Smart LED selector

smart LED selector I In this tiny circuit, for use in, for in- stance, a two-lights model railway signal, one of two LEDs may be selected with either a single pole switch or a series transistor, as shown in the circuit diagrams. Note that the LEDs are fed via a common current limiter resistor, while a switch is con- nected in series with one of the LEDs. Why do not both light simultaneously when the switch is closed? Because, apart from their colours, the two LEDs also differ as regards their for- ward voltage drop; when connected in parallel, therefore, the LED having the lower voltage drop should be fit- ted with the series switch; this ar- rangement causes the high voltage drop LED to light when the switch is open and to go out when the switch is closed, at which moment the other LED takes over. Two of the accompanying four small circuits show the use of a series switching transistor rather than a real switch, but the difference hardly re- quires further detailing, since apply-...