Safely powering a relay from same source as attiny/arduino?

Question

My project involves using ATTINY to control a 5v relay (specifically JZC-11F and will be ground triggered) in an automotive setting.

I keep seeing the warnings of powering a relay from the same source as the ATTINY (or arduino), everything I can find says you need a completely separate source.

I'm still working on a 12~v to 5v power circuit that is safe from the issues that arise from being in an automotive setting (load dumps, noisy, reverse polarity protection..etc) but saying I have that figured out and I have a clean 5v source for power, do I have to duplicate that circuit just for the relay? or are there a ways of preventing the relay from eventually shorting out the ATTINY?

Plan B: What if I scratch the whole 5v relay and set it up to trigger an external standard 12v automotive relay (this is probably more ideal)? Is the ATTINY capable of triggering a 12v relay via ground? if not what are the recommended ways of making this happen?

side note: I'm know code I don't know EE but I'm trying. The end goal is to combine everything onto a custom pcb, so any suggestions about the 12v-5 using an off the shelf car usb adapter or a premade board are unnecessary (seems to be the go-to answer on that subject). I have a separate post regarding that circuit so if at all possible please stick to the relay questions.

• Edgar Bonet: This question may be more appropriate for EE stack exchange. It will all depend on how beefy is your 5 V power source, on how much current the relay uses, and on your supply decoupling capacitors.
• aaa: Why not use a solid state relay? Often they work on 5V/the output pins of a microcontroller.
• dandavis: @Paul: most solid state relays are for AC (triac-based) and won't work in automotive application. a MOSFET or MOSFET-based SSR designed for DC would work.
• dawm: @EdgarBonet shooting for 5v 500mA, the relay I mentioned has a coil draw of 40mA and attiny is around 13mA

Answer 1

do I have to duplicate that circuit just for the relay?

No, not really, no.

Is the ATTINY capable of triggering a 12v relay via ground?

The ATTtiny is capable of triggering a transistor - the transistor is capable of triggering a 12V relay.

Unless your relay is incredibly high powered and the coil takes (and generates through back-EMF) massive amounts of current, there really is no problem running it alongside a small microcontroller. Just be sure you have the flyback diode, and maybe a damping capacitor as well to absorb noise.

TBH you're probably going to be getting more noise from contact spark than you are from the relay's coil.

• dawm: Decided to drop the onboard relay and go with an external 12v relay w/ built in diode (301-1C-C-D1)