Sound Sync Disks

Warning – Do Not Bend

Do not bend the wires on the supplied reed switch without supporting the wire with pliers, or similar, between the glass and your bend. Failure to do so will result in the glass cracking, the vacuum being destroyed and the reed not working.


Right having got that out of the way on with information about the installation and use of the sync disks. They come in a variety of flavours but they all work on the same principle so these words will cover the general principle, common features, general problem solving and issues to watch for before details of installation specific to the type.

How it Works

The Magnets in the sync disk are arranged with their polarities alternating. If I followed my usual routine you’ll see alternating red and black (or plain) ends to the magnets. Each of these magnets radiate a magnetic field of their given polarity so when the axle rotates a fixed point adjacent sees an alternating magnetic field.

These magnets create a very strong magnetic field such that if we were relying on the pull of magnetism to close the reed or its absence to open it the reed would have to be placed a long way from the disc. That’s not practical.

Instead what is happening here is that somewhere midway between the red pole of the magnet and the black pole of the adjacent one the two magnetic fields cancel each other out resulting in zero magnetism and the reed opens and then closes again.

Reed Placement

The reed can be placed at various locations adjacent to the Sync Disk as shown in this graphic. Option 1A is often best  is placed off centre as there is also a theoretical zero point between the ends of the magnets.

I would recommend testing your placement with a simple battery and lamp or with the continuity setting on a multi-meter to make sure it works before you connect it to any expensive electronics like your decoder.

Here are a couple of installation photos to give you a clue. They do feature a style of disk that we no longer make (making me realise its time for some new photographs.

The first is an installation in an 0n3 K27 with the reed solder to a PCB sleeper and glued to the cross member.

Hopefully, its obvious that the disk is on the axle and will appear when the loco is assembled.

The leads on the reed do allow a little bit of bending up and down to optimise the operation.