when a zener fails me.

[ja421]

I have a small project that I need to have a digital trigger when the voltage is above 12v.  so I bought a few 12v zener diodes.
The problem is they trigger closer to 15v than to 12v.
Here's the problem I am trying to find a solution for...
I need a 12v+ "trigger" signal; but the source has the V+ always hot, the ground floats, or goes to ground.  When it is floating it's possible that it's allowing 6 to 7v thru.   So I can't use the source's V+ alone, I have to use it conjunction with the ground.  My idea was to use the zener and an optoisolator so that the optoisolator triggers when there's 12 or more volts (and then pass 12v thru the other side).  I wanted to use the zener in hopes that it would use less energy than a pull up resistor... but it seems a pull up resistor is going to be the better route to take.  before I redo it; wanted to check if that's the best way to go, or if there's another simpler way I am overlooking.
Thank you.

[tatus1969]

you may use a TL431 instead of a zener, that will improve your setpoint accuracy and makes it also adjustable.

[Zero999]

Please post a schematic.

In fact the TL431 has an internal op-amp which can be used as a comparator +to trigger something when the voltage exceeds a predetermined threshold.

See page 28 of the data sheet.
http://www.ti.com/lit/ds/symlink/tl431.pdf

[ja421]

Thank you... I ordered a tl431 ... thanks for the idea!