GCA102 Reverse loop switch with current detectors and turnout control


By Peter Giling


Many railroaders have problems with reverse loops.
The majority in available units for this purpose are working with a short circuit detector
It does not need any explanation hat that would be rather destructive for your precious material.
This unit gives an ultimate solution in digital systems.
It uses the normal current detection for switching the loop into the right polarity.
It is NOT suitable for analog systems.
It is provided with several feed-back outputs to connect GCA50 (LocoNet) or CAN-GC2(CAN), for full compatibility with Rocrail, and/or LocoNet/MERGCBus.
Also in manual digital control, this unit will provide the correct switching for your reverse loop.
4 current detectors are on board to enable correct swap of rail power lines.
2 extra current detectors are on board to get a rail occupied indication inside the loop.
Opto couplers are on board to separate rail power from Loconet.
Auto reset multifuses are on board to protect current sensors from overload in short circuit situation.
The standard 10 pin header connector is compatible with the 10-pin connector on GCA50 (LocoIO) and CAN-GC2 (CAN).
One output is available, which makes it possible to change the position of the turnout via LocoNet (GCA50) or CanBus(CAN-GC2).
This is mostly useful when the loop is used in manual control, with certain turnouts, which definately need to be in the right poaition before a train can pass.
The turnout is always set in the right position, just before the train reaches the turnout.
On remark on that: you should take care that the turnout driver is fast enough to make sure the turnout is in correct position BEFORE the train arrives there.
A bi-stabile relays is used, to avoid problems after power off/on.
Relay will stay in last chosen position, and on power on, the GCA102 sets itself in the last used postition before power down

How does it work ?

3 different railplans for connection
The blue lines show, where the rails should be cut.
Each section should have at least the length of the longest Locomotive used.
The longest train should fit between points C and E.
There are 6 sensors , all direct available as output on J2 ( for connection to GCA50 or CAN-GC2 ).
There are 4 sensors needed, to make this work: 1, 2 5, and 6.
As explained obove, these sensors are current sensors, reacting on a train that enters that area.
Lets assume the turnout is set to 'go right'.
The first sensor then is sensor 1.
This part is still part of the normal railway., but when sensor 1 is activated, it will inverse polarity of the
whole loop (including sensors 2-5) in such a way, that the polarity matches when the train is entering sensor 2.
Sensor 2 will also have the same effect as sensor 1, but since that situation is already set, nothing will change.
When the train conntinues it will enter sensor 3 and 4, which have no further effect for this unit.
It is important to realise that your longest train should fit totally within the length of sensor 3+4 together!
If that is not possible, you can NOT use this unit! When train enters sensor 5, the whole loop will be polarised to match with sensor 6.
Also the output tc will change , which can be used in LocoNet or CBUS to change the turnout position.
This is specially very useful when you drive manually. Entering sensor 6 will have the same effect as sensor 5, so there will be no chance.

This sequence also works if a train enters at sensor 6.

Sensor 3 and 4 could be used as Block.
In that case, the turnout will be part of the route and thus commanded by the program in the PC.

How it looks like

Older versions

MGV102 description is found here.
GCA102 (Blue PCB V1.3) files:

The schematic
The partslist

Schematics and Layout

These files correspond with the latest version V2 (Red pcb).

The schematics
The pcb + parts placement
The partslist
The sources and hex files
N.B. Self made pc-boards are not supported!

Remarks on GCA102 Version 2 used with hall sensors

This (red) board is also equipped with two connectors for Hall-sensors TLE4905.
On each connectors, two hallsensors on each side should be connected parallel.
There are a number of parts no longer needed, if GCA102 is used with hall-sensors only (instead of current detection).
A few parts has to be different like this picture explains.
This partslist is made for use with hallsensors.
Hall=sensors at each side of the loop should be connected parallel.
Be sure that a train, approaching a rail separation, first activate a hall-sensor.

Cable to GCA50 / CAN-GC2

Important remark on connection to GCA50 / CAN-GC2

When GCA102 is connected to one of these boards, it is important that first 7 connected ports
should be programmed as input !
Refer to this table:

Connection table
GCA102 connect to port# Railsection# (GCA102)
GCA50( J5 pin 3 ) 1 1
GCA50( J5 pin 4 ) 2 2
GCA50( J5 pin 5 ) 3 3
GCA50( J5 pin 6 ) 4 4
GCA50( J5 pin 7 ) 5 5
GCA50( J5 pin 8 ) 6 6
GCA50( J5 pin 9 ) 7 turnout ctrl
GCA50( J6 pin 3 ) 9 1
GCA50( J6 pin 4 ) 10 2
GCA50( J6 pin 5 ) 11 3
GCA50( J6 pin 6 ) 12 4
GCA50( J6 pin 7 ) 13 5
GCA50( J6 pin 8 ) 14 6
GCA50( J6 pin 9 ) 15 turnout ctrl
CAN-GC2( J4 pin 3 ) 1 1
CAN-GC2( J4 pin 4 ) 2 2
CAN-GC2( J4 pin 5 ) 3 3
CAN-GC2( J4 pin 6 ) 4 4
CAN-GC2( J4 pin 7 ) 5 5
CAN-GC2( J4 pin 8 ) 6 6
CAN-GC2( J4 pin 9 ) 7 turnout ctrl
CAN-GC2( J3 pin 3 ) 9 1
CAN-GC2( J3 pin 4 ) 10 2
CAN-GC2( J3 pin 5 ) 11 3
CAN-GC2( J3 pin 6 ) 12 4
CAN-GC2( J3 pin 7 ) 13 5
CAN-GC2( J3 pin 8 ) 14 6
CAN-GC2( J3 pin 9 ) 15 turnout ctrl


The firmware for U2

The very simple program is written in PicSimulator IDE basic

Led funcions

Always one of the two led is constantly on, depending in which polarisation position the unit is.
Just to show that the board is active, the other leds will flash with a frequency of approx 1 Hz.


** IMPORTANT NOTE !!!!!! Some users seem to know better, and solder wires directly to the pins or the pc-board.!
That is totally unacceptable !!
Wires will easily break off, and cause a lot of damage in many cases.
So use connectors !!!!!

For buyers of complete packages, there is a special offer: This special plier will be sold for net price of € 18,25 if ordered together with complete kits. crimpzange.jpg
Buying this , it will save you much trouble.

Stand-alone use

This unit can also be used as a stand-alone for your layout.
Please consider it is NOT suitable for analog drivers.
Only digital systems can be used.
For stand alone, the unit needs ONLY 5V supply connected to J2. (max 30 mA)
All 6 feed-back points are also connected to J2 and can be used for external indication
The table below explains all connections.
Pin #1 of J2 is marked.

Pin# function
1 +5 Volt
2 0V ground
3 Feedback 1
4 Feedback 2
5 Feedback 3
6 Feedback 4
7 Feedback 5
8 Feedback 6
9 Output for turnout ctrl
10 Not connected

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