I am making a Clock Module that should, in theory, run the LittleBen code from quinie. Here is my schematic so far (*Updated based on discussion below):
And my 75mm panel. I tried fitting it into 50mm, but that would be very tight for the components:
I am not sure that I have the start/stop interrupt input circuits correct. Does that look like it will work?
What I used in a recent design:
Note I used +5 V and a resistor on the collector. I guess the input could use the internal pullup, I didn’t really think about that. I also used a weak 1M pulldown to avoid having the base floating.
I will add that 1M to ground. Probably won’t change anything, but seems like a good idea.
My circuit has the collector connected to 5V through a 100k resister at the switch, which will pull it to ground when pressed. I think the two circuits will work the same way? Yours just makes the transistor pull to ground while mine does it with the button.
Right, I didn’t see the other instance of START. I have the push button connected (via Molex) to CLK_IN — and I should have put a resistor there, or connected it after the 100k, in case the input is connected to ground or some other voltage. Duh.
You should connect the *OE input of the 74HC595 to ground, CMOS inputs should never be left floating.
I will do that, and It seems I should also pull the SRCLR high. Also, maybe I should power with the 3.3v regulator on the nano?
Yes, the *SRCLR input should also be driven, I missed that.
Using 3.3V to power the '595 is probably a good idea as the input high threshold voltage would be marginal if powered by 5V but driven by 3.3V outputs from the micro-controller.
You might want to adjust the gain of the opamp output buffers to compensate for the lower output voltage of the 74HC595 when powered from 3.3V.
Looks like flipping my resistors would get me close to the same output voltage at 8.25V. I have considered configuring the opamps as comparators and getting 10.5V clocks.
Opamps as comparators, to save on resistors?
You would use 0V and 12V to power the opamps? Then you would have outputs that don’t go all the way down to zero.
Unless I’m missing something, doesn’t seem like a good idea.
It would be +12 and -12, and a diode could be added to keep the -12 to ground. Still probably a more complicated circuit if I consider that I would have to make sure my threshold voltage was correct.
Even though Analog Output made a module that covers a lot of what I want out of this one, I am far enough along to go ahead and finish it out.
Also, I needed more weird on the panel:
This could be used along with that clock to give more divided outputs.
Very nice weird! 
Updated schematic in OP based on discussion.
Do you really need the shift register? Without it you have D4–6, D12–13, and A0–A3 (which can be used as D14–D17) available, that’s 9 pins.
I have concidered that. I would lose compatibility with the Little Ben code if I removed it. I do plan on altering the code anyway, so it is still an option.
I posted the module and arduino code to git for anyone interested.
How is external clock supposed to work with this? I was assuming that it would expect to find clock on the “play” input but that doesn’t seem to be working correctly. The UI says the module is always “paused” in external mode and I just see sparse random LED flickers on the numbered outputs. Apologies if this info is elsewhere, but I haven’t been able to find any documentation for this or the LittleBen firmware anywhere.
