With two common NPN-transistors and two resistors you can build this simple adaptor to connect bipolar switch machines with the Märklin Digital system. It is connected to the k 83 decoder.

The best way to build this adaptor is to use a small Veroboard-PCB (pre-drilled holes). You can get a good idea of the size needed from the layout below, where the view is from the component side. The component-legs are mounted through the holes and soldered to the copper ribbons on the solder side. Use transistors TIP 41 or similar. If the adaptor is doing heavy duty work a small cooling sheet profile should be attached to each transistor, be sure they are not touching each other. That is the reason for separating the transistors on the pcb.

Both transistors should normally have 5.6 kilo-ohms resistance value( 1/4 watts).That corresponds to the green-blue-red color code. Some older switch machines move very slowly and require more amps than the average ones. You detect this as a slow,hesitating movement. Sometimes you even need to press the button on the keyboard twice.

There is a way around that problem. Change the value of the resistors to 4.7 kilo-ohms ( yellow-violet-red color code)