Electronic – Connecting a microcontrollers ground pin to a reference above ground

Yes, you can connect the EN pin to ground through a transistor, and control the transistor with the microcontroller. You are correct, the 100k resistor prevents a short circuit and the current will be quite low, so it will not damage anything.

You may use an NPN BJT or an N-Channel Enhancement mode MOSFET, assuming you are using a positive voltage to do the switching. If you wanted to use a negative voltage (which you probably wouldn't since you're using a uC), then you would want to use a PNP or P-channel Enhancement mode MOSFET.

As already mentioned, since the enable threshhold rising-edge voltage only has to be 1.8v, and the disable falling-edge voltage has to be a maximum of 0.4v, then you can control it directly with your microcontroller, but make sure you don't forget the pull-up (which you already have). If you left it floating, it could cause some funky behavior and possibly even damage your circuitry.

I realize I am not the first person to answer your question, so only accept my answer if the others who already posted comments don't re-post as answers.

What you're talking about is not huge risk for a hobby project.. a few things to think about.

It's not really the \$V_F\$ at the operating current that matters, it's the \$V_F\$ at the maximum 'off' brightness that is acceptable. That voltage might be as low as 1.4-1.5V volt for a low-current red LED- in a dark room they can be quite visible with microamperes of current. Driving the output to the 3.3V (nominal level) gets us 3.3V on the output. A fresh alkaline cell with minimal load might be 1.63V/cell at room temperature (just measured one), so 3 would be 4.89V. That leaves you with 1.59V across the LED + resistor (nominal, not allowing that the 3.3V might be a few percent low).

That's way too much to ensure it's not emitting a whole bunch of light.

So, we tri-state it- that allows the output to go maybe a bit above Vcc without much current flowing. 300mV is safe, the datasheet says 500mV absolute maximum. At 500mV, we'd have 1.09V across the LED, probably enough to ensure it's off, at least under nominal conditions. The 'absolute maximum' figure is never a good thing to design to, but usually there's a caveat on this particular figure that allows that voltage or a bit more if the current is limited.

So, I do think this will work (with tri-state, not push-pull), and I also think it's acceptable enough for a hobby project assuming nobody is going to be using a battery eliminator on the circuit in the future*. Keep in mind the margin on the red LEDs is minimal and consider eschewing red in favor of yellow or orange. Or, simply add a silicon diode in series with the red LEDs (one diode can be used for several LEDs).

• If the ESD protection network in the ATMEGA328P does begin to conduct it will tend to raise up the 3.3V supply, out of control of the regulator. This is not a good thing for stability and could conceivably damage something, though the ATMEGA328P-M itself is rated for 5V operation.

I once did something like this in a commercial product (to drive a series string of LEDs with high Vf using a 5V constant-current output), but I designed a power supply with just the correct oddball voltage and appropriate temperature coefficient to match the LEDs and thus optimize the situation. I think the supply was around 8-9V. Worked a treat, easily from -20°C to 80°C (spec was 0-50°C).