Well, I could not leave well enough alone, so I modified my brand new openevse to read the line voltage. My main motivation was to accommodate my house voltage which is 240v and the voltage at work which is 208v. The 208v comes from the fact the work is fed with 3 phase.
This post will detail the hardware modifications. I expect to post another detailing the mods to the atmega firmware.
I wanted to be able to safely add the high voltage sense circuit to the existing board. I looked at Nsayer's method (very clever to use the opto-isolator in its linear mode), but I thought it too difficult to implement, and lots of watts wasted in the voltage drop resistors. So I did the next best thing, I used a step down transformer to get the ac voltage closer to what the atmega could handel. So off to the local electronics junk store (there are advantages to living in the silicon valley, besides living in a high crime neighborhood in a million dollar house, aka a shack). There I found a 240vac to 5v power supply. I ripped out the 7805 linear regulator and added a voltage divider... vola a linear high voltage sensor for both 110 and 240/208.
Here is the schematic for the 5v power supply, now a voltage sensor. I included an alternative transformer from Digikey, for those of you without a junk store a mile away. I thought it would be smaller, but it is just about the same size, so I just used the junk store one.
I then modified the Openevse to read the voltage on ADC3. This was by far the most difficult part. The pin I needed was not routed at all. Note: please route unused pins to test points to make this kind of modification easier! Therefore I had to solder my red wire to the micro's pin. This took a steady hand, a small wire and a microscope! The other side of the wire was routed to a convenient connector, FTDI pin2 was unused on the pcb side, I don't know about the FTDI cable, but I have the WIFI so I don't care. This is connector is used by the WIFI for RAPI commands, and this and the ground pin next to it are unused. Also note: next turn of the boards please use keyed shrouds on the connectors, it will prevent the magic smoke for leaking out of components.
The voltage source from the ac to dc transformer is hooked up to pins 1 (gnd) and 2 (ADC3). My junk power supply came with wires terminated in connectors that I just popped the pins out of and installed in the wifi/ftdi connector. Very convenient.
I bought an OpenEVSE 2 from Geppetto electronics. This is my second one building. First one was is the original OpenEVSE and works great.
I am now testing the new build that has voltmeter embedded.
I have already uncommented //#ifdef OPENEVSE_2 in firmware and uploaded it to the board.
I like to see Realtime volt in Web GUI instead of static.
I am also getting Stuck relay error, Ground Monitoring error and not reading Amp (Shows 0). If I disable the safety, EVSE works just fine and charges the car without showing the amp.
Let me know if any one has more information that can help.
@Robert We would recommend contacting Nick at Geppetto electronics. We are not familiar with the particulars of his hardware.
Thank you for your response. I did reach out to him, but did not get a straight answer. He said he doesn't remember changes he made to the code. I was hoping to get a more sophisticated board to use in my project, but documentation and support is not there yet.
I can't get it to pass all tests.
@Robert try firmware 5.0.1, maybe older firmware will work.