Connecting to common ground after a relay.

Hi. Hope this isn't a very noob question but I am a beginner. I have a question about connecting a component to common ground after a relay. I'll explain. I have an output pin that in connected to the gate of an NPN transistor that will power up a relay and switch on a 12 v DC door strike. My question is, should I conect the door strike's 12 v ground to the arduinos ground as well ( I heard that youshould connect to a common ground even if there is diferent voltage levels, but I want to be sure before I fry something), or since the relay isolates the circuits is this not necessary? ( Or maybe you really should do it since it would damage the arduino). I haven't build it yest since I want to make sure I don't damage anything. Thanks.

First though, no. But make a little diagram (photo/scan of handdrawn one) so we can be sure that we understand you correctly.

or since the relay isolates the circuits is this not necessary?

That is the case here.

Or maybe you really should do it since it would damage the arduino

No.

It is only when there is a common circuit should you connect the grounds to make the currents flow. The relays totally isolate the two circuits, so don’t throw this isolation away.

Thank you for your help. Here is a pic of want I want to do sterretje. Though instead of a electric door strike I put a light bulb cause that’s all I could find. So just to make sure, I should NOT connect the 12 v power supply to common ground?

Number 1:

If you do not use opto-electronics to seperate 2 circuits you will need a common ground. If your voltages are not higher than 30 or 40 Volts, you will not have a problem. The circuit design will be much simpler.

Do not mix AC and DC when using a common ground.

Number 2:

Fritzing diagrams are a real nuisance when it comes to troubleshooting. A hand-drawn circuit diagram is a lot more helpful. You see a device like a transistor but cannot distinguish between emitter, collector and base. A relais is just a black box.

If you do not use opto-electronics to seperate 2 circuits you will need a common ground.

I would disagree with that. Why would you want / need to do that?

arduinoaleman: Number 1:

If you do not use opto-electronics to seperate 2 circuits you will need a common ground.

I agree with Grumpy. The coil and contacts of a relay ARE isolated. It's the whole point of a relay. The opto isolation only isolates the digital control pin from the voltage used to energize the coil side. Perhaps that is what you were referring to, in which case you would need common ground on the coil circuits.

I do agree with the fritzing diagram....it just makes it hard to follow and distinguish what is what

Thanks for your answers. I'll be sure to do a drawing of my circuit next time. I going to not connect the 12v power supply to a common ground. If anything blows up I'll let you know.