PSU schematic

[ppppenguin]

I've used a different trick to split rails from an AC wall wart with just one winding. I've used a 12V regulator to make the "outer" rails, then a -5V regulator to split the supply into +5 and -7. When doing this sort of trick you need to be very clear about the relative loads on the 2 rails. Regulators can source current, they can't sink current. If necessary you preload one rail with a resistor to ensure this is true. The attachment is from another design I did that went into production. The 22R resistor was necessary and got quite hot. Might have been able to avoid it by using a pair of 7905 -ve regs but I'd need to think about that.