I have a lot of experience about noisy (Heavy RF) environments.

Here is how I would do it PNG Image

Note that there are two resistors in the relay wires. If there is a possibility that the wires pick up noise then no wire should be connected to the circuit directly. It is a better idea to split the resistor that way.

The schottky diode absorbs any negative spikes which could damage the opto LED. This arrangement also dampens any ringing in the line quite effectively.

The opto in my example is one of the best there is for noisy environments.


The rest of the circuit is really normal and simple stuff.