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IRDASC two aspect signal controls

IRDASC is the acronym for Infra Red Detection And Signal Control. These boards have built in infra red detection and the electronics to control a semaphore or 2 aspect colour light signal automatically in conjunction with the positions of trains and points.

IRDASC-1, IRDASCD-SS and IRDASC-3 are automatic signal controllers for semaphore and 2 aspect colour light signals. They operate signals in the the traditional way when the signal is normally at danger (red) and only changes to clear (green) when a train is approaching and the line ahead is clear. There are four versions of the IRDASC. IRDASC-1 is for operating (common negative) LED signals, it has built in resistors.  IRDASC-DSS is specifically for operating the Dapol Semaphore signal and has contacts to replace the push button switch used for manual operation. IRDASC-3 has a double pole contacts for operating both the signal (LED or bulb) and isolating the track to prevent trains running through signals at danger (red). The only difference between all three IRDASCs is how they connect to the signal. Otherwise their control connections are identical and if required different IRDASCs they can be combined together.

How IRDASCs operate

diagram to show how the IRDASC board operates 2 aspect signals

There is a train detector (IRDOT-1 for example) at A, an IRDASC at B wired to signal 1 and an IRDASC at C wired to signal 2. The signals are initially at red. The detector at A will first detect a train travelling from the left. A wire connecting A to B will tell the IRDASC at B that the train is approaching (entering the block section). This causes Signal 1 to change to clear (green). After the train passes signal 1 it reaches the IRDASC at B. When the IRDASC at B detects the train it sets signal 1 back to danger (red). Simultaneously it will tell the IRDASC at C via a wire that a train is approaching (entering its block section). This causes the IRDASC at C set its signal to clear (green). When the IRDASC at C detects the train it returns signal 2 to danger (red). This chain of signals can be extended indefinitely. Each additional signal will use an extra IRDASC to control it. When the signals are on an oval track, wire the first and last IRDASC together and omit the train detector at A. It can be seen that the IRDASCs form block sections. They each have a terminal to add a block section occupied LED to the control panel if required. Further wiring can be added so that the signals only change to green for approaching trains when the next block section is clear. Interlocking to points can be added so the signal stays at danger until the points are set in the correct position.

The IRDASC boards may also be used so that the signals are manually set and automatically cleared. This is done by wiring a push button switch to terminal h (train entering section).This method is useful for starter signals at bay platforms. The push button switch being pressed when the train is due to depart. If the IRDASC-3 is used it can also connect power to the trains route.


Each IRDASC is located after the signal at the position where the signal is required to return to danger (red) when the front of the train is detected. The IRDASC is fitted beneath the baseboard in the same way as the IRDOT. It is supplied fitted with a "train detected" LED so the correct operation of the infrared train detector can be checked.

Wiring IRDASC-1 to the signal

Diagram showing IRDASC-1 connected to a signal and to the internal wiring of LEDs within the signal

The IRDASC-1 is designed for operating 2 aspect (common negative) LED signals. It is suitable for use with CR signals, Traintronics, Eckon, Berko and possibly other signal manufacturers. If the signal instructions show the common of the signal going to the negative of the power supply then the signal will work with the IRDASC-1, if not use the IRDASC-2. The IRDASC-1 has built in resistors to prevent the LEDs in the signal being damaged. When used with the Traintronics signal the signal is wired dierctly to the IRDASC-1 board and the circuit board supplied with the signal is not used. (if you want to use the traintronics circuit board then it can be used with the IRDASC-2. CR signals are supplied with resistors soldered onto there wires. It is worth initially trying the signal with these present and if the signal is found not to be bright enough then they can be removed for use with the IRDASC-1.The upper diagram shows the actual wiring to the LEDs the lower how the 3 wires from the signal are connected to the IRDASC-1 board. The right hand diagram shows how to wire an LED signal using only two wires.

Some 2 aspect signals are supplied with just 2 wires. These signals are connected to the IRDASC-1 board by connecting the wires to the A and B terminals.


