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DCC Bouncing Semaphore Controller


Full size semaphore signals are usually connected by a steel wire to a lever in the signal box controlling them. Due to slack in this connecting wire and the weight of the signals arm and balance weight the signal arm makes a characteristic bounce when the signal returns to danger. Another noticeable feature is that when the signal is changed to clear the signal arm moves a little beyond its clear position and then immediately falls back into place. We have called this movement the overshoot. These factors vary from signal to signal and depend partly on the distance of the signal from the signalbox controlling it. As these movements add a realism to the signals operation we have designed the Bouncing Semaphore Controller to replicate these movements for model semaphore signals.

realistic bouncing semaphore signal for model railways

Adjustments are simple to make. The clear and danger positions of the signal arm, the raise (upward) speed of the arm, the fall (downward) speed of the arm, the amount of bounce and the degree of overshoot past the clear position are all adjusted with six push buttons on the Bouncing Semaphore Controller board.

The Bouncing Semaphore Controller utilises a servo motor to provide movement to the signal arm. A linkage of piano wire (springy hardened steel wire) from the servo motor arm is connected to the signal arm. Servo motors are relatively small and are very suited to operating semaphore signals.

Electrical Setup

The DCC Bouncing Semaphore Controller is connected to the track with 2 wires. This provides it with power and its built in DCC accessory decoder detects the commands to raise or lower the signal arm. There are screw terminals for these connections. To change the signal arm between the clear and danger positions a DCC accessory command is sent. There is a link wire in the two righmost terminal blocks. This link is removed if you wish to use an on/off switch or a contact worked by the point motor to change the signal. A switch or contact can also be used in addition to the DCC command if the link is left in place. This allows interlocking with the points so that the signal remains at danger when the point is wrongly set. The servo motor has a short electrical lead with a connector which pushes onto a 3 way plug on the DCC Bouncing Semaphore Controller.

Programming the DCC decoder

To program the accessory address the push button switch marked PROG is pushed, the green LED will start flashing, the next accessory address sent by the DCC controller will be programmed into the DCC Bouncing controller and become its address.

Mechanical Setup

The Servo motor is usually mounted beneath the base board. We have a bracket which can be used to secure it in position. A small hole is required in the baseboard for the wire linkage. When first powered up the servo motor arm will be near its mid position. The motor arm can move nearly 90 degrees either side of this. You will get more precision in setting the signal arms movement by fixing the wire to an inner hole on the servo arm. If you use an outer hole the amount of rotation will be less and play in the linkage will be more significant.

Lower quadrant signals

Lower quadrant signals also bounce. They can also be automated with the Bouncing Semaphore Controller which works equally well for the reversed direction of movement.

Push button adjustments

With the decoder address programmed send a command to light the green program LED. Now use the adjust1 and adjust2 push buttons to move the signal arm to its danger (horizontal) position. These pushbuttons move the signal arm slowly. One moving it upwards amd the other downwards.

When the adjustment to danger is achieved send another DCC command to the DCC Bouncing controller. The green program LED should now be unlit. The DCC Bouncing Semaphore Controller will now remember the danger setting of the signal.

The adjust1 and adjust2 push buttons are used to move the signal to its clear position. Sending another DCC command to light the green program LED will cause the DCC Bouncing Semaphore Controller to remember this position in its memory. The memory remembers these settings when power is switched off.

The signal will now move between danger and clear positions when the DCC commands are sent.

The speed of the arms movement as it travels from danger to clear is now adjusted with the raise speed pushbutton. Each time this push button is pressed the speed increases. Eight different speeds can be selected. On reaching the eighth (fastest) speed the next press of the push button returns the speed to the slowest setting. Above the pushbutton is a red LED. This gives a series of flashes from one to eight corresponding to the speed.

The fall speed push button adjusts the speed the signal returns from clear to danger in the same way as the raise speed button. The green LED gives an indication of the speed setting.

The bounce push button gives an adjustment of the amount of bounce of the signal arm when it returns to danger. In the first setting there is no bounce and each subsequent press of the push button increases the amount of bounce. The bounce increases in 1/32nds of the amount of movement between clear and danger. The current setting is shown by the number of yellow LED flashes.

The overshoot push button adjusts the amount the signal arm passes the clear position when it is switched to clear. The overshoot is switched off in the first setting. subsequent settings increase the amount of overshoot. The setting is shown by the number of flashes of the blue LED above it. (this LED appears clear on the photograph).

Resetting to Mid Position

In the event of becoming confused you can reset the servo motor to its mid position. This is simply done by holding down the raise speed button until it resets. When reset the servo will be in its mid position. The button needs to be held down for at least 5 seconds.

Servo Motors

There are two types of servo motor, analogue and digital. This is not related to model railway control systems but to the internal workings of the servo motor. Both types work with the Bouncing Semaphore Controller but the analogue type jitters (the servos arm makes a rapid movement and may travel beyond the normal settings for the signal) on power up whilst its internal circuitry settles down. The digital type of servo do not have this problem and so these are the type we now stock.

how to interlock points and signals

Motorising the Ratio LMS upper quadrant semaphore

Although the signal will work with the right angle crank at the base built up this does seem to add extra play particuarly as the hole in the crank is quite large so it is a lot simpler to omit this and just to extend the wire straight down. The cylindrical cover was still fitted as this makes it easy to mount the signal into a hole in the baseboard. We also found that we needed to open up the hole in the post which the arm pivots on. This just needs a small diameter drill. Otherwise if the arm is stiff the post tends to bend a little.

Power supply

Power is supplied by the DCC powered track.


Length 81mm, Width 62mm, Height tallest component 22mm.