multi aspect model signal controller

IRDASC-4s control colour light signals exactly like the full size railways. As there is no connection to the track they work equally well with either DC or DCC controlled model railways. Although simple to wire they are flexible in use. For a summary of all the units for controlling 3 and 4 aspect signals please see the 3 and 4 aspect signalling page.


Unless a train has just passed the signal or an interlocked point is set for a derailment, the IRDASC-4 sets its signal according to the aspect of the next signal along the line. However, if the IRDASC-4 has detected a train passing its signal it will set the signal to red. The signal will stay at red until the next unit up the line detects that the train has passed its infra red detector. Unlike the MAS-Sequencer and MAS Sequencer RI, the IRDASC-4 does not work on its own. The next unit up the line may be either a MAS Sequencer, a MAS Sequemcer RI or an IRDASC-4, IRDASC-5, IRDASC-4RI or IRDASC-5RI.

The wiring is very simple. The wires from the power supply connect to terminals + and 0V and the wires from the signal connect to the terminals shown in the diagram.


The IRDASC-4 is available in 2, 3 and 4 aspect versions and these may be used together. For 4 aspect signalling at least four IRDASC-4s are required (with three 4 aspect IRDASC-4s only red, yellow and double yellow would be indicated). When less signals are used or the line does not form a continuous oval a MAS Sequencer-4 operates the last signal in the chain with the IRDASC- 4s operating the other signals.

The board currently available is a redesign of the IRDASC-4. The new board is compatible with the old and functions in the same way but the terminal arrangement has changed.


1. Power

IRDASC-4s may be powered from either an AC or DC supply of 12 to 16Volts. Every unit controlling a chain of signals needs to either use the same power supply or if it is necessary to use more than one power supply the negatives but not the positives of the power supplies should be linked together.

2.Wiring to the signal

How a 4 aspect multi aspect signal connects to the IRDASC-4 signal controller for model railroad signal control

The IRDASC-4 operates LED signals. Nearly all model signals produced use LEDs. As LEDs require current to flow from anode to cathode in order to light and as signal manufacturers usually common one side of the LED to a common wire then there are two possible types of LED signal. These are known as common negative (the common goes to negative) and common positive (the common goes to positive). We can supply IRDASC-4s for either type of signal. Generally all signals produced in the UK are common negative.

UK manufacturers (CR signals, Eckon, Bercko, Traintronics etc) wire their signals common negative. ie the common wire from the signal connects to all the cathode (negative) legs of the LEDs within the signal. European and American signals are usually common positive. Please specify which type of signal when ordering so that we can supply a compatible control board. We also need to know how many aspects (number of lights) your signals have.

The IRDASC-4 has built in resistors for the signal LEDs. This means that LED signals can be connected directly to them. If the signals already have resistors attached then they will still work but will be dimmer. If they are not bright enough the resistors supplied with the signal can be safely removed. Without any resistors the LEDs in the signals will probably be destroyed if the signal is powered.

3. Interconnections between units

A single wire is used to send information about the aspect of the signal back to the previous unit. The Send "S" terminal is wired back to receive "R" terminal of the IRDASC-4 controlling the previous signal. This wire sends back information about the aspect that the next signal is set at and whether the train is in its block section. Only one send "S" can be connected to the "R" terminal.  At junctions one "S" may be connected to two "R" terminals.

The above are all the connections necessary for the signals to operate correctly.

Diagram of wiring between MAS Sequencer and IRDASC-4 units for non continuous signals

The diagram above shows the connections to wire 3 signals. The signal on the right is controlled by a MAS Sequencer. It could equally be controlled by a MAS Sequencer RI

connections between IRDASC infra red detection and signal control units to control 4 signals on an oval of model railway track


This diagram shows how IRDASC-4s (any combination of IRDASC4 IRDASC5 IRDASC4RI or IRDASC5RI) can be used to signal a continuous oval.

Junction with lines converging.

The IRDASC-4 displays a red when there is no connection to the R terminal. This is useful at junctions. A contact operated by the points movement routes the S terminal to one of two R terminals according to the points setting. The result is that the line with the adversely set point will have a signal displaying red because its R terminal is disconnected due to the points contact and the signal on the line the point is set for will display in the usual way depending on position of trains and the signal at C. This method can be extended for more complicated junctions.


converging junctions are easily signalled

N.B. The IRDASC-4s controlling signals A and B can be located on the single track.


