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[Reply #15 on:]

The 12V is a wall pack, but again it's a cheapy ebay one. The 12V in is reading ~12.24V on the DMM.

That's the DC reading, but you should also check that the AC reading when its under load - this should be small if
the wall-wart is regulated - if its not regulated it might explain the odd readings.  Certainly worth putting 0.1uF on the input
of the 5V regulator though, those caps are specified for a reason.

[Reply #16 on:]

I was told an input cap wasn't necessary

Then stop talking to morons.

[Reply #17 on:]

I was told an input cap wasn't necessary but that may be the problem.

Voltage regulator datasheets typically say that a separate input cap isn't necessary if the smoothing capacitor for the power supply that is feeding the regulator is close by - which means that the wires to it are no more than a few cm long. If you are feeding the regulator from a wall wart with a typical length of output cable, then you haven't met that condition.

[Reply #18 on:]

I once had over 2000 set top boxes scrapped in production because someone changed the manafacaturer of the regulator used and that type needed more that the 1uF cap that was fitted to the input. It oscillated.

[Reply #19 on:]

Thanks for all the help guys, sorry it took me so long to reply.

As a finisher to this post, it turns out that I had pulled the power cable out of the Arduino without realising. The Arduino was running off USB power and referenced a different ground to the boards. Also, I will add the input cap ASAP.

[Reply #20 on:]