Wiring IRDASC-DSS to the signal

Shows Dapol Semaphore signal wired to the IRDASC-DSS board to give automatic signal control

This is for operating the Dapol Semaphore signal. The IRDASC-DSS uses a built in contact to do the job of the push button switch described in the semaphore signals wiring instructions. This contact closes for a short pulse to change the aspect of the signal. The signal requires 16 Volts AC to operate and this supply may also be used to operate the IRDASC-DSS although a different supply could be used for the board if required.

Wiring IRDASC-3 to the signal

IRDASC-3 shown connected to model railway signal with bulbs

In addition to contacts for controlling the signal the IRDASC-3 has contacts intended for wiring to control an isolation section in front of the signal. This allows the track to be isolated in front of signals at danger (red) and live in front of signals at clear (green). As the aspect of the signal can be controlled by both occupancy of the next section and the position of points then this can prevent derailments. If several IRDASC-3s are used on an oval it also allows several trains to run on the same oval and to automatically stop when they catch up with the train in front. The contacts will switch up to 5 Amps.

The diagram shows the arrangement for using a separate supply for the signal. The signal could also be powered from the same supply as previously shown for the IRDASC-2. Note how the contacts connects either B and e or B and f to light the red or green bulb. Like the IRDASC-2 if a resistor is wired into the supply LED signals can be easily operated.

Semaphore signal operation

The IRDASC-2 or IRDASC-3 can operate Semaphore signals. Instead of bulbs these boards will switch low current solenoids, memory wire or a relay with a moving arm (our relays are not suitable) to move the semaphore signal.

Wiring to a Servo motor controller

If you wish to control servo operated semaphore signals automatically the IRDASC-1, IRDASC-2 and URDASC-3 will operate either the the Single, Dual or Bouncing Semaphore Servo Motor Controller boards.

Train entering section (h) terminal

Activating this terminal "tells" the IRDASC that a train is approaching.  It causes the signal to change to green (provided terminal i the override interlock terminal is not operated..  The "train entering section" (h) terminal is usually wired from the "train detected" (j) terminal of the previous IRDASC.  It can also be operated by terminal 2 of an IRDOT-1, a reed switch or a normally open push button switch.  The push button switch is useful for a starter signal.  It is pressed when the train is due to depart. After passing the signal, the train will reach the IRDASC.  The IRDASC returns the signal to red when it detects the train.  Whilst the train is detected the IRDASC operates the "train detected" terminal.  This terminal can be wired to the "train entering section" terminal of the next IRDASC.  This sets the next signal to clear (green). These boards provide very sophisticated signal control but if you wish to minimise wiring you may find the MAS Sequencer more suitable.

Summary of wiring

Wiring 3 IRDASC-1s to signal power TES and train detected terminals

To make the signal change to clear (green wire on diagram) as a train approaches and return to danger (red wire) after it passes the signal the following connections are made.  Two wires to power the IRDASC. (orange and blue wires on diagram)  Three wires to the signal (blue, red and green wires on diageam).  One wire to the TRAIN ENTERING SECTION terminal from the previous IRDASC, (or from an IRDOT-1, a reed switch or a push button switch) (brown wires on diagram).  The diagram shows the first signal is set to green by pressing the push button switch the others by the detection of the train by the previous IRDASC. Using the remainder of the terminals to provide more advanced operation is optional.

Extra features of IRDASCs

Override interlock (i) terminal

As well as its use for block sections this terminal provides:

INTERLOCKING TO POINTS: This causes the signal to stay at red when trains approach if a point is wrongly set. It is useful at junctions.

MANUAL OVERRIDE: A switch can be operated to cause the signal to stay at red for trains stopping at a station. Similarly the relay of the Dual Timer can operate the "OI" terminal. The override/interlock terminal operates when connected to 0 volts. To experiment fasten a piece of wire on to terminal V0 then touch the override/interlock terminal with the wire. If the signal was at green it will change to red. Removing the wire returns the signal to green. Points and signals are interlocked by wiring the override/interlock terminal to a contact operated by the points movement. The contact is wired to connect terminal V0 to the over ride/interlock terminal when the points are wrongly set. When the point is correctly set, no connection is made. A toggle switch can also be used to connect terminal V0 and the override/interlock terminal to give manual control of a signal at a station. The switch is operated for stopping trains causing the signal to change to red until the switch is released. For through trains the switch isn't changed and so the signal changes to green for approaching trains. A double pole switch can be used; the second pole isolates the track in front of the signal. To make the signal remain at red when the next block section up the line is occupied; connect from the "train in section" terminal of the next IRDASC to the override/interlock terminal. Many wires can be connected to an override/interlock terminal. When any one of these connections operates the override/interlock terminal it causes the signal to remain at danger for approaching trains. The signal will only change to green when none of these wires is at 0 volts.

