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GCA129 Led-indicator interface


By Peter Giling


In some large displays, it might be nice to indicate a route, where a train is sensed.

But the larger the track on display is, the more leds you will need to have a nice indication.

These leds of course can be connected all parallel together, all with separate resistors, but the GCA50 or CAN-GC2 is not able to supply that.

With this interface it is possible.

Depending on the separate connected power supply, (more about that later) it can switch on and off a serial chain of multiple Leds.

The current throught these leds is limited (with the listed resistors) to approx 18 mA.

For example: with 24 V dc power suplly it will be possible to make a chain of 7 green/white/blue leds or 10 red/yellow leds.

The power supply must be a dedicated one for this interface only, but the power could also be taken from the LocoNet power line.

Simply connect point 1 of DB9 connector of GCA50 or CAN-GC2 to J4(pt2).

The + (plus) of the led-chains are connected to J4(pt3). The - (minus) of the 16 led-chains are connected to J1.(pt1..16)

The board needs all 16 outputs from GCA50/CAN-GC2.

GCA50 is connected via J2 and J3.

Important notes :

* It is strongly discouraged to exceed 24 Volt supply, to protect anyone from possible physical harm.

* We refuse any responsability of not complying to this remark!

* Also precaution has to be taken that power supply is adiquate fused, to prevent fire hazard in case of fault connections.

* Since each chain has a transistor which regulates the current, this transistor can become seriuosly warm.

To be on the safe side, the total power of each transistor should not exceed 1/4W.

This power is calculated by this formula :

P = (V-1.2-(Vl x Leds)) x 0.018

where P = total dissipated power in transistor in Watts.

V = the power supply in Volt

V1 = depending on type of leds > each colour has different voltage see table below

Leds = total amount of leds in the given line.



V1= 2.2 (red led)

Leds = 4

P= (24-1.2-(2.2 x 4)) x 0.018 = 0,245W THIS JUST FITS!

Table of Voltages for Leds This figures are general info and can slightly be different.

led colour Volt
white 3.1
warm-white 3.2
blue 3.2
green 3.2
yellow 2.2
orange 2.2
red 2.2
purple 3.7

There is a nice info site about leds, originated in Dutch, but also in English available:

The hardware

* The pcb-layout also contains the parts displacement

The protoype

Thanks to Walter Pohler I have a picture of the first made item.

Bill of materials

Qty Ind Description
16 R1..R16 4K7
16 R17..R32 33E
32 D1..D32 Diode 1N4148
16 T1..T16 BC547b
1 J1 Header 2x8 pins
2 J2..J3 PS254/10G
1 J4 AKL012/3

Connection cable GCA50 <> Interface

Two possible options can be used for this connection. (see picture below)

Option 1 (left on picture)

This cable should be made with wire 0,25mm.

Qty Materials supplier
2 PSK254/10W
1 PSK-Kontakte (set of 20 pcs)
1 Psk-Crimpzange
Option 2 (right on picture)

Flatcable 10x2 + connectors. The side without the notch will fit easily.

Qty Materials supplier
2 PFL 20
1 Flatcable AWG 28-20G 3M
1 Crimpzange MWZ 214
Both options are used here
mgv129-en.txt · Last modified: 2018/11/12 08:56 (external edit)