Thanks RyanI (@StuckInSynth) I will have to find that LVM321.
The closest opamp I have on hand is the nte928m, It works with a +5 supply, and has a rail-to-“almost rail” output, zero to 3.8 volts. I had noticed the littleBits all use 321s, but have not been able to get my hands on one, nor any other single-supply & rail-to-rail amplifier (do others exist?), so I’ve compromised my designs so far to use the 928. For things like lighting LEDs, it works fine, since the voltage needs to be dropped for the LEDs anyway.
But some applications require the full 5 volts however, so the 321 will be needed. For example I spec’ed the fan for my co2 sensor to run on 5 volts to give 0.2 L/min through the detector. It was difficult to go as low as 5 volts. So if I put an opamp that drops the output below 5 volts, I’ll need to find (and calibrate!) another fan (which I don’t want to do. 321 to the rescue!)
… to be continued.