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 am building a charging station. I also wanted a 24/7 method of measuring voltage, amps & power to the car. This $15 device shipped directly from China will do the trick without changing the original hookup. This same unit is available from other vendors for as low as $12. I have both the EVSE display and this power monitor display on a 12 X 12 X 6" NEMA housing.
We have had a few builders use that unit from China. It works okay, however the OpenEVSE Current measurement setup with WiFi provides great control and visualization. Energy Data is sent every 30 seconds, the Energy Monitoring solution EmonCMS provides extensive graphing capabilities.
I agree that built-in solution is more elegant. I spoke that advice from my viewpoint because I do not own a cell phone. It was also concerning that an independent solution that does not require cuts or jumpers on a working new module would be safer.
The energy monitoring solution does not require a cell phone or modification, just the current measurement coil (included with all Advanced and Standard kits). The modification listed in this topic provides better accuracy by measuring voltage instead of assuming a fixed 240v and measuring current.
The output is Current in Amps on the top line and Session watt hours followed by kwh totals.
Wifi adds the ability from any web browser (cell phone not required) to track usage by day, week, month, year plus chart parameters such as power and temperatures.