Hi, thanks, so the relay coil is 12V/80R = 150mA?
I guess the power supply for the variable resistor and "circuit I need" is 12V/
?A, and is not going to be drawn from the same battery in the picture (a car battery?)?
Pure interest: how come the buzzer needs 30A?
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Hi, this is a basic version of the general idea. There may be mistakes in the calculations or concept related to your circuit, so get a second opinion.
If you use a comparator like the LM339 (instead of an op amp as a comparator), it has an open collector output, so needs a pull-up resistor, and the circuit would need reconfiguring a bit, perhaps just by changing the NPN to a PNP.
Probably want to limit the op amp output swing so its output doesn't hit the supply rails.
Might find that a +12V supply is not quite enough for the relay, and it will chatter, and +13V or so may be necessary.