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Semaphore Sequencer



electronic model railroad control to automatically operate Dapol semaphore signal

The Semaphore Sequencer is designed to operate Dapol semaphore signals automatically. The signal changes to clear as a train approaches it and returns to danger an adjustable time later. This gives the same type of operation as full size semaphore signals which are normally at danger and moved to clear only whilst a train is passing.

Apart from the four wires from the signal only two additional power wires need to be added to wire the Semaphore Sequencer. Extra connections can be added to give an override to keep the signal at danger whilst a point is adversely set or when the train is stopped at a station.

Built into the Semaphore Sequencer are an infra red detector, an adjustable timer and contacts (between C1 and T2) which close momentarily to give the same action as the pushbutton switch described in the Dapol instructions.

A train approaching the signal crosses over the Semaphore Sequencer and is detected by the infra red detection built into the Semaphore Sequencer. When the Semaphore Sequencer first detects a train it causes the signal arm to move to clear and starts the adjustable timer timing. When the adjustable timer finishes its timing cycle the signal moves back to danger.

Positioning the Semaphore Sequencer

infra red train detection positioned to operate Dapol semaphore signal on model railroads

The Semaphore Sequencer is positioned where the train is required to change the signal to clear. As soon as the infra red detects the train T1 and C2 will be momentarily joined by internal contacts thus making the signal arm move.


The Semaphore Sequencer is usually fitted to the underside of the baseboard in the same way as the IRDOT-1 etc. The infra red detectors fit in a hole drilled in the baseboard The unit works with the normal sleeper spacing of Z gauge and all larger gauges. If the baseboards are thicker than the 22mm the infra red devices extend or there is an obstruction on the underside of the baseboard the Semaphore Sequencer-EW can be used. This has the infra red devices attached to 18 inch long wires.

Basic wiring connections

semaphore sequencerwired to Dapol semaphore signal for automatic signalling

The Semaphore Sequencer and signal are both powered by 16 volts AC. This is a requirement of the Dapol signal. The AC supply is connected to terminals + and 0V of the Semaphore Sequencer. The red and black wires from the signal are also connected to these terminals. The yellow signal wires are connected to terminals T1 and C2.

Train detection and timing LED

The Semaphore Sequencer is fitted with a train detection LED. The long leg of the LED goes to the "LED" terminal and the short leg to the "C" terminal. This LED may be wired to the control panel or it can be left in position on the board. In addition to showing train detection the LED also indicates the length of time the train stops for by flashing more quickly for short pauses than for long ones. The time the signal stays at clear is adjusted by rotating the "pot" (mounted on the board) with a small screwdriver.


The Dapol signal is designed to use a single push button switch (push to make) this is pressed momentarily to both move the signal arm to clear and dangerl. Because of this the signal must be at danger when first installed. If it is at clear when the Semaphore Sequencer is inactive it can be set to danger in a number of ways. A push button switch could be wired to connect between T1 and C2. A wire could be used to touch between T1 and C2. The unit could be activated and the power switched off whilst it is timing. When the power is switched back on the signal will now be in the correct danger position.

Provided the Semaphore Sequencer is not switched off whilst it is still timing the signal should never need resetting to danger. After returning the signal to danger there is a dead tine during which the Signal Sequencer will not operate the signal. The purpose of this dead time is in case the rear of a long train moving slowly is still over the infra red detector at the end of the timing sequence. Without the dead time this would cause the signal to return to clear.

Station and Junction wiring

push button switch to synchronise dapol model semaphore signal operation

For stations or junctions the signal may need to remain at danger for some time after the train has been detected approaching the signal by the infra red. For example stopping train approaching a station or a train approaching a signal followed by an adversely set point at a junction. Terminals A and C are provided for this. A manually operated switch (station etc) or contact operated by the points movement (junctions) is connected to A and C as shown in the diagram. More than one switch or contact could be connected if required.

Whilst the switch or contact is operated the signal remains at danger when a train crosses the infra red. Once the switch or contact is released the board will have remembered the train has crossed the infra red and change the signal to clear and start the timer. If no train has crossed the infra red the signal will remain at danger regardless of the switch position.


Timing is adjusted by rotating the "pot" on the Semaphore Sequencer with a small screwdriver. Timing can be adjusted between 10 and 120 seconds.


6 x 1.25 inches 150 x 32mm


Dapol have changed the recommended voltage for their signal. Follow their instructions for the signal power. The Semaphore Sequencer will operate with 12 to 16 Volts AC. If necessary both the signal and semaphore sequencer could use different power supplies although 12 volts DC should be suitable for both.

Dapol junction and bracket signals

These signals are operated by servo motors built into the signals base. They are supplied with a SPDT switch (single pole double throw aka changeover switch). We have several boards that can operate this type of signal. The simplest wiring is with the IRDOT-3D which is located in front of the signal. When a train reaches the IRDOT-3D the signal arm will change to clear and the IRDOT-3D will start its built in timer once the train has crossed the IRDOT-3Ds sensor. Once the timer has finished timing the signal arm will return to danger. To wire the IRDOT-3D all that is required are 2 power supply wires to power it and the brown orange and yellow wires from the signal being connected to terminals e (common) D (normally open) and f (normally closed).

A more sophisticated control of the Dapol Servo operated signal would be provided by the IRDASC-3. This would provide interlocking with points and the occupation of the next block section as well as the signal returning to danger when the train leaves the section rather than using a timer.

push button switch to synchronise dapol model semaphore signal operationJunction signals can be operated automatically so that as a train approaches the junction the signal arm corresponding to the route set by the points changes to clear The wires to operate the signal arms are connected to contacts on a Dual Timer board. When the timer is timing the signal arm will go to clear and when the timer finishes timing the signal arm will return to danger. The timers are started by activating the IN1 and IN2 terminals. Terminal 2 of an IRDOT-1 infra red detector will activate IN1 and IN2. As the diagram shows a contact operated by the point motor so indicating the position of the point is used to connect terminal 2 of the IRDOT-1 to either IN1 or IN2 depending on the points setting.