Breaking a circuit with an arduino and without a relay/isolator?

Is this possible or do I need to get a relay?

Perhaps you had better explain your requirement.

arduinobewnie: Is this possible?

It depends.

PeterH: It depends.

Ok, how do I do it?

arduinobewnie: Ok, how do I do it?

You explain what it is you wish to do.

Paul__B:

arduinobewnie: Ok, how do I do it?

You explain what it is you wish to do.

I'd like to break a circuit, like an off solid state relay would do except with just an arduino.

Well, you can get the Arduino to control a Solid State Relay quite well, as long as it does not require (or draw) more than 30 mA to operate.

Other than that, you need to explain what circuit you wish to control - what voltage, what current, AC or DC, what isolation is required, is it an inductive load or incandescent lighting or uses input capacitors like a switchmode power supply?

arduinobewnie: I'd like to break a circuit

What circuit?

PeterH: What circuit?

Frustrating, isn't it?