Märklin’s recently introduced light signal Type 74391, designed for HO model railway sets, offers an affordable option at 10 euros (£7·00). However, there’s a catch – its operation necessitates the use of the signal keyboard Type 72750, increasing both cost and rigidity of the setup. Thankfully, a simpler and more economical alternative exists, requiring only a switch and two diodes. This solution capitalizes on the fact that, from an electronics perspective, the light signal comprises two LEDs connected in anti-parallel with dropping resistors. The diagram’s lower section on the right, represented by Da, Db, Ra, and Rb, illustrates the typical internal circuit of such a light signal. When the light power source’s output voltage is applied to the circuit, both LEDs illuminate.
Selective Color Control with Diodes
To achieve either a red or green signal, voltage is applied through diodes D3 and D4. Switch S1, a change-over switch, determines which color signal will be visible. This approach offers a cost-effective solution for the desired functionality.
Enhancing Control in Complex Setups
For more intricate setups requiring direct control over the track’s light signal color, additional anti-parallel-connected LEDs, D1 and D2, can be integrated into the connection between switch S1 and the light signal. This modification provides greater flexibility in managing the light signals.
Considerations for LED Types and Circuit Compatibility
Light signals typically consist of a standard green LED and a low-current red LED. Consequently, the dropping resistors have different values due to the LED specifications. In this circuit, D2 must also be a low-current type to maintain compatibility. Modern Märklin light signals often feature two low-current LEDs. This can be verified by connecting the circuit temporarily and measuring the current for both switch positions, ensuring the appropriate LED type is used for compatibility.
Scalability and Current Requirements
The choice of diode types, such as 1N4148 or 1N4001, impacts the circuit’s compatibility with multiple light signals. When using 1N4148 diodes, the circuit supports around five light signals with standard LEDs or up to twenty with low-current LEDs. If 1N4001 diodes are utilized, the circuit can handle currents up to 1 A, expanding its application for larger setups.
Adaptation for Different Power Sources
For analogue railway tracks utilizing 16 V AC power sources. Märklin light signals can be connected directly or through D1/D2. In contrast, digital tracks employing higher supply voltages require correspondingly higher-value dropping resistors or an additional resistor in series with D1 and D2 for proper functionality.
Practical Tip for Analog Track Systems
To maintain the convenience of analog tracks where the train stops automatically when the signal is red. Isolate the power line to the last rail before the light signal. Power this rail via the second contact of S1, as illustrated in the circuit diagram. This straightforward modification ensures seamless integration with analog track systems, enhancing the overall user experience for model railway enthusiasts. For additional guidance and advice, the author’s website, www.koerber-home.de, offers a wealth of information tailored to model railway enthusiasts.