Hi goodpickles,
I found this,
http://www.mpja.com/productview.asp?product=14026+QT
it certainly looks like the component you're using.
It actually is already a 'darlington pair' encapsulated as
a single transistor.
You may not have realised that when you arranged these two units
as a darlington pair.
Have you tried the system using just one of these ?
Also it is apparently a PNP unit, so i will amend my drawing
accordingly.
After a bit of searching, i found a PDF for TIP145 on this site:
**broken link removed**
This PDF includes the pin connections, something which is missing
on the other PDFs that i have found. I consider this an omission
of such magnitude that the people responsible should be punished
by being publicly ridiculed on the internet by anyone who wanted
that information, and downloaded their PDF for it.
Could you also give the details on the photo sensitive gap unit
you're using, it might be that choosing an appropriate resistor
would 'bias' the signal to the optimum level.
On the face of it this should be straight-forward.
I have corrected my little copy of your drawing.
Please check with the PDF mentioned that i have it correct.
John