Electronic – arduino – Connecting an external ADC to Mega 2560

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.

Your problem is likely logic level mismatch. Since your RAM IC is powered with 3.3V, its output HIGH level on the SO pin can't be larger than 3.3V. Your microcontroller however seems to use 5V (looking at the schematics). Now I haven't consulted AVR datasheets, but most CMOS chips recognize a HIGH level when the voltage on their inputs is larger than 0.7Vcc. This threshold is 0.7*5V = 3.5V, so the microcontroller has every right to not recognize 3.3V as HIGH. This may well explain why you're reading all zeros from your RAM.

Note that the maximum input HIGH level your RAM tolerates is Vcc+0.3V (table 1.1) which in your case equals to 3.6V. That's much smaller than the 5V your MCU outputs, so you might have cooked your RAM already. The difference is certainly enough to bias protection diodes into conduction. Some 3.3V ICs have 5V tolerant inputs, this one does not.

Your options are:

• Run your MCU at 3.3V. This is often best if your MCU supports that voltage (I think it does) as it avoids the need for level translation completely. Unfortunately this hardly seems possible within the confines of your platform.

• Increase Vcc of your RAM (datasheet says it can tolerate up to 3.6V). However, this is going to be unreliable since the threshold is only marginally exceeded, so any voltage variations (and even internal variations between individual chips) may well lead to problems. This also doesn't solve the input level mismatch, so half of the problem remains.

• Do logic level translation in both directions. Many solutions to this problem exist, you should search the web (including this website) to get the picture. In short, it's easier to lower the voltage from 5V to 3.3V (a divider is enough, perhaps with a diode) than to do the inverse (you'll need to use a FET or an IC as a buffer). This appnote shows some circuits, as does this page. See also this product from Sparkfun for a similar idea. HEF4050BT is a buffer IC with absolute input thresholds (HIGH is 3V) and can tolerate up to 15V on inputs. I'm sure more possibilities exist, but I personally believe it's best to power your microcontrollers with 3.3V and save the trouble. Many useful ICs aren't 5V compatible (or tolerant) these days.