data logging after an event

i want make data logger monitor input , store data in fifo way.
storing last 30 seconds of readings or so.
then if there abrupt change, write last section sd card , start save 10 minutes after last abrupt change.

this ac voltage measuring.

i want put on panel , when air conditioner compressor kicks in, , there brown out of value, store last 30 seconds prior brownout , keep recording 10 minutes show effects happened past last major change.

another question how many readings per second/minute needed.

we have brownouts on occasions , want track down cause.

if want record ac sine wave need lot of data readings , wonder write them sd card fast enough - have never tested that. suggest writing short sketch test how long takes write data. far know sd card writes faster if done in block rather 1 byte @ time.

on other hand if have way detect rms voltage few times per second there lot less data handle.

the other important issue how long brown-out - need sample enough sure don't miss it.

you may have sram problem on arduino because 30 seconds of data may never want keep should saved sram in circular buffer - if taking lots of samples 30 seconds run several kbytes.

maybe 10 seconds before event enough - after all, definition, nothing happening in period.

...r

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