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SA1, SA1-S and SA1-DSS


control board for shuttle to run a train backwards and forwards along a single track

The SA1, SA1-S and SA1-DSS are used for the automatic control of a train moving backwards and forwards along a track. They are very effective in running a branchline service automatically. The SA1-S functions identically to the SA1 but has the addition of controls for a starter signal at each end of the line. These signals change to clear just before the train departs and return to danger after the train has left. The SA1-DSS also controls the train identically to the other boards but has controls for Dapol Semaphore Signals at each end of the line.

No controller is required although one may be wired into the board to override the operation. Alternatively a DPDT switch could be used to switch between manual and automatic operation. See switchesfor more information about this.

They are suitable for all analogue N and OO gauge engines. All the SA boards can be used with Z gauge provided a regulated 12 Volts DC power supply is used to power the board and Heathcote Electronics makes a small modification. This is to ensure the output voltage is 9 volts or less so that the locomotive's motors are not damaged. Modern O gauge locomotives work fine as they have efficent electric motors, small LGB engines also work fine. With larger LGB locomotives or old O gauge ones check that the current consumption is not more than 1 amp. If it is we can custom build boards to supply much higher currents.

The SA1 like all the SA boards has an adjustable maximum speed, an adjustable minimum speed, an adjustable wait time and an adjustable rate of gradual slowing/acceleration.

The maximum speed sets the top speed of the train after it has finished accelerating. When it decelerates it reduces its speed to the minimum speed setting which is usually set to a slow crawl. The reason for this is that if it decelerated to a stop it might not consistantly stop in exactly the same place due to its motor warming up, track getting dirty, etc. The minimum speed overcomes this. The rate of slowing acceleration can be adjusted to allow the train to slow to minimum speed in any distance between a few inches and 10 foot or more.


Train Detectors

shuttle arranged along a single track with starter signals

The SA1 needs to know where the train is so that it can slow or stop the train. Either IRDOT-1 infra red detectors or reed switches and magnets can be used to detect the trains. D1, D2, D3 and D4 refer to the terminals on the SA1 board. The locations of the train detectors which wire into their terminals are shown in the diagram. D2 starts the braking for trains travelling to the left, D1 starts the braking for trains travelling to the right. D3 and D4 stop the trains and start the (adjustable) wait time before sending the train back in the opposite direction.

Location of train detectors

If IRDOT-1 detectors are used then the stop (D3 and D4) train detectors are positioned where the leading ends of the trains stop. In other words about 1 inch from the buffer stops. If reed switches are used there positioning must correspond to the position of the magnet on the train.  Allow the same braking distance at each end of the line.

For a short line of 3 metres or less then a single start braking detector can be used for both directions. This detector is located at the centre of the line and wired to both terminals D1 and D2.

If you do not require braking at one or both ends of the line then the stop detectors can be used on their own provided they are also wired to the corresponding start braking detector. For example if no braking is required on the right hand end of the line then terminal D1 and D4 need to be linked together and the detector in the right hand stop position wired to these terminals. The reason for this is that the SA1 works in a sequence and will not stop the train until it has been told to brake.

Wiring IRDOT-1

Wiring for SA1 shuttle and IRDOT-1 infra red train detectors

The IRDOT-1s must be powered from the same Power supply as the SA1. Every  terminal 6 connecting to "o" and every terminal 1 connecting to the "+"terminal. Terminal 2 connects to the appropriate terminal D1, D2, D3 or D4 of the SA1.

Reed switch

One end of each reed switch is connected to the "o" terminal and the other end to the appropriate input ie D1, D2, D3 or D4.


Operating sequence

The train is positioned at the left end of the track at the start of the sequence.  On powering the unit it operates as follows:

1.Adjustable time delay indicated by the red and green LEDs flashing.

2.Signal 1 switches to green (SA1-S only).

3.Red LED lights for approx 4 seconds

4.Green LED lights and Train gradually accelerates to maximum speed.

5.On reaching the train detector connected to terminal D1 the red LED lights to indicate braking, and the train gradually slows and signal 1 returns to danger.

6.On reaching the train detector connected to terminal D4 the train stops and the red and green LEDs flash for the adjustable delay. (the length of flashes is proportional to the length of delay set)

7.Signal 2 switches to green (SA1-S only).

8.After a short delay indicated by the red LED, the green LED will light and the train will accelerate to the left.

9.On reaching the detector wired to terminal D2 the train will brake and the red LED will light on the SA1 board. Signal 2 will return to danger.

10.On reaching the detector wired to termina D3 the train will stop and the sequence will continue from 1.

