Sending signal over long distance wire

Question

I need to control a remote relay over a 1km (max) distance. Here is a simple block diagram that represents what I'm trying to achieve:

^{simulate this circuit – Schematic created using CircuitLab}

The control signals will generated with a microcontroller. The relay modules needs to be electrically isolated from the controller, and each of them have their, independent 12V supplies. Standard CAT5 cabling will used for connecting modules to controller.

The control signal frequency needs to transmitted is less than 1 Hz (mostly on or off).

What is the best way to reliably transmit the control signal to the relay modules?

Clarification / additional info:

• Relay current is 40mA @ 12V

Some thoughts:

• Using an optocoupler in relay module, I can buffer my TTL signal with a darlington array and drive that opto then relay.
• I can use some kind of current loop?

I also assume that EMI will be a problem with long cabling, it can create false triggers or damage control/receiver sides. Is there anything other than using TVS / clamping diodes on each end of wiring?

Answer 1

Relays already make great current loop devices. Your issue though is the voltage drop along the wires.

The solution to that is to drive it with a constant current / current limiter circuit and use a larger supply at the relay to cover the voltage drop.

^{simulate this circuit – Schematic created using CircuitLab}

Note, in the schematic above I added a smaller relay on the logic side to provide full isolation.

Alternatively, you could flip all that around.

^{simulate this circuit}

Of course these methods rely on the relay current being small. If need be you could add a smaller, lower current, 5V relay to drive the large 12V one.

Answer 2

In view of the distance involved, I suggest that you need a differential line driver and receiver for each signal - typical examples of the type are MC3487 and MC3486. The optocoupler goes between the microcontroller and the line driver, which means a separate isolated power supply for the driver. Unless it is indoors there is a risk of lightning strikes, so you need suitable protective devices on the lines. You probably also need similar isolation at the receiving end, and it may be necessary to regard the line driver/receiver combo as expendable under extreme conditions.

If there are spare lines, some form of monitoring would be a good idea so you know that signals are getting through. However, in view of the complexity and risk of damage, a radio system is probably more reliable.