Hi fifahaywire,
you really haven't left out much to do wrong with your circuit.
The TL494 connections are wrong by about 90% according to TI datasheet page 6.
I suppose you chose that circuit for your design.
An optocoupler, even if overkill for your application requires a certain Vf and If for the on chip IR LED. The one on your chip requires typically 1.5V at If=10mA. With a 12KΩ resistor it won't do anything.
Pin5 of the optocoupler should be connected to ground. Before doing that you should remove the Zener diode between pin8 and pin5 to avoid the magic cloud of black smoke.
Boncuk