Designing PWM controller for coral aquarium

Projects General Guidance
1

Hey All,

A co-worker of mine is building a coral aquarium and I'm thinking of helping him with a light system for it. I want to design this around an Arduino, using PWM to control the LED intensity. My thought is to measure the current through the LEDs, compare this to the value measured from a potentiometer and modify the PWM accordingly to increase or decrease the intensity. I realize that I'll have to play around with the mapping of values but what I've got here is just a rough sketch. I'll use the PWM output to drive an NPN transistor that will control the gate of a P-channel FET which will drive the LED's. Then I'll measure the current through a resistor at the end of the LED string and vary the duty cycle based on this. Exact amount of LED's in the string will depend on their voltage drop and the power supply I end up sourcing (these LED's run in the range of 3-5V and draw a lot of current).

Attached is a rough sketch of the circuit I'm planning to use. The end design will have a few (at least 3) of these circuits, again depending on how many LED strings I end up using, which will depend on the number of total LED's he needs, their power requirements and the power supply I can source.

***I screwed up a bit on the schematic, cap should be on the other side of R4.

Any thoughts/info/suggestions here appreciated!

PWM_LED.png
PWM_LED.png417×552 2.82 KB

2

I don't see any reason to measure the current through the LEDs since that should be entirely predictable.

The only feedback that makes sense to me would be to measure actual light level so that you can adjust the LED brightness to compensate for changes in ambient light levels, but I don't know whether that's something you want to do. Otherwise, just use the potentiometer to specify the brightness and calculate the required PWM value to provide that brightness, using a hard-coded relationship which you will establish by trial and error.