I just did a search on the Internet to use the so-called logical outputs of ESU decoders. What I am interested in is to equip a locomotive with a Lokpilot Micro V4.0 which would make it possible to separate the white and red lights. Indeed, this decoder has only two wired outputs (front and rear lights), but it has two additional outputs on soldering pads, AUX1 and AUX2, which ESU calls logical outputs. According to this manufacturer, these outputs cannot be used directly on any load.

Here, as an electrician, I rebelled! Even if there is a need of transistors to amplify these outputs, they will have to provide a minimum of current. The problem is that ESU doesn’t give the maximum value of this current.

Personally, being a fan of discrete lights, I am often satisfied with LED circuits consuming only a few milliamperes, and I believe, but it’s to verify, that the decoder can provide such a current on its logical outputs. But this is not the only question: it’s also necessary to know these outputs active state.

Let me explain.

What is a logical output?

Contrary to what ESU suggests, all the outputs of a decoder, including the motor outputs, are logical outputs. Indeed, in electronics, a logical output is an output that can take only two values ​​(with a certain tolerance of course), distinctly different. This behaviour is contrasted with that of an analogue output, which can take an infinity of values ​​between two limits, such as, for example, the voltage supplied by a conventional rectifier transformer.

Perhaps you think: OK for the auxiliary, lighting or other outputs, but for the motor one? Well, it’s the just the same: the output voltage will change quickly from zero volt (approximately) to 14 V (depending on your command station configuration) and vice versa. An oscilloscope will show a succession of pulses more or less close together. This is the very principle of variable duty-cycle control, which I will not give here a complete explanation: this would bring us a little too far.

To be more rigorous, it would be necessary to distinguish between amplified and unamplified logical output.

If you want a more detailed explanation of the internal operation of a decoder, it is here.

Logical state and logical level

What is this yet? A logical state is a familiar notion: a door is open or closed, a lamp is switched on or off, a transistor is turned on or off, and so on. Note that this is actually logical: one does not bother to know if the door is more or less ajar, or if the lamp shines little or much. To ease reasoning, one of the states is assigned the value 0 (zero) and the other the value 1 (one). But when one wants to wire an electronic circuit, one needs to know the electrical values (voltage, current) corresponding to these states. These are the logical levels, which are usually voltages, expressed in volts. The lowest of these voltages is called low level, and the highest… high level, bravo!

Active state of a logical output

The active state of a logical output is the one for which it is capable of actuating a receiver (for example, lighting a lamp). An output can be active at high level (case 1) or at low level (case 2).

In both diagrams, the decoder’s internal circuit is represented by a simple contact (in fact, it is a transistor, but it doesn’t matter). In the first case, the output, when active (= contact closed), applies a 14 V voltage to the receiving circuit, represented by a bulb: it can be seen that the other terminal of the bulb must be connected to the Common negative, denoted GND for ground, so that it can light up. In the second case, the active output connects the bulb to GND. It is therefore necessary that its second terminal is connected to the positive common, denoted VCC or VDD.

Output active at high level

Case 1: output active at high level.

Output active at low level

Case 2: output active at low level.

This may sound a bit complicated, but it’s essential to understand to correctly wire the decoder outputs.