The chip that I had hoped to use for the EL wire power supply is not going to work. It puts out about 75 VRMS as measured on an RMS meter and checking with a scope, this is correct considering the duty cycle and the wave shape (close to square wave with big dead times). Unfortunately, it is not enough.

I looked at a commercial EL wire inverter and it is putting out about 130VRMS (300VPP at 3KHz) and it is fairly sinusoidal in appearance. The voltage slew rate is high enough that it may prevent the opto-triacs from switching off as the voltage goes through 0V. It is still worth trying the commercial inverter, if it does not allow the opto-triacs to switch off, I can continue with building my own inverter.

Numerous sources say that you are not supposed to disconnect the load from an EL wire inverter while it is powered up. My solution to this is going to be to put a segment of wire on the board that is always connected, so the inverter will never be run without a load. The "always on" segment will be long enough that the smaller loads from the wire segments should be negligible.

When I designed the board, I planned to cut the EL wire to the proper length and strip both ends. The center (driven) end of the wire would have both leads soldered to pads visible on the board. The outer end would have the center conductor soldered to a pad just to keep the wire in place mechanically. After stringing a couple of wires in this way, it is a huge pain to get the lengths accurate after stripping, and the wire will not lay very straight. It looks bad. The new plan is to print some little guides that will be glued to the board and the wire will only be connected to the board at the center end. The guides will have a channel for the hour and minute wires and keep the wires straight. This cuts the number of prepared ends on the wire in half so it should be a lot less work. It should look much better as well.

I started the software development so that I could test the SPI slave interface that drives the opto-triacs. It is a 24 bit slave interface built from 6 octal D flip flops, using three configured as a shift register and 3 configured as an output latch. It works as expected. A series of switches and buttons are on the board to handle manually setting the clock and setting the time zone when the GPS sets the clock. The switches and buttons go into GPIO inputs on the LPC1114 board. I wrote and tested the code handling the switches over the weekend.