External 3.3v PSU

I am using a large number of ambient light sensors which I will be running at 3.3 volts as that seems to be recommended over 5 volts.

Since this is an external power supply and not the 3.3 v provided by Arduino I assume I must tie the external 3.3 volt (-) to the Arduino 3.3v (-, com) so that the digital sensing of the sensors state will be poper, that is have a proper reference point.

Please advise,

Barry Gordon

Generally you would tie only the power supply grounds together. But it's not clear what is powering your Arduino. Is it the external 3.3V supply or something else? It really would help if you could post a drawing of your setup so we don't end up giving you bad advice.

BarryGordon:
I am using a large number of ambient light sensors

Which sensors? How many? How far apart?
Without this information we can't say much about your project.

Since this is an external power supply and not the 3.3 v provided by Arduino I assume I must tie the external 3.3 volt (-) to the Arduino 3.3v (-, com) so that the digital sensing of the sensors state will be poper, that is have a proper reference point.

Definitely don't try to connect an external 3.3V to the Arduino 3.3V pin, unless you're looking to destroy stuff. You will have to share grounds.

Which Arduino?