VEX Limit Switches are wired as SPST switches.  The actual switch inside the plastic housing is an SPDT switch, and a simple wiring change allows you to make use of the alternate contact.

After the modification, the switch will continue to function as a normal limit switch for the VEX Micro-Controller.  However, the additional circuit allows you to also use it as a limit switch for VRC motors using a simple adapter.

This page walks you through the steps necessary to make the modification.  It isn’t particularly difficult, but basic soldering skills are required.  The mod is easily reversible, if necessary.

Why should I make this mod?

    Two modified switches are needed for the VRC Motor Limiter.

Why shouldn’t I make this mod?

    This modification will void the warranty on your limit switch.

    Some competitions don’t allow the use of modified electronics.

    You won’t be able to plug modified limit switches into Y adapters to “wire-or” their signals.

Before

After

Step 1)

Remove the two small screws that hold the limit switch enclosure together.

After removing the grey back, examine the red-half of the enclosure.  It should look something like this:

Step 2)

Carefully lift the switch out of the red plastic enclosure.  Straighten out the wire leads.

Note the small labels above each switch terminal: ‘NC’, ‘NO’, and ‘C’.

The red wire and the ‘NC’ contact are currently unused.

Step 3)

Unsolder the black and white wires from the switch terminals.  Simply pressing the iron tip to each terminal for a few seconds should soften the solder enough to allow the wire to fall away.

There is a risk of the solder splattering, so make sure to protect your eyes.

Step 4)

Prepare the red wire for soldering, and attach the three wires as shown:

    Red goes to ‘NC’

    Black goes to ‘NO’

    White goes to ‘C’

Note that the black and white wires are swapped from their original positions.

Step 5)

Insert the re-wired switch back into the red enclosure half.

Carefully route the wires through the pegs, letting one wire take each of the three possible paths.  It doesn’t matter which wire goes which way, as long as they all fit.

Step 6)

Re-attach the grey enclosure back.

Make sure to label it, since you can’t otherwise identify a modified switch by sight.

One Vex Limit Switch, and some solder.

Small philips-head screwdriver, Soldering iron, Wire stripper

Basic soldering, Simple disassembly & reassembly

About half an hour, depending on experience level

Materials Required:

Tools Required:

Skills Required:

Time Required: