I forgot I drew this earlier... I'll bet my diode polarity is backwards, too. W/e
The 180 ohm resistor limits the base current to 20mA, which is fine for most small transistors. When the voltage is above that of the Zener diode's rating, the circuit is on. The diode stops passing current when the voltage drops too low, and the circuit shuts off. Just need to find a suitable NPN (could be PNP as well with reversed batt. polarity) transistor to handle the current.