Controlling LED's with external 12vdc power

Projects General Guidance
1

I'm not looking for a "sketch" but for a wiring diagram (not schematic) in words or drawing on how to make a "SIMPLE" circuit that consists of 12vdc LEDs, powered by an external 12volt power supply, controlled via the Arduino board (Uno or Mega256).

Background: The LEDs are purchased as a "12v LED" (proper resistor and LED married) on a twin lead (R+, B-) wire.
I have both a "UnoR3" and "Mega256" Arduino. Configured properly and tested using SEVERAL sketches from Aruuino.cc I want to use an external 12vdc power supply to power the LEDs, not the built in Arduino 5v circuit.

Project: When completed the LED's (10 separate functions) will be controlled through the Arduino board via a Windows7 program that incorporates the proper sketch.

My specific problem at the moment is "How the circuit gets wired" in that I created a 10 LED parallel interface, all 10 (+leads) connected to a common connection point and all 10 (-leads) as separate wires. Bench testing interface works properly (Arduino omitted). Intention of using the Arduino to open/close the 10 independent circuits via a separate pin is not working. What am I missing? Having no formal Electronic or Electrical training (but quite a bit of practical knowledge) the logic escapes me. My logic - "I don't need to know how a carburetor works, I need to know where the gas pedal is".

ModWirArdTs.png

2

What you need is a circuit to drive the LEDS using a relay or MOSFET. I am assuming these are powerful LEDS otherwise simply put in place of the relay below.

The simplest way would be a ready made relay shield. The Arduino is used to drive the relay via transistor.

The do it yourself method involves wiring up a transistor to switch the relay. Use a 10k resistor to the base of a 2N2222 or BC388 or similar NPN transistor. The collector is connected to one side of the relay and the emitter to gnd. The gnd of the relay board is connected to the gnd of the Arduino.

If these are simply indicator LEDs, they still need drivers as above to isolate the 12v from the Arduino.

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