The scenario
When the car ignition is turned on, a 12V signal from the vehicle electrical system must close the laptop's power button contacts for about half a second to boot the laptop automatically, then open again. The circuit's 0 net is the vehicle chassis ground.
What is in this drawing
Read the decisions behind it.
01
Use a series RC network with the relay coil to generate a momentary pulse instead of a latching relay
02
Place a flyback diode across the relay coil to clamp inductive kick
03
Use isolated dry relay contacts across the laptop power button terminals
Reuse when the control signal is a switched DC rail such as ignition or accessory, and the load is a low-voltage, isolated momentary switch input like a laptop power button.
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