Is it ok that the resistors aren't connected directly to the positive? Any advice, warnings, or rule of thumbs?
Are there any problems with this simple Arduino LED Wiring
arduinoledresistors
Related Solutions
You definitely do not just have a 6V battery and 6 LEDs in series. Regardless of bad design or not, this just wouldn't work. Eash LED would only get 1V, which is well less than the 3V or so most white LEDs need to operate at reasonable brightness.
So let's forget how you think the board is wired and treat it like a black box. You say it seems to work correctly when the two wires coming off the board are connected to 6V. It would be good if you had actual specs on that or explained where you got this from, but for now let's assume this is the correct way to power the board.
In that case, what you are really asking is how to switch 6V with a microcontroller. Part of the answer will depend on how much current. Are these little 20 mA LEDs or high powered lighting LEDs? Either way, a low side N channel FET switch is probably the easiest way to interface this with a microcontroller output, but the choice of FET depends on the current. For example, I like the IRLML2502 since it can be directly driven from a logic output and has decent current capability and low Rdson for such a small part. It can handle over 3A at 70C ambient temperature. If your board draws 3A or less when 6V is applied to it, then that's all you need. If more, then you'll have to carefully chose a different FET and perhaps different drive method than simply connnecting the microcontroller output to the gate.
The ATmega32U4 datasheet recommends not connecting (unused) pins directly to ground in case the pin gets configured as an output. VIL is defined as approximately 0.2VCC maximum, which with a pull-up of 20k means a pull-down of no more than about 4k7. With 5V this means approximately 200uA at most will be consumed by each switch when held on. As long as your PSU can accommodate all switches being on, there should be no problem with this configuration:
simulate this circuit – Schematic created using CircuitLab
Best Answer
No problems.
It doesn't matter what side of the LEDs the resistors are on. The positions of each LED and resistor could be swapped without affecting operation.
Because the LEDs share a common ground, you will need to use digitalWrite(pin, HIGH) to make them light up. If the LEDs shared a common positive instead, you would need to use digitalWrite(pin, LOW) to make them light up.
The only thing that disheartens me about the photographs is the way the component leads have been bent, and how the components are at random angles. I would have a resistor from ground to j30, then an LED from h30 to h29, then the cable to the Arduino in f29.