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Manually switched DC control systems

By Terry Flynn 10th December 2004

Updated 9th February 2007


Before computers and Command Control, modellers used 2 basic DC systems to control more than one train independently on their layouts. The first system is commonly called Block Control. In this system the layout is divided electrically into sections or blocks. Each block has a separate controller. In the most simple system 2 people are required to move a train from one block to the next, and both controllers need to be set to the same speed and direction. This system can be difficult to operate as I found out on a model railway club layout many years ago. This system is still used today. A model railroad that has overcome these problems using simple rules. The second basic system is commonly called Cab Control, and became popular in North America. Again the layout is divided into electrical blocks. This time each block is switched between controllers. The simplest system uses toggle switches. The result is the driver can move with his train more easily, if you have a person looking after the switches. The above system is only easy to operate for 2 trains. When more than 2 trains were required to operate on a layout, people often used rotary switches. The advantage of Cab Control is it is the cheapest method, as you normally have more blocks compared to trains. Both systems as described above leave allot to be desired from an operators view point, hence the move to more sophisticated control systems. One disadvantage with cab control is as you need more trains, the Cab selection switch wiring becomes more complex.

When I first built my current layout 15 years ago I decided to use a Command Control system (Dynatrol). I found that using this system had some draw backs, one was it was hard to fault find problems, especially if the problem required removing the body from brass steam locomotives. The advantage of Command control is flexibility, allowing many locomotives to move any were on the layout without the need of electrical blocks. One disadvantage is layout operation can degenerate into tail chasing to easily. Because my prototype used signals and block working, I had another look at DC control before committing to the new Digital Command Control System my friends were changing to. I decided in order for DC control to be easy to use the signals would need to be interlocked with electrical blocks. Following the cab control method I decided I could build a cheap walk around cab control system, using plugs and sockets to do the necessary switching without the need for fancy electronics, extra relays or computers. This is the control system I currently use.


Link to Cab Control wiring diagram




The selection of the same cab for adjoining blocks will set the signal indication to go. The advantage of the system is you do not have to have separate signal switches. The main draw back is if operating, you tend to get lazy and not return the signals to stop. The other problem is it is still easy to make the wrong cab selection if trying to run more than one train by oneself. Its more suited to layouts with a central control panel because if you have 2 control panels, you need to select 2 blocks to the same cab before moving from one to the other, which means the operator has to operate both panels.

In order to improve the above system I decided to look at a more technically advanced hardware based automatic DC cab control. After coming up with a number of designs, it was clear such a system would take a fair bit of time to get right, and in order to keep costs down would be limited to around 8 to 16 cabs. It would be as expensive as DCC. Then I decided to have a serious look at progressive block control. After some arm chairing It became clear it was a better option. The answer was similar as I did with DC cab control. Interlock signaling with block selection. . When you switch a signal to go, the adjoining blocks control voltage becomes the same, and can be operated from the first block. The other trick is to build a memory walk around controller for each block and have layout mounted push button controllers. This decreases the need to constantly unplug and re plug hand controllers. It also solves the problem of operators getting tangled up in hand controller cables. A much cheaper option than infra red or radio control. This means the system is suited for walk around style layouts and layouts with signalmen. It has no maximum train limit, the larger the layout, the more blocks, hence more trains. Operating the layout becomes like the operation of steam era mechanical signaled prototypes. Again it does not require computers or lots of relays, just a modification of bi polar conventional inertia DC controllers.
Rob Paisleys Toy train throttle for comparison It does require you to operate signals. With the installation of track detectors, the switching between blocks and signal operation can become automatic. Alternatively, if using the walk around controller option a latching relay per block per direction could be used to isolate the block and set the signal to stop when the walk around controller is plugged in to the block

Block Control wiring diagram

Power supply circuit diagram using battery chargers

Block circuit diagram

Hand controller circuit diagram

Push button control circuit diagram

I am currently building and testing the above circuits.

Power Supply

Because we are using a bi polar transistor circuit, we need to use one 24V centre tapped transformer or two 12V transformers for track power. The output from these transformers needs to be converted to DC using bridge rectifiers. The minimum required current draw in Amps can be calculated by dividing the maximum number of locomotives running on the layout at one time by 3. You can also use 12V lead acid battery chargers for the track power supply. These produce unfiltered DC and have over load protection. They are an ideal off the shelf model railway power supply. They can also be used to run stall type point motors as well as track power. We also need a regulated +15V 15V supply. Fortunately this only needs to be of low current rating. We can use 1A voltage regulators for this and it can be used for running track detectors.

Block controller

The block controller circuit I have decided to use has been kept as simple as possible, using easy to get components. Because it has few parts, it is fairly easy to build the circuit without going to custom made printed circuit boards, however this would make manufacture easier and quicker. My new version uses only 1 OP amp in the LM324, therefore you can build 4 block controllers on each board, using all the OP amps in the LM324. To keep the circuit diagram simple I have only drawn the circuit for one block. The circuit is designed so the maximum current flow in the control voltage circuit is limited to 12mA. This does introduce a small amount of inertia into the system, but it is minimal. The control voltage is back to + / - 12 V as I have decided to keep computers away from my steam era model railway. It is necessary to use heat sinks on the transistors. I have used the same transistors on my Cab Control system for 15 years without a failure, using aluminium heat sinks of around 50mm square.

Block controller overload protection

The new circuit as drawn uses the old and proven 12V 20W lamp on the transistor outputs. This results in a short circuit current of 1.5A . The reason for abandoning my earlier more modern style circuit protection was it failed to protect the output transistors when a voltage was placed on the output. This situation can occur if you accidentally run into another block against a red signal. The new circuit is a simple low component count solution. If you want more output current simply use a 36W 12V lamp and larger heat sinks, or safer still higher current capacity transistors.

Simple capacitor memory walk around.



The block hand controller uses a simple memory system that I use with my existing Cab Control system. A 100uF capacitor remembers the voltage when the hand controller is unplugged. You need have the control voltage pin in the plug the last to connect or disconnect. Using D connectors it might be possible to cut one pin shorter than the rest .I have not tried this yet. Instead I have used a normally closed switch in the D plug back shell. This works well and is what I currently use. When you grab the plug, you naturally operate the switch. The only thing to watch with this simple capacitor memory system is over time the capacitor voltage changes. You need to plug in your controller within a minute otherwise the speed difference becomes noticeable.



These are my current Cab control walk around hand controllers. After years of use they still work. Like the block control hand controllers they only need to contain a few switches and resistors.

Simple Inertia Push Button Control

The circuit diagram I have provided shows only too push buttons and a reverse switch. The acceleration and braking rates can be independently adjusted by replacing the 100k resistors. You can use a variable resistor in series, so you can easily fine tune the acceleration deceleration rates. The larger value the slower the change in speed. Because this is a build your own system, there are many variations on this theme. For example an emergency stop button can be included. A smaller value resistor of around 1k would give the desired result.

Email: tgflynn@unsw.edu.au