I was waiting to see what people would say in answer to this, but there doesn't seem to have been a rush!  Most people these days seem to use some kind of electrical servo mechanism, but I use gravity in much the same way that real semaphore signals are usually worked.  A crank below the baseboard, with a lump of lead on one end and the other end attached to the signal.  The crank is connected to a lever in a lever frame by fishing line.  Some pictures might help.

Firstly the mechanism for a signal with two arms.  The pivot for the cranks is a 6BA bolt, and the holes in the cranks are tapped 6BA which means the cranks are screwed onto the pivot and no retaining collars or washers of any kind are needed.  In this picture, no 'stops' have yet been fitted.  The mounting is a section of aluminium angle; the mounting holes are tapped 8BA and the fixing bolts pass through the base of the signal and the baseboard, and then screw into the aluminium angle.

Next, the parts for a mechanism for a gantry which has six arms, although there are only parts for five in the picture.

The next two show the same mechanism attached below the signal on a test rig.  In the first picture, you can see the holes in the 'long' arms (which are made from lengths of old rail) where the cord from the lever is attached.  The cord running from the lever also incorporates a tension spring somewhere along its length, so that the lever 'over travels' slightly; this also gives a nice smooth action to the arm.

You can also see the 'Off' stop which is another bolt, slightly below the pivot bolt.  I also provide an 'On' stop on the other side of the cranks, against which the cranks drop when the signal is retuned to danger - if you get this 'just so' the signals will bounce in a random way when they are replaced to danger.  The stops should prevent the fragile scale parts of the actual signal from any stress from the cord being pulled.

This shows how the linkage was connected to the signal in this case.  Before installation, the upper parts of the connecting wires can be blackened to make them less obtrusive.

I hope this is useful!