A zener can be used as a simple voltage regulator when the voltage source has sufficiently high resistance. This is called a "shunt regulator". It regulates by loading the power source so that it doesn't exceed some voltage. Shunt regulators have their purpose, but are used in special applications because they are pretty inefficient in most applications.
I took a brief look at the schematic, and there seems to be signficant confusion there. Post it here instead of a link to make it easier to talk about.
Added now that the schematic is more accessible:
I don't know where to begin explaining so I'll just enumerate some of the problems:
- LEDs need to be driven with current, not voltage. The current as a function of voltage is rather unpredictable near the intended operating point. With the LED test switch closed, you are applying a fixed voltage to the LED. That could be bad if the LED would normally operate at a little less voltage than the battery.
- Since the LED requires about the same voltage as the battery, you won't get much of anything from it with a zener in series. Even a single forward diode drop would drive the LED with so little voltage it probably wouldn't put out noticable light. The 2.5V zener will drop all the battery voltage and leave nothing for the LED, so the LED will never come on.
- You say you don't want to run the LED at 1 mV, but that won't cause any problem since it won't draw any current at that voltage.
You need to step back and explain what you are really trying to accomplish. This circuit it too broken to bother with it as a starting point. If you provide clear specs as to what you want the LED current to be as a function of the photoresistor resistance, we can probably help. It would also help if you provided a link to the LED datasheet since 2.5V is unlikely for a IR LED.
Added after more infomation from the OP
You now say: I want a voltage regulator that allows current through if it is _ volts or amps or whatever unit it uses.
Learn the units. That's how we communicate. That will also force you to think more clearly. A big part of the problem is that you are contradicting yourself, probably in part because you are using the wrong terms. Whether you understand the terms fully or not, you can at least be careful. Above you say if it is but once again it's not clear what "it" is referring to. There is good reason we don't allow hand waving in engineering.
I'm looking for something similar to a "crowbar" circuit.
OK that's more clear and is what I started out thinking you were asking about but you later contradicted. As I said originally, such things are called "shunt regulators". A true crowbar is something else, but I don't think you mean that. It now seems you want something that conducts no current until a voltage threshold is reached, then conducts current after that with the fixed voltage offset accross it. That is basically what a zener diode does, and what a shunt regulator does. A zener diode can be used as a simple shunt regulator. More complicated shunt regulators use active electronics to make a sharper knee.
However, once again, I think you are getting bogged down in details of whatever implementation you suppose instead of the true problem. That apparently has something to do with driving a LED a particular way depending on light level. I asked before what the LED current was supposed to be as a function of the photoresistor value. Since you have refused to answer that, I am giving up now.