Initial prototype version ready

A project log for Raspberry Pi EVSE Hat

Use a Raspberry Pi to build an EV charging station

Nick SayerNick Sayer 09/12/2019 at 16:210 Comments

I've fleshed out the preliminary schematic and it's now complete. In addition to what was sketched out as a design before, there's one more feature. There is now a hardware watchdog whenever the power relay is turned on. This watchdog requires toggling a pin every so often (my own recommendation will be at least once every 10 ms) whenever the power is on as a proof of life. If the watchdog is not triggered, then it will eventually cause the equivalent of a GFI event, forcing the power off. The watchdog is disabled whenever the power is off. Whenever the power is on, it's an EVSE's duty to continuously monitor the pilot impedance feedback to check to make sure the vehicle still desires power. Leaving the power on when it's not requested is a serious safety failure.

The ADC I've chosen is the MCP3004. It's a 10 bit, 4 channel ADC and it's pretty fast. It's connected up to SPI channel 0, so for Linux, you'd use /dev/spidev0.0 to talk to it. You perform a 3 byte bidirectional transfer, sending 0x1, 0xn0, 0x00 (where n is a code that selects which channel you want to read), and at the same time you'll receive 3 bytes, the last 10 bits of which will be the ADC reading. You can use a clock speed up to 3.6 MHz, though with the diode-and-resistor level shifter, 1 MHz is probably about all you can expect (this will have to be tested). Channel 0 is the pilot pin, channel 1 is current transformer and channel 2 is the AC voltmeter from the HV board.

I'm not sure where to go next. I don't really have a *need* for a Raspberry Pi based EVSE. I'm sort of hoping I can sort of put this design out there and a great horde of hackers will descend and run with it, but that's probably wishful thinking.