Model Electronic Railway Group

Leading Model Railway Electronics since 1967

Over 50 years of Innovation: 1967-2019

Diode Matrix Interlocking

A suggestion from Colin Glenister (26/2/2000)

plan.gif (1666 bytes)

Single Track Station with Passing Loop & Yard

Locking Table

Lever Locks Releases matrix.gif (9324 bytes)
1 2, 3  
2 1, 3  
3Normal 1 2
3Reversed 2 1
4Normal 6 2, 8
4Reversed 8 6
5Normal 9 8
5Reversed 8 9
6 4, 5, 8, 9  
7 4, 5  
8 4, 5, 7  
9 5, 7  

Matrix Operation

The matrix comprises a number of unconnected horizontal & vertical electrical ‘lines’.

The vertical lines carry the matrix inputs or interlocking signals from point motor/signal relay contacts. In this example the common terminal of these contacts is connected to, and switch, the (-ve) terminal or circuit earth.

The horizontal lines are the matrix outputs to the point motor or signal relay coils. One end of all these lines is connected to the (+ve) terminal of the power supply; the other end of each line is connected to the coil of a point motor or signal relay.

Connections between the input and output lines of the matrix are made via diodes such that the (+ve) supply to a locked point or signal is fed through a diode and a closed point or signal contact to the (-ve) terminal. In practice each of these input lines would carry a lamp or LED to act prevent the (+ve) and (-ve) terminals of the power supply being shorted together.

A Simple Example

Let’s Signal 1 and Point 3 …….

example.gif (2806 bytes)


  • Signal 1 controls entry to upper platform only.
  • Point 3 locks Signal 1 on when Normal
  • Point 3 releases Signal 1 when Reversed
  • Signal 1 locks Point 3 when Off

In current state:

Contact on Signal 1 open therefore Point 3 is released:

(+ve) voltage travels along output line to Point 3 motor coil (No connection to (-ve) through diode A and contact)

Point 3 is Normal therefore Signal 1 is locked On:

(+ve) voltage is prevented from reaching Signal 1 relay coil as it is rerouted to (-ve) terminal via the diode B and the closed change-over contact on Point 3.

Route to Platform Set

Point 3 Reversed:

Change-over contact on Point 3 opens 3N line and connects 3R line to (-ve)

Signal 1 is released: (+ve) voltage now flows along output line to Signal 1 relay coil

Signal 1 pulled Off:

Contact on Signal 1 is closed and connects Signal 1 line to (-ve)

Point 3 is locked: (+ve) voltage now flows to (-ve) terminal via diode A and closed Signal 1 contact.


  1. Add lamp or LED to input lines above the contacts to show the state/position of the Signal or Point.
  2. Add an extra, switched, (+ve) feed to all Point/Signal coils for interlocking override.
  3. Incorporate Single Line Staff by adding an input and output line for each staff
  4. Add level crossing interlocks