Sorry…can't think of a succinct question for the subject line!
I am working on a project on my car. The car's existing ECU is connected to a pressure switch, which opens when the A/C refrigerant pressure is low. This causes the ECU to switch off the A/C (via a different circuit). By opening this circuit myself, I will be able to switch off the A/C using a logic output from an MCU. The attached picture shown an NPN transistor in the place that I intend to insert…something. My testing shows that opening the circuit works as intended, and that a simple NPN does the job.
I want to ensure the component is immune to or protected from the dreaded load dumps and other transients/events I know to expect in the environment. However, I don't want to use a relay, as inside the cabin the clicking will be annoying. My MCU is running off an automotive LDO voltage regulator (LM2937), which is protected from overvoltage, reverse polarity, load dumps, etc.
I have two questions:
1) In this specific case…do I have anything to worry about anyway? I pretty much assume the ECU's circuitry should be insulating this line from these events. Maybe the NPN I'm using for testing will be good enough – perhaps with a big diode in reverse?
2) If I DO need to worry…what would be the best solution?
Best Answer
You can't go far wrong with an opto-isolator or opto-MOS relay.
At < 1mA, something cheap like this one.