I have a project where I need to engergize a 40A relay in a car. My uC based circuit is running at 5V via a 7805 and is mainly doing its own thing, but I need a digital out to energise the 12V relay, which in turn is driving an air conditioner clutch (as it happens).
My thoughts were (discounted in turn, possbly incorrectly) 1) a transistor (worried about 5V on base, 12V across CE, and if there's sufficient current) 2) an opto-isolator / opto-darlington (worried if it can drive the current, and at 12V if the power dissipation will be too high) 3) a relay (feels a bit silly using a relay to energise a relay, but otherwise seems sensible) but even so I'm likely to need more coil current than the uC can drive (or do I?) 4) an opto-isolator driving a relay (handles the 5V/12V disconnect nicely and the currents should be high enough) but now I feel like I'm badly overengineering.
I should mention that while I (and everyone) says 12V, in a running car it's usually >14V.
What would be my best and easiest approach, bearing in mind I'll be building a fair number, so cost is also a consideration...?