I'm trying to design a mood light system using 4 RGB LEDs which will be powered by an external 9V source and an Arduino for processing.
- 3 x 2n2222 transistors (NPN)
- 4 x 5mm LEDs (Common Cathode)(http://www.jaycar.co.nz/products_uploaded/ZD0012%20-%20AL-50-30RGBC-C-004.pdf)
- 3 x 10k pots
- 3 x 1k resistors (PWM to Base)
As you can see, what I've attempted to do is:
- connect each LEDs common cathode to ground
- 9V to each (R)(G)(B) collector
- Arduino PWM output to each (R)(G)(B) base
- emitter to each (R)(G)(B) anode
What I was expecting is that the PWM output of Arduino would regulate the emitters voltage between 0~9V therefore being able to dim the LED with my pots.
What I have instead got is that the voltage is 0.7V short of what the base is receiving from Arduino and the voltage supplied to the collector is being ignored. In fact, I can even disconnect the 9V battery, and the LEDs would still be lit.
From a previous question, I believe this is emitter-follower behaviour?
Is there a way I can re-wire this and get the expected behaviour? Or will I need to get some different parts. I got given the wrong LEDs (common cathode instead of anode) but the shop isn't open for another few days to go get a replacement so I'm trying to make do with what I have.
Please ignore the lack of resistors between the emitter and LED. I have omitted them for simplicity.