Simple Signalling System

The Simple Signalling System was developed as a guidance system for operators of model railroad layouts. It is not typical of any prototype but like the prototype is there to guide the operator and warn of the situation ahead. There are several unique problems confronting the 'train driver' when in a model railroad situation.

This system can be set up to present to the operator only the information he requires to proceed along the track safely. In the following sections the components are described which will allow you to customise a signalling logic to your particular requirements. Each signal head can be set to do the specific job required for it's layout location.

Although the occupancy detectors are designed for DC conditions they will work as well on DCC block circuits too.

This Signal System uses searchlight heads with 2 or 3 aspects. Red and Green, if a dual LED (2 lead bicolour) is used and Red/Yellow/Green if a three lead LED (common cathode) is used. Note "N" scale can only use 2 lead LED's.

Each signal head LED driver can have one yellow (caution) input and up to 3 red (stop) inputs. All inputs are Low active. If no input is Low (0v) then the aspect is green.

Each head has one power control (approach) input to enable the signals when a train is approaching. These can either be wired as 'signal ahead' type (prototypical USA) or as a three block 'island' of signals that follows the trains progress (more interesting for spectastors in a large layout situation)

Red	Yellow	Green
Block is Occupied OR	IF NEXT block	IF None of
Turnout reversed OR	is occupied	the other options.
CAB's not coincident OR
Manual over-ride on

The Block Detector is based loosely on the twin diode type of current sensing detector. It operates on a 3 wire power supply , +9 to +12 volts, 0 volts and -9 to -12 volts.
A sensitivity adjustment with a LED presence indicator allows a minimum sense current to be set from 0.1ma to 5 ma. (for caboose resistor detection) The diodes limit the voltage developed across the detector track input to 0.6 volts although in operation the voltage drop can be less than this, depending on the sensitivity adjustment.

The Output is a relay/switching contact; either closed (train detected) or open. There is a LED output that can be used for a remote train location display board. The closed contact when a train is in the block will go to the logic circuits of the approach signals for the block to set them to red should a train enter the adjacent blocks.

There is an option for directional approach operation also, where train direction controls the signal power. To only light the signals in the direction the train is going.

Cab selection coincidence between adjacent blocks is an operational feature (although non prototypical). The entry signal into a block will only be green if the two blocks are set to the same CAB selection. This will warn operators of a possible uncontrolled power condition. The Coincidence detector also provides an output for driving a LED indicator that can be placed besides/between the appropriate CAB selection switches. (CAB selection is not applicable to DCC operation)

The Input is from the three positions of the CAB switches. However a six (or 9) CAB selection can be provided by cascading the PCB outputs. Output is a relay/switched contact that is made when the CAB's are not coincident, this will be used by the signal head logic to set them to red should a train enter the adjacent blocks.

This is a 4 block, CAB switch position detector which will generate 4 coincidence outputs from 5 CAB switches.
[Blocks = External to A, A to B, B to C, C to D ]

Inter connection is by screw terminals but could be changed to a multi-pin plug (DB 25/37) or edge connectors, by hard wiring.

Inputs are:- 4 wires from each CAB switch (3 positions), 3 wire power, 3 or 4 wires from one adjacent detector or CAB switch and 3 wires going to the next detector.

Outputs:- 4 logic low levels for signal head logic use, 3 wire power and 4 optional LED indicators.

A single block board and a 4 block board have been developed. Both require 3 wire power supply (+9 to +12, 0volts, -9 to -12)

The Single block board contains the drive for three heads with fixed logic connections that will cover the situations shown in the block types A, B and E above. Basically designed for use on long single line or junction blocks. The single block occupancy detector has an output for an indicator LED. Screw connectors are used. A further head drive could be added with an outrider board to allow for block types C and D.

Inputs:- 2 coincident signals (indicating block ahead for both east and west travel), turnout normal and reversed, approach power control from East and West adjacent blocks, 3 wire power.

Outputs:- to the 3 signal LED's, 2 wires to the turnout switch, 0 volts for turnouts and LED returns, East and West approach control power and 3 wire power.

This board contains four block occupancy detectors (with optional LED outputs) that provide 'occ' and 'app' logic levels. These and inputs from the turnout switches, CAB coincidence detectors and a manual switch are hard wired to the appropriate head drivers as required. There are twelve head drivers on the board. This board is designed for use at loops and crossovers.

Inputs:- 6 coincidence logic, 4 approach control, 3 wire power, up to 4 turnouts.

Outputs:- 2 wires to each signal head LED, 0 volt return wire for 3 aspect heads, East and West approach control power and 3 wire power.

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