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Voltage Reading modification to V4 board

Hi all,

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.


Good Luck. 

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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. 100A Power Meters Monitor Volt Amp kWh Watt Digital Combo Meter AC110v 230V

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.

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Good comment. The web site display appeared to be on a cell phone and I thought about being in the garage and monitoring the charging while it was in progress. But having the wireless capability connected to the internet would be of greater global value. I can see that clearer now.



Hi! Can somebody post the necessary changes for the atmega firmware to turn on voltage measuring ?

Voltage measuring hardware in not currently present in controllers from OpenEVSE. Voltage is assumed to be 120v if L1 is detected or 240v if L2 is detected.

If you add the required circuits or are using a board from Geppetto Electronics with the experimental voltmeter circuit you can enable bu un-commenting the //#define line in the source code.

// Support for Nick Sayer's OpenEVSE II board, which has alternate hardware for ground check/stuck relay check and a voltmeter for L1/L2.

//#define OPENEVSE_2

I should unmark the whole function //#ifdef OPENEVSE_2 or only #define VOLTMETER ?

@zhuchella Do you have a controller with voltage measuring circuits? 

If you purchased OpenEVSE II from Nick Sayer at Geppetto electronics on tinde than the firmware should be setup already. If you need an update you should contact Geppetto electronics directly.

If you have a stock OpenEVSE board you shoud NOT unmark either OPENEVSE_2 or VOLTMETER, the voltmeter circuits are not present.

If you have created something of your own design you will need to enable the hardware you added. OPENEVSE_2 has a different ground monitoring hardware in addition to the voltmeter.

I made as chuck kamas on the top of this topik: made the voltmeter and conect it to the ADC3 on MEGA 8.

We do not have any experience with the hardware modification by Chuck Kamas or the modified code for Nick's hardware. You can try to enable Voltmeter and see what happens. You will likely need to experiment with the values to get the readings accurate.

Please do realize that I have a degree in Electrical and Computer Engineering, and this is an area that I am very comfortable working it. There are voltages backed up by currents in these boxes that can kill you. Please be VERY careful! Let me repeat, be VERY VERY careful. Expect it to blow up and be fine with that as a possibility before you proceed.

I know that currant transformers have a high voltages on output (if i correct understand your "There are voltages backed up by currents in these boxes that can kill you"). I only want to know your chuck kamas changes in atmega firmware for implantation of valtage measurements.

Well I bought my second openevse and I am about to modify this one to also read the AC voltage. I will try and post a more detailed description of how to modify the FW and the HW this time!

For the HW I am planning on using a ZMPT101B. Only $7 from Amazon!

For the FW I will evaluate the current code base and perhaps just turn on the voltage measurement flag or incorporate the code from:

I'll post when I am finished.

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