(on a long line the signal will return to danger before the train has reached the braking detector as there is an internal timer to time out the time it stays at clear)

LED indications

Green LED lit = Train accelerating or travelling at maximum speed.

Red LED lit = Train braking or stopped.

Red and green LEDs flashing alternately = adjustable time delay.  The length of time of the delay is proportional to the duration of the flashes.

Red and green LEDs both lit = Overload. ie short circuit on track.

Power connections

The SA boards are designed so that the control electronics and track power can be separated.  This allows either the whole unit to be powered from a single AC or DC power supply; or the track power to be supplied from a controller and the control electronics supplied from a separate AC or DC power supply.  The reason for using a separate controller is to allow the train to be slowed or stopped manually and the maximum speed to be set by the controller setting.  This option is of more use with other units such as the SA5, SA6, SA7 and SA8.  Feedback controllers will not work with the SA units.

12 volt DC supply

This option powers both the control electronics and track from a 12 Volt DC power source.  The external diode and capacitor supplied with the unit can be left in place or the diode can be replaced with a link wire and capacitor removed. Connect a 12 volt DC supply with positive to the "+" terminal and negative to the "o" terminal.. When first powered the train must move from left to right (move towards D1 and D4). If it moves in the other direction swop over the two wires to the track.

Using a smoothed regulated DC power supply will give pure DC to the track suitable for N gauge locomotives and coreless motors. We can provide suitable power supplies.

AC Supply

Replace the wire link with a diode, the band on the diode must face the direction shown in the diagram.  The supply to the track with an AC supply will be unsmoothed DC.  This may cause damage and the overheating of N gauge locomotive motors but is fine for most OO gauge and may be preferable to a smoothed supply. The DC on the track may be smoothed by using a (electrolytic) capacitor. The capacitor must be attached with its negative leg to the rightmost terminal. Using the capacitor will make the track power smooth and suitable for N gauge and coreless motors. The capacitor is supplied fitted to all the SA boards.

Train controller

wiring shuttle to controller

Remove the link or diode and capacitor and connect as shown.

The automatic control  only works with one direction setting of the controller. The other direction setting will allow trains to be reversed manually.  Feedback controllers will not work with the SA board.

Connections to signals (SA1-S only)

To give a realistic effect there is a short pause between the points changing and the signal changing to green then another delay before the train moves away.

Contacts on the SA board switch the signals. These contacts have no electrical connection to the rest of the SA1 board. Effectively wiring signals to the SA1-S board is electrically identical to wiring them to single pole changeover switches. The internal relay contacts are shown as white lines.  "sc" is the common to which one wire of the signals power connects. This is switched to one of the adjacent terminals by the SA1-S to light either the red or green.

The signals can use the same power as the SA1-S or a separate supply.  The diagram shows wiring for common negative LED signals (most UK signal manufacturers make this type of signal) using the SA1-S supply.  A diode is required if an AC supply is used as AC may damage the LEDs in the signals. The diode converts the AC to DC preventing this damage. A resistor (usually supplied with the signal) is used with each signal to limit the current through the LEDs. There may be a single resistor in the common return (blue wire) as shown or two resistors one in the red and one in the green wire. Either method reduces the current to the LED and prevents it being damaged.



The diagram on the left shows bulb signals which use the same power supply as the SA1-S. Use of resistors depends on the voltage of the bulbs and the voltage of the power supply. Refer to signal manufacturers instructions.

LED signals


These are wired in the same way as bulb signals but the LED polarity must be correct and current limiting resistors used. (two resistors are shown as rectangles on the diagram). The diagram shows common negative signals. This means that the signal has been internally wired so that the two short legs (cathodes) of the LEDs are wired together within the signal  If an AC supply is used the diode shown in the diagram is required.  If a DC supply is used the diode is not necessary.

using 2 wire signal with SA1 shuttle

The diagram shows how to wire to an LED signal with just 2 wires. This type of signal requires the polarity to be reversed to change its colour.

Semaphore signals

The relay contacts of the SA1-S can be used to operate semaphore signals by using them to operate a slow motion point motor or a relay.  The relay is the type with a moving arm  which can be adapted to move the signal. For slow motion point motors use the circuit shown for the 2 wire signal. This circuit reverses the polarity to the point motor. Alternatively it can operate the Servo Motor Controller or Bouncing Semaphore Controller for semaphore signals operated by servo motors. See below for connection of SA1-DSS to Dapol semaphore signals.

