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An example design for an I2C-based solar slave uses I2C differential redrivers running at 5V. This requires 6 wires, with power/ground being provided from the solar master. We can use a 4-channel unidirectional digital isolator on the differential lines, so the LV provided would be powering the isolator and the redriver and ADC would be powered from the MPPT. We could probably use an LDO since we shouldn't be drawing much current. To minimize current draw from the MPPT, we could also use a 2-channel bidirectional digital isolator for a few more dollars and place the redriver on the non-isolated side.

For LIN, we would probably use an STM32F030 (Cortex-M0) or STM32L011 (Cortex-M0+) and LIN transceiver. Since LIN is a 1-wire interface, we only need 3 wires. We could use much cheaper ADCs since we don't have the addressing requirement. We could even use an analog optoisolator with the MCU's built-in ADC, although it may be very close price-wise for a digital isolator + ADC. That needs to be investigated more. We could probably get a ~$3 digital isolator and a ~$2 8~10-bit ADC.

LIN Slave

If we go with LIN, I propose a low-cost variant of the controller board that uses an STM32L0 + LIN transceiver as its core. Since this should be a low-power system, an LDO is probably fine and saves money. These slaves should be designed to daisy-chain, so we can use 2 6-pin DuraCliks (12V, ground, LIN, IO1/IO2 for auto-addressing, optional alert). Instead of a high-density mezzanine connector, we can use standard 0.1" programming headers since we'll probably only have 10 or so GPIOs. The target price should be less than $10 each since we don't need a crystal or dc-dc. A good chunk of the cost will likely be the connectors.