As the original use of Hoffman motors proved unsatisfactory these have been replaced with inexpensive R/C servo motors controlled using a kit available from MERG. This gave the following benefits: Speed of movement is adjustable and so all signals can be set to the same speed whereas the Hoffmans proved extremely variable; Limits of travel are set in the controller so physical stops were no longer required; Up to three 'bounce' positions could be set at each end of travel.
The signals and servos are installed so that the signal arm and servo are both half way through their travel with their respective operating arms vertical. This gives the optimum amount of travel in either direction away from the midway point. Experimentation showed that if either end of travel for the servo lay in the horizontal plane it was difficult to get a good 'bounce' because the servo was moving the operating link up and down rather than pushing or pulling the operating arm of the signal.
Experimentation also showed that it was best to arrange for full travel of the servo to be slightly more than full travel of the signal operating arm. In the case of the Ratio signals used the signal operating arm moved about 8mm from 'off' to 'on'. The hole in the servo operating arm was placed so it had about 10mm of travel from end to end. This allowed for trimming the 'on' and 'off' position of the signal arm using the settings in the servo controller so that installing the linkage from servo to signal did not require watchmaker like precision engineering. Of course it is important not to have to great a difference in the amount of travel or else subtlety of movement is lost due to the limited amount of travel available to the servo.
Lastly make sure the operating arms are aligned in the same plane, like the coupled wheels of a locomotive . This ensures the action from the servo is just moving the signal operating arm and not pulling the linkage from side to side. If they're not nicely lined up then twisting force may cause the linkage may pull out of one of the operating arms or the signal post to wobble about.