 230*0.5 (7, 85, 115).
The LEDs might have different non-linear responses, so you might need to tweak the scaling for perceived colour
In particular, at very low levels, you will suffer from rounding error. There is no easy way around this except by increasing the colour resolution (e.g. to 0-1000 instead of 0-250) or selecting devices with a more appropriate dynamic range for the application (brighter or dimmer LEDs).
Another alternative if you are using multiple devices is to switch a subset of the devices on to give different intensity levels, for example if you have 6 RGB LEDs you could switch 3 to (1,10,10) and 3 to (0,10,10) to add resolution, effectively (0.5, 10, 10) overall.