Speed controller for model train

The Type ZN419CE servo control IC from Ferranti is ideal for use in model railway direct-current engines. The internal circuit diagram of this device is shown in here. The IC varies the pulse width of input pulses in direct proportion to the displacement of a potentiometer. It operates with positive-going input pulses which can be coupled directly or via a capacitor to input pin 14. The advantage of a.c. coupling is that should a fault occur which causes the input signal to become a continuous level, the servo will remain in its last quiescent position, whereas with direct coupling the servo output arm will rotate continuously.