fischertechnik has its own reed contacts.
For a few cents and a little work you can also build this yourself.
The reed contacts should be as small as possible so that it can be integrated in fischertechnik building blocks.
Step 1: Required Parts
Reed switches are available for little money in the electronics trade. Depending on the purpose you have to select the allowed voltage and current. 2 A switching power should be enough in the toy environment.
For this example, I use a fischertechnik Building block 15x30x5 with groove and pin.
An additional components, we only need two 2.6 mm fischertechnik (or Marklin) plug and 20cm twin wire.
A magnet can stick with epoxy resin to any fischertechnik building block.
So that the sensitive glass body is well protected.
Turn the reed switch so that takes place when the magnet is in the contacts before.
The groove we pour out with some epoxy. An alternative is also hot glue.
Step 3: Another Version
In the hole were two 2.6 mm jacks Märklin.
Note that is the vitreous of the reed contact unprotected and is therefore easily damaged.