Ian Field wrote:

Worst case scenario for whatever wall-wart happens to be ready to
hand.

It's great to design for the possibility that the user will substitute the
wrong power supply, but then you have to take it to the limit and allow for
adapters with reverse polarity, AC output or a much higher voltage. So a
zener protecting only the 555 isn't enough for either the 555 or the MOSFET
pair. Either you have to use a better regulator or assume they will use the
right adapter.

Better protection would be to use an unusual connector, which many devices
do.


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