In ESU decoders, as for all other brands, because the DCC standard imposes this, the amplified outputs are active at low level. This is why all wiring diagrams show receivers directly connected to the positive common (blue wire when it exists). Okay, in the example below, the front and rear lights are connected to loco housing, to the chassis, but they could be connected to the blue wire too.
For unamplified outputs, this is a mystery. ESU doesn’t indicate their active state nor, as already mentioned, their maximum current, nor their logical levels. This is where my little research on the Internet comes in, notably on the Loco Revue Numérique et Digital forum (funny title for that matter, since it's the same thing!), on page Fonctions AUX décodeur ESU et Zimo. According to the given diagrams, one with MOS transistor, the other with NPN bipolar transistor, the logic outputs are active at high level. I restore the two diagrams, using symbols a little more standardized!
According to a German forum diagram, the bipolar transistor circuit operates with base resistors of 4.7 kΩ. By neglecting voltage drops, this implies a maximum output current of around 1 mA, assuming that these outputs high level is 5 V (see below). If the operating circuit doesn’t consume more than this current, it will therefore be possible not to use a transistor and directly connect the circuit to the output, according to the third diagram opposite.
Attention: in Lokpilot V4.0, the common positive (blue) is directly accessible, but not the negative, which is on a soldering pad; in Lokpilot Micro, the two commons are on soldering pad. With the AUX1 and AUX2 outputs, there will be in this case four micro-soldering to make!
N-MOS transistor control.
Bipolar transistor control.
Direct control without transistor.
To calculate the resistance to put into the circuit of the LED, if there is of course an LED, we must know the output logical high level. If the output comes directly from the microcontroller that equips the decoder, it is likely that this voltage is 5 V or less, because most microcontrollers operate under this voltage. And to be sure, there is only one way: to measure it. Something I will not fail to do… on occasion.
Having a free ESO Lokpilot 4.0 decoder with 21MTC connector, I was able to test the famous logical outputs. For this, I have prepared a 1.27 mm pitch two row male connector on which I soldered the minimum number of wires. This is small: you have to carefully solder the wires and make sure there are no contacts between the pins. Here is the wiring diagram, with the six wires needed to test the AUX3 output.
The terminal numbers are those of the 21-pin connector. It is necessary to connect a resistor to the motor output so that the decoder can operate. Here is the decoder equipped with the connector.
General view of the test assembly. The decoder is gently tightened into the vise to prevent it from moving. The two wire grips are connected to the Lokprogrammer, which allows to configure the function keys and control the AUX3 output level.
The output voltage is actually 5 V. The internal resistance is approximately 20 Ω. The test was carried out with a current of 6.4 mA without any problem. So a current of at least 5 mA could be used without worrying. With a 1 kΩ series resistance, a 3 V forward voltage LED will consume 2 mA. The detailed test flow is recorded in a PDF document which can be consulted here (in French).