Testing and set up adjustments

The SA1 can be tested without the detectors. To do this, touch a piece of wire between the "o" terminal and appropriate "D" terminal. The terminals operate in a sequence.  Operating D4 will not stop the train if D1 has not been operated first. The signal applied to the inputs is 0 volts.  Hence the operation can be verified by touching the input terminals in turn with a piece of wire connected to terminal "o".

There are adjustments on the board for maximum speed, minimum speed delay time and rate of acceleration/braking.  The train will accelerate to the maximum speed setting and brake to the minimum speed setting.  Turn the variable resistors with a fine screwdriver to the end of travel positions.  Initially adjust to give:

delay time=minimum (turn pot clockwise),

min speed=minimum (turn pot clockwise),

max speed=maximum (turn pot anticlockwise)

rate of braking / acceleration = abrupt (turn pot clockwise). 

All the variable resistors turn through 3 quarters of a revolution.

With a train positioned at the left hand end of the line switch on the power.  After the variable delay (green and red flashing) followed by the green LED lighting, the train should travel towards the right (towards D1 and D4).  If it travels towards the left, swop over the two track power connections. On reaching train detector D1 the red LED should be lit.  And the train should rapidly slow to a halt.

Adjust the minimum speed control until the engine is moving at its slowest reliable speed.  When the engine reaches the train detector at the end of the line (D4) it will stop and the red and green LEDs will flash

The purpose of the minimum speed control is so that a braking train does not slow to a stop before reaching the end of the line. Whilst the green LED is lit adjust the maximum speed control until the train is travelling at the desired maximum speed. When both the minimum and maximum controls are set then adjust the rate of braking/acceleration so that the train slows to its minimum speed at the place where it is to stop.

Finally adjust the delay time to give the desired waiting time at each end of the line.

Overload protection

Overload protection is built in to the SA1.  If a short circuit occurs on the track due to a train derailing etc then the unit switches off power to the track and lights both the red and green LEDs.  Power is restored once the fault is removed.  The overload protection is triggered when the current rises above 1 amp.


Sidings can be added to one or both ends of the track. Trains will travel alternately along the line. It is possible to add sidings at one or both ends using IRDOT-Ps to switch the points, so allowing any number of trains to alternately run backwards and forwards between the sidings.  The IRDOT-P "P" terminals will replace the train detectors at the ends of the lines.

Using the SA1 with IRDOT-P controlled sidings

The shuttle line can have any number of sidings at one or both ends of the line. Suppose there is a point at each end of the line, this will allow 3 trains to run alternatively as there will always be an empty line for a train to enter. An IRDOT-P is installed at the end of each siding.  The IRDOT-Ps both switch solenoid (Peco, Seep, Hornby, etc) type point motors and notify the SA1 that the train has arrived and is to stop. The P terminal of the IRDOT-P does the job of terminal 2 of the IRDOT-1. The point settings isolate the stationary trains, both insulfrog and electrofrog will do this. All the "P" terminals of the IRDOT-Ps controlling the left hand sidings are wired together and connected to terminal "D3" of the SA1.  IRDOT-Ps at the right hand sidings also have all the "P" terminals connected together and connected to the "D4" terminal of the SA1. As the IRDOT-P P terminal gives a short pulse rather than a continuous signal then there is no problem with stationary trains over IRDOT-Ps masking newly arrived trains. The IRDOT-Ps have a contact that connects together their R and R1 terminals

The A terminal of the SA1 stops the IRDOT-Ps operating when a train is departing

A connection is made from terminal "A" of the SA1 to every IRDOT-Ps "I" terminal. The "A" terminal switches to 0V as a train is about to depart. Its purpose is to prevent the IRDOT-Ps falsely triggering when a train is halfway across the points.

A single point motor is automatically operated by two IRDOT-P

This diagram shows how the two IRDOT-P are wired to operate the point motor. A CDU could be used for the power supply. One terminal of the power supply connects to the common of the point motor. The other connects to both R1 terminals. When the IRDOT-P detects a train its internal contacts will momentarily connect R to R1 so energising the point motor. An arriving train should cause the point motor to set the point to the other line. If this does not happen swop over the wires to the R terminals.


3.7 x 3.7 inches

93 x 93 mm


12 to 16 Volts AC or DC




Apart from the signals the operation of the SA1-DSS is identical to the SA1 and SA1-S. This diagram shows the two Dapol semaphore signals connected to a 16 Volt AC supply. The signals are designed to only work from AC and are damaged by DC.

The yellow wires intended for connection to push buttons are wired to the contacts on the SA1-DSS. These conracts close then re-open when the signal is to change so performing the same action as a push button switch.


3.7 x 3.7 inches

93 x 93 mm


12 to 16 Volts AC or DC