Also, it would be safe to remove the LED from the drawing ?
So drawing 1 must be deleted as the optocoupler is not capable of powering any external devices without something like a MOSFET or Relay to boost the power to the device.
The link is to the opto, but never mind I googled the FET.It has a typical Vgs(th) (threshold voltage) of 1.5v so that means it will be well turned on by 5v. Apart from that the second bullet point on the data sheet says "logic level gate drive".
2, I'm not a FET expert, but I think as you are actively pulling the gate down (off) this needs a pull up resistor on the gate to ensure it turns on when the opto is not active. Now that we've determined this is a logic level FET I guess that pull up could go to either 5v or 12v. In this situation I don't think you need a series resistor on the gate, the small and infrequent inrush current will not harm the opto.
Another thing to think about is the line length and environment for the wires on the FET. FETs are quite prone to ESD so while this is OK on paper it is not a good design I think for the real world.I'd either use a transistor instead or have the FET throw a small (onboard) relay that in turn operates the remote siren etc.If you do this you can ditch the opto as you get isolation with the relay.
I think...(pauses for FET expert to step in)...that ESD is a problem on all terminals and that 40 metres is asking for trouble.If you drive the FET directly from the Arduino and have it drive a small relay then the C and NO terminals of the relay can control the siren and they are immune to everything.I may not be right about the ESD but at least I'm safe ______Rob
So would need either :1. the pin driving a transistor ( like 2N2222A ) driving the relay, or2. the pin driving a FET driving the relay.
I was hoping for a silent option,