Train in section (k) terminal

This terminal switches to V0 Volts when the IRDASCs TES terminal is operated and remains at V0 Volts until the train is detected by the IRDASCs infra red detector (or the Reverse Running terminal is connected to V0 Volts). In other words it is at V0 Volts whilst a train is in the IRDASCs block section. When inactive time this terminal is open circuit (ie it is an open collector transistor).

The TRAIN IN SECTION terminal can be wired to the OVERRIDE/INTERLOCK terminal of the previous IRDASC. This will cause a train activating the previous IRDASCs TRAIN ENTERING SECTION terminal to not get a clear signal whilst a train is in the next section. The IRDASC will remember the train has entered the section and as soon as the next section is empty the signal will change to clear (green).

Train in section (l) LED terminal

This terminal is used if you wish to have a signal box style illuminated track diagram. The terminal lights an LED whilst a train is in the block section (the track between 2 IRDASCs). As trains travel around the layout you will see each LED lighting in turn as the train travels from one block section to the next. The main use of this LED is to check the infra red detection is working correctly during installation.

REVERSE RUNNING (g) terminal

This terminal is used to allow correct signal operation when there is bi-directional running along a line. When a train is to run against the signals the reverse running terminal sets the signal to red. Unlike using the override/interlock terminal the "reverse running" terminal clears the IRDASCs memory of a train appeoaching (in its block section). If this terminal is not used then a reverse running train will cause each IRDASC to detect a train leaving the block section before the train entering section message; the result of which is to wrongly leave each signal at green. The "REVERSE RUNNING" terminal can be activated with a changeover switch. For analogue (DC) trains it can be operated automatically with the Train Direction Detector. A single Direction Detector or changeover switch will operate all the "reverse running" terminals of a chain of IRDASCs.

The "Reverse Running" terminal has a second use as a reset to set all the signals to red. In this case all the reverse running terminals are connected to a single push button switch.

Train detected (j) terminal

This terminal switches to the voltage at V0 (negative supply) whilst the IRDASC board is detecting a train by infra red detection. When not detecting a train this terminal is open circuit. (It is controlled by an open collector transistor) The main use of this terminal is to tell the next IRDASC up the line that a train is approaching by wiring it to the TRAIN ENTERING SECTION terminal.

Train detected LED terminal

This terminal lights an LED whilst the IRDASC board is detecting a train by infra red detection. The LED is wired with its long leg (anode) to terminal m and its short leg (cathode) to terminal V0. The main use of this LED is to check the infra red detection is working correctly during installation.

Isolating the track with the IRDASC-3


The IRDASC-3 can isolate the track in front of a red signal. Power is switched onto the track when the signal is green. Hence trains will be automatically stopped by the signal if the points are incorrectly set; the next block section is occupied etc. Relay contacts (rated at 3 amps) isolate the track so keeping the track, signalling and IRDASC electrically separate. The isolated section should be just in front of the signal where the engine is to stop and slightly longer than the longest engine. For bi-directional running a diode will feed current into the isolated section for trains running against the signals. Several trains can be run on the same line by using a number of IRDASC-3 controlled block sections. When one train begins to catch up with another it will automatically stop at the next signal until the next block section is empty. This works with both DCC and analogue.


Both facing and trailing junctions can be correctly signalled. Signals, points (and even isolating sections if the IRDASC-3 is used) can be interlocked. This is explained in detail in the instructions.

Diagrams of IRDASC Terminals


4.85 x 1.2 inches, 124 x 31mm


12-16 Volts AC or DC