I'm building a cue light system (think traffic lights for theatre, media and entertainment industries) and I need to connect outstations (the traffic lights) to the control station (Arduino).

Each control station consists of one button and two LEDs - so I use 4 wires in total (ground, 2x 5v for LED and a pull up for the button). In selecting a connector I noticed that RJ45 is abundant (especially with EtherCon connectors) and cheap to obtain.

What is the danger, both to my system and to switches and networking gear, of using these cables in my system? With the low voltage DC current within the rating for the cables I'm confident it works great for my system on its own - but what would happen if someone accidentally plugged my base station into a laptop for example, or an outstation into a switch? Can I use certain pins to mitigate this or should I use an entirely different connector all together?


1 Answer 1


... but what would happen if someone accidentally plugged my base station into a laptop for example?

Probably nothing. Damage at 5 V is extremely unlikely.

You need to watch for inadvertent cross-over.


simulate this circuit – Schematic created using CircuitLab

Figure 1. A suggested layout.

At 5 V it's going to be hard to do any damage.


simulate this circuit

Figure 2. Reversal.

If the cable is inadvertently crossed over all that will happen is:

  • D1 and D2 won't light until SW1 is pressed.
  • IN1 will still work as intended.
  • \$\begingroup\$ Don't forget that the magnetics in the Ethernet interface will block the DC anyway. \$\endgroup\$
    – Majenko
    Apr 4, 2018 at 0:08
  • \$\begingroup\$ Thanks ! If I wanted to use all 8 pins for example for more lights/switches does that mean that the magnetics in the interface will mean that's also fine @Majenko ? \$\endgroup\$
    – JBithell
    Apr 4, 2018 at 11:37
  • 1
    \$\begingroup\$ The magnetics remove any DC component from the ethernet signal, so any DC voltages you inject into an Ethernet port (within reason of course) will be ignored by the port. Every ethernet port basically has a set of small transformers that only let through the AC ethernet signals. \$\endgroup\$
    – Majenko
    Apr 4, 2018 at 12:35
  • \$\begingroup\$ If you're just using the connector and cable, there are no magnetics involved. You'd know if you put magnetics in your design, they are a separate component (some ports have them built in, avoid those ports). \$\endgroup\$
    – user253751
    Apr 7, 2018 at 6:57

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