protection - Circuit to protect against undervoltage?

What is the simplest way to regulate the min DC voltage in a circuit? Is it possible to do with zener diodes?

Desired performance:


Input > 3.3 VDC     Output = Input
Input = 3.3 VDC     Output = Input
Input < 3.3 VDC     Output = 0.

Answer

According to your description, something like this should work:

The TLV3012 has the reference voltage built-in, so that's 1 6-pin IC, 1 FET, and 2 resistors. Not sure if you consider that "simple". :)

Similar:

Using a pMOS to switch a load is summarized here.

You could probably use a Zener as the reference for the comparator, too, or maybe rig something up to drive the MOSFET from the Zener more directly, but I don't know how precise that would be.