User Tools

Site Tools


GCA125 Interface for turtoise type turnout drives


By Peter Giling

Features and facts


  • 8 individual inputs, commanding 8 individual outputs
  • 600 mA max load per output.
  • Max 42 Volt per output.
  • Time selection for output duration.
  • Outputs are activated one by one for power reduction.
  • Processor on board is isolated from the board by opto-couplers


Controlling a LGB 1201, Kato or Fulgurex turnout-drive, in fact also some type of Hoffmann turnout motors, only have one coil or motor, that needs to be controlled by polarity.
Even the Bohler WA2 is very nicely useable with this interface.
The normal driver like GCA77 is not suitable for that.
The complexity of this unit is slightly higher.
For GCA drive-units it is standard, to have an isolation between power for the coils or motors and the used network system by means of optocouplers.
That is also done on this board, but in a bit different way.
The processor runs on 5V from GCA50 / CAN-GC2 or GCA_PI02, and the processor drives two optocouplers to send I2C commands to the I2C expanders.
This unit is able to control 8 turnouts, and need 8 inputs from the GCA50 / CAN-GC2 / GCA_PI02.
It is also possible to use a switch for direct activation without GCA50 / CAN-GC2 / GCA_PI02.
You will need a 5 Vdc to activate the processor, connected to pin1(+) and pin 2(-) of J3.
In that case pin3(=input 1) to pin10(=input8) of J3 are to command the motors.
One other option is to use GCA174 DCC or Motorola function decoder, to drive directly from the central unit.
The software in the PIC16F628 is made in such a way that the processor provides a correct timing for the coil, and switch it off after the chosen time.
This will take care that the turnout coil or motor will not be overheated by too long powering.
The time selection is done with the Dipswitch, and counts for all outputs.
The processor in the circuit needs 11 inputs and 18 outputs, and 3 extra for dip-switch reading.
Instead of these 29 I/O ports, smaller PIC processors have only maximum 16 I/O.

Old version

The hardware files

The schematics
The pcb Layout
N.B. Self made pc-boards are not supported!

Cable to MGV50 / CAN-GC2 / GCA_PI02

Using GCA125 without GGCA50/CAN-GC2/GCA_PI02

It is possible to use the GCA125 as a stand alone unit.
A 5V supply should be feeded to J3 (pin1 +, pin2 -).
Pin 3(=1) to 10(-8) should be switched between +5V and - for position select.


Led functions

Green led: is on when no output is activated.
It is off when one of the ouputs is active.

Red led: will flash when one of the ouputs is active.
If flashes as many times as the binary value of SW1.
Led is off when no output is activated.

Time delay settings

SW1 is used to set the delay time after which the output will be swotched off.

Delay table
Motor activation
in Sec
SW1.1 SW1.2 SW1.3
0.1 Off Off Off
0.2 Off Off On
0.4 Off On Off
0.8 Off On On
1.6 On Off Off
3.2 On Off On
6.4 On On Off
12.8 On On On


Inputs, if activated, should remain activated, to keep motor/coil in the chosen position.
GCA50/CAN-GC2/GCA_PI02 ports should be programmed as switch, with C2 NOT checked.
Checking C2 will inverse action.

Induction knowledge

Motors and single coil dirves, also the ones in turtoise engines, are using coils.
And a coil, feeded with a current, will push back a large power surge, when it is switched off.
That behaviour, for example, is used in cars for creating the ingnition spark.
But in modelrailroad, we do not want any sparking, execept the look of the layout of course.
In dc systems. it is rather easy to suppress that spark by connecting a diode straight on the coil,
and connected in such a way that the cathode is at the positive side.
Any electronic system will thank you for that. It saves you a lot of trouble.
These diodes are in GCA125 inside the L293D chip, so we do not have worry about that…..
WRONG!! because in most motor driven tortoise units, and also the turnouts, moved by (single-) coils, there is a limit switch
which will disconnect the motor/coil from the driver (in GCA125 the L293D chip).
This switching off then also disconnects the on-chip diodes from the motor/coil!
This problem, by the way, causes many limit switches in turnout coils to fail after some time, due to the generated spark.
Now, one option is to bridge these switches, because with the available timer on the GCA125, the motor will switch of after a chosen timing.
But if we like to keep the switch in function, we need to find another solution.
And specially on motors and single coil units, that is a problem, because there is no +/-. because they will be reversed for reaching the oppossite position.
So a diode cannot be used here\\. The other solution for that is a VDR or varistor.
These VDR's, (useally here something like a 30V type), will short the motor/coil as soon as the voltage exceeds 30V.
So if you have this situation, use a VDR like VDR-0,25 30, directly connected to coil or motor, and your problem is solved.

gca125-nl.txt · Last modified: 2018/11/12 08:56 by