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GCA77 Interface for turnout coils and illumination



This board is available as board only or complete parts package at .

By Peter Giling


- 8 outputs each 0.5A max .
- Opto-coupler isolation between Loconet and outputs.
- Simple connection to GCA50 / CAN-GC2 / GCA_PI02 / WIO_*
- For switching turnout coils.
- Also to be used as light switch.
- Each one of 8 outputs is separately useable for switching lights or other items.

Older version

For older MGV77 version 1.3 look here


First question may is: why should we use interface and MGV50 LocoNet, since there are turnout decoders available that connects straight to the rails, and will be commanded by either Motorola or DCC commands.
There is a simple argument for that.
If you ever used turnout drives , commanded with Mot. or DCC, you also will have seen that everytime you, or in our case Rocrail, switches a turnout, you can see that light in Locs are flashing, due to the sudden drain of power from the rails. And the longer the connection between central station and the turnout decoder is, the lower the available power for that turnout will be.
One more important reason for this, is the fact that in HO scale, the standard voltage is about 15V, which can be too low for many type of turnouit coils.
Since we did not like these effects, we prefer a separate supply for turnouts.
This supply is even not used for anything else on the railroad track i.e. lights etc.
In that case it is better and far more reliable to send commands for turnouts and signs by means of LocoNet, CBUS or RocNet.
The standard I/O module GCA50 / CAN-GC2 / GCA_PI02 has limited capacity for switching outputs.
These I/O modules can only deliver 5 Volts and max 25 mA on each output which is suitable enough for connecting LEDs and small relays.
Turnouts and arm signs with coils use much more power, in voltage and current, than GCA50 / CAN-GC2 / GCA_PI02 are capable to supply.
For that purpose the first available interface is GCA77.
It is a good interface for using with low current turnout drives, mostly in N scale systems.
On one side (top on picture) it is connected with I/O module, by means of 10 wire cable.
At the bottom screw terminals 8 outputs (4 turnouts) are to be connected.
An addional power connection needs to be connected to transformer/adapter 12-16 Vac or 18-20 Vdc.
Optocouplers on board will separate the I/O module circuit from the addional power.
This board also can be used to switch lights on your railroad setup.
It is to be considered, that total max load of the printed circuit can not exceed 1 Amp.
If more than 300 mA is to supplied by the circuit, the Fuse F1 has to be adapted.
For heavier loads, refer to GCA76 (double coil system) or GCA125 (single coil or motor system).

The hardware

Cable to GCA50 / CAN-GC2 / GCA_PI02

Known current of switch coils

For detailed info about different makes of turnout solenoids please refer to this page

Safety precaution

A short circuit at one of the outputs the fuse F1 will trip .
It resets itself after problem is solved, or ac power has been shut off.
With complete kits, a 0,5 Amp fuse is included, which will normally trip when more than 0,8 Amp is absorbed.

Instructions for controlling GCA77

The GCA77 must be activated with a positive pulse.
That means that there will be 5 volt feed into the input, wich must be shut off shortly after that.
On GCA77 itself , there is no electronic device that will take care for this shut off.
Standard turnout decoders usually (there are exceptions) do not have this facility also.
Shut off is done by software in central units or programs, which will activate the coil, and then inactivate after approx 200 miliSecs.
With an GCA77 connected to GCA50 /CAN-GC2 this can be done more safely by GCA50 / CAN-GC2 hardware itself.
Examples below show how settings could be done.


:!: *It is essential to do the settings of GCA50 / CAN-GC2 first, before connecting switches!*
:!: *Always repower GCA50 / CAN-GC2 after programming !*
The best way to check if programming is done in the right way, and pulses are working, is using a small 12 volt lamp (from automobile » no more that 2 Watts !! ) to check each output.
Each ouput should only shortly light up on activation.

Example of GCA50 settings.

Adjust GCA50 The GCA77 is connected to J5 on GCA50
J6 on GCA50 is used for connection
of GCA93 current detector.

Example of .CAN-GC2 settings.

The first 8 ports are set to control 4 turnouts (8 coils)
Adresses of Turnouts in Rocrail to be set to 25,27,29,31
Ports 9-16 are set to be connected
with GCA93 current detector (Adress 1-8)

GCA76 is connected to J4 on CAN-GC2
GCA93 is connected to J3 on CAN-GC2

Using the test-button
will change position of turnout on any toggle.

Setting example for Rocrail

Output 6 is used to command this turnout.

Using GCA77 for Lighting functions

GCA77 is also suitable for switching or functions in your layout, like lighting.
The programming is only slightle different in GCA50 / CAN-GC2 / GCA_PI02, because you need a switch, and not a pulse.
Please consider that total current delivered by all 8 connections together, should not exceed 1 Amp.

Info on various switch drives.

gca77-en.txt · Last modified: 2024/01/23 11:08 by phg