Here are my thoughts…
(1) It’s very rare that you have a situation where the 32 devices you want to control are all in the same physical area. Usually, these devices are scattered around a large space, and in this situation its often better to have multiple devices with a smaller number of relays connected to them, in different locations around the building.
In my case, I control many individual devices with plug-in “Sonoff” type smart switches that are all controlled via Blynk. If you do indeed have 32 devices all wired back to one location then go with your initial plan.
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Using a singe 32-way relay board is generally not a great idea. If one relay fails then you have to remove the whole board and replace the entire board or the faulty relay. I find that it’s far better to use 4 or 8 relay boards instead, and have a few spare (and some replacement relays that you can use to repair a faulty board).
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Your choice of MCU board is awful! I wouldn’t accept one of these boards if someone gave me one for free. Far better to us a NodeMCU and two MCP23017 expansion boards instead, as described here…
Using the MCP23017 IO Expansion Board -
If you want physical buttons to control each of the relays (often a good idea) then you’ll need 64 GPIO pins (so four MCP23017) and the Mega doesn’t have enough pins.
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Offline functionality isn’t currently available in Blynk IoT.
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Your Mega (or NodeMCU if you go down the MCP23017 route) counts as a single Blynk device.
Pete.