Old Grey wrote:It's showing 2.4Vmax, -0.9Vmin, 3.3Vpp and 1.77Vrms on the scope - on a multi it's 1.7V on AC -. I read that it has to be a 5V sq wave, so is that just the positive component, Vmax or the full wave, Vpp.
The caps came off the LM7805 datasheet, should I change them.
Well, I feel somewhat vindicated since you are getting a voltage reading on the AC scale. 1.7VAC sounds about right considering the readings you're getting on the scope. Having a negative voltage does seem suspicious to me though. That might imply some back-EMF is occurring, and that could be a result of how you have the 7805 set up. Non-pol caps are a must.
2.4V will probably not register as a logic high to Speeduino. Like I mentioned before, 2.7V is about the bottom end of what would be considered high with 5V CMOS logic signals.
As for the 7805, I'm looking at this data sheet- https://www.fairchildsemi.com/datasheets/LM/LM7805.pdf
, and it's not calling out the capacitors as electrolytic type so that may be part of your problem since you are using polarized. I've nothing against that but there should be non-polarized caps there too. They will help block any negative voltage signal from affecting the regulator and since you're seeing that, I'd suspect that may be part of your issue.
So I suggest you do, as a modified version of the datasheet I referenced above:
V+ -> 10uF/33V electrolytic cap -> 0.33uF/33V non-pol cap -> 7805 -> 0.1uF/16V non-pol cap -> 10uF/16V electrolytic cap -> load
I'd be cautious about the caps on the input side and their voltage rating: you may want to go up a bit just in case the voltage may exceed their rating. Heck, maybe up-rate all the caps just to be safe.