Junction with tracks diverging

A changeover contact operated by the points movement is required. This connects the IRDASC-4 marked A with the send S of whichever IRDASC-4 the route is set for. Diverging junctions are often signalled with a feather (route indicator), to operate the feather an IRDASC-4RI, IRDASC-5RI or MAS-Sequencer-RI board can be used in place of the IRDASC-4 at position A. If the signal is close to the point and the infra red detector is positioned before the track is doubled the signal (which changes to red as soon as the leading edge of the train reaches the infra red detector)will change to red when the train has only moved a short distance past the signal. Full size signals which are usually automatic work in this way, changing to red as soon as the train reaches a track circuit. The track circuit can be very close (20 feet) to the signal. However IRDASC-4s can be supplied with 2 sets of infra red detectors for these locations, one set for each line so regardless of which line the train travels down it will reach an infra red detector and the signal will change to red. These are known as IRDASC-4-2EW.

Older colour light signal junctions used 2 signal heads on the post in a similar way to semaphore junction signals. To automate this type of junction with IRDASC-4 units you need to treat the 2 signal heads as 2 seperate signals each with an IRDASC-4 controlling them. The diagram below shows how this type of signal can be operated with connections to signals either side. If the signal was on its own it could be controlled with 2 MAS Sequencer units. The switch operated by the points movement could be wired to switch one or other of the MAS-Sequencers RR terminals to 0V so that the signal ahead of the line not travelled down is held at red.

signalling a diverging junction

Much more complicated junctions than those shown above can be signalled simply by routing the send to receive wires through contacts worked by the points. Also signals can be set to red via the RR terminal which can also be operated by the points movement.

4. RR Terminal

toggle switch wired to override IRDASC 4 signal contoller to red

The units will work without any connections to this terminal but it is useful for certain operations.

Uses are:

When a train is running in the opposite direction to the signals.

For junctions (interlocking with points).

For manually setting the signal to red for station stops.


Terminal "RR" is activated by connecting it to the 0V (negative) terminal and this sets the signal to red. This connection may be made with a switch or with the Train Direction Detector. One switch or Train Direction Detector may be connected to any number of "RR"  terminals.

Another use for the RR terminal is on bidirectional lines. When trains run in the opposite direction to that signalled, the signal should be at red. If the control system is DC then the Train Direction Detector can be wired into the RR terminal. For DCC a switch can be used. The switch may be connected to more than one MAS Sequencer or IRDASC-4.

Combining different aspect signals

IRDASC-4s controlling signals with different numbers of aspects can be combined together. For example as a railway line approaches a terminus station there could be a 4 aspect, then 3 aspect and finally 2 aspect signal.  It would be incorrect for the signal immediately before the end of the line to be 4 aspect because it would mislead the driver into thinking the track was much longer. Similarly it would be meaningless to have a 4 aspect signal before a 2 aspect because double yellow means the next signal is at yellow and a 2 aspect signal cannot display this.

Position and fitting of IRDASC-4

the IRDASC-4 is positioned to determine where the signal is to change to red

Each IRDASC-4 is positioned past the signal it controls. This position is important because the signal changes to red when the front of the train is detected by the IRDASC-4.

The unit is screwed to the underside of the baseboard with the infra red emitter and detector located in a hole between the sleepers. It is easiest to install the units after the track is laid.  Drill a small pilot hole between the sleepers.  Fit an 8mm drill bit marked with tape for slightly less than the base board thickness.  Drill from underneath the baseboard following the pilot hole.  Cut or file the small amount of baseboard material left between the sleepers.  Install the unit, and then fill the remainder of the hole with modelling material.  Blue tack will hold the units in place temporarily.

When fitted to Z or N gauge track the gap between sleepers will be less than the diameter of the infra red detector and emitter.  However, the modules work well provided they are adjusted so that the infra-red devices touch the underside of the sleepers. This positioning prevents reflections off the sleepers causing detection. The modules will also operate on their side placed alongside the track. For thick baseboards and restricted space we have the IRDASC-4EW. These have the emitter and detector fixed to 18 inch long wires.

If you use them on their sides make sure the red LED is not lit on the board. If it does light it is because the infra red is reflecting off the top surface of the baseboard.

The IRDASC-4 is positioned past the signal as shown in the diagram.

Dimensions of IRDASC-4


Length 135 mm 5.3 inches


Width 32 mm 1.25 inches


Voltage 12 to 16 volts, AC or DC