What about semaphores?
Semaphore signals are normally set at danger and only change to clear when an approaching train is to pass them. Interestingly, colour light signals are normally at clear unless a train has just passed, or the next signal up the line is at danger. The simplest way to replicate semaphore operation is to have a detector before the signal which will change the signal to clear and also start a timer which, on timing out, will change the signal back to danger. A more sophisticated method is to have a detector before the signal to change the signal to clear and a second detector after the signal to return it to danger. The IRDASC-1, 2, 3 and DSS work in this way. In fact, the return to danger detector is also used as the ‘set to clear’ detector for the next signal along the line. Dapol’s signals feature a mechanism that moves the signal arm, operated by a toggle switch which changes the signal between clear and danger. Heathcote Electronics’ Semaphore Sequencer can control Dapol signals, and is positioned before the signal. When the train reaches the Semaphore Sequencer, it closes a contact for one second and starts its timer. At the end of the timing interval the contact closes for one second, and the signal returns to danger. Heathcote Electronics’ boards come with control servos, including the Bouncing Semaphore Controller, which gives the signal arm its characteristic bounce.