556 Timer Stall-Motor Switch Machine Drivers

Two Input Circuit

Single Input - Alternating Output Circuit

  This circuit is designed to drive Tortoise™ and Cobalt™ slow motion type switch machines. Other makes of stall motor switch machine should also work with this type of circuit. The circuit can also control signal LEDs and low power relays.

  The circuits on this page are Switch Machine drivers that can be used to operate slow motion Stall-Motor type switch machine motors. The circuits use LM556 dual timer chips to provide a PUSH-PULL type output to reverse the polarity of the voltage to the switch machine motors. The output of the drivers is equivalent to a DPDT toggle switch.

  The 556 timers also provide a logic function equivalent to a SET / RESET type Flip-Flop. This gives the circuit a memory function that allows the turnout to remain in its last position when the control switches are open.

A printed circuit board and parts are available for this circuit.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: 556 Tortoise Switch Machine Driver

Basic Switch Machine Driver Circuit

  The first schematic is for the basic Stall-Motor Switch Machine driver circuit. There are four Switch Machine drivers on each circuit board.

Typical Switch Machine Driver Circuit

  The following schematic is for a typical Stall-Motor switch machine driver circuit controlled by two normally open push button switches. The NORMAL and REVERSE push buttons cause the TRIGGER and THRESHOLD pins of their respective timers to be connected to the circuit common.

  A 4.7uF capacitor is connected to the NORMAL input, this causes the driver to go to its NORMAL state when power is applied to the circuit. The capacitor also reduces the chances of false triggering by electrical noise in the control wiring.

  In some control circuits the 4.7uF capacitor is left out of the circuit, in these cases the switch machines may not return to the same position when power is applied to the circuit.

  NOTE: - If the polarity of the power supply to the switch machine drivers is reversed or the circuit is connected to an AC or DCC source this circuit could be damaged. The maximum supply voltage for this circuit is 15 Volts.

Rules For The Circuit Above

  The direction of the motor and the operation of the indicator LEDs depends on the direction of current flow through the output circuits. This can be changed to suit the particular situation by reversing the motor and LED wiring as needed.

Full PCB Circuit Schematic

  The next diagram shows the full circuit of the PCB, including a 4.7uF supply capacitor that is common to all of the drivers.

Control Examples For The Switch Machine Driver Circuit

  The driver circuit can also be controlled by other input arrangements, some examples are shown in the following diagram. The examples are in block form to simplify the diagrams.

  The 3.3K resistor in the SINGLE INPUT circuit prevents a short circuit if both switches are closed at the same time and also limits the current that can flow into the THRESHOLD terminal of the A timer.

  The transistor inputs could also be from voltage comparator outputs directly or optoisolators. The PNP transistor's input signal voltage should be above 2/3rds of the supply voltage.

Single Input - Alternating Output Circuits

Original - Single Input, Alternating Output Circuit Using The 2008 Circuit Board

Original - Single Input, Alternating Output Circuit Notes

  1.   Because there are two switching levels, 1/3rd and 2/3rds of the supply voltage, the 556 timer is not ideally suited for this type of circuit, therefore the capacitance range of the input capacitors that will work reliably is quite small; between 0.1 and 1.0 microfarads.

  2.   If a circuit does not function correctly, the capacitor and resistor shown at the lower right corner of the of the schematic can be added to the input of the circuit. This may happen under some slowly changing input conditions.

  3.   The 4.7uF capacitor at the NORMAL input of the typical driver circuit is removed for the Single Input - Alternating Output circuit.

New - Single Input, Alternating Output Circuit Board Page

New - Single Input, Alternating Output Circuit Notes

  1.   The new - Single Input, Alternating Output circuit has be specifically designed for this application and will work with a wider range of bipolar capacitor values in the input.

  2.   The output options for the new circuit are identical to the old circuit.

The turnout changes alignment each time S1 is pressed.

Stay At Last Position Switch Machine Driver Circuit

  In the next schematic, one set of the switch machine's built in contacts and a 10uF capacitor are used to force the circuit to stay in its Last Position when power is applied to the circuit. This would allow a train to be stopped over a switch and not have the switch move under the train when power is applied to the circuit.

Stall Motor - Reverse Loop Switch Machine Control Circuit.

556 Switch Machine Driver Notes

PCB Parts List

8 10K OHM 1/4W Resistor 10KQBK-ND
4 4.7uF 50V Miniature Aluminum Electrolytic P5177-ND
1 100uF 35V Miniature Aluminum Electrolytic P5165-ND
6 2 Position Terminal Block - 5mm ED1601-ND
2 3 Position Terminal Block - 5mm ED1602-ND

  T42-1 Push-In Terminals Push-In Terminals can be substituted for the terminal blocks to reduce costs if desired.

556 Stall-Motor Driver Printed circuit boards

  Stall-Motor Driver circuit boards sold - 570 (2280 Drivers).

  A printed circuit board and parts ar available for this circuit. There are 4 identical 556 drivers on each board.

  The following is an image of the circuit board which is 3 inches square and is drilled to fit terminal blocks with a 5mm pin spacing.

  The INPUTs are on the left and the OUTPUTs on the right side.

  The board has been commercially made and is tinned. The NORMAL (N) and REVERSE (R) terminals are marked on its copper side.

556 Stall-Motor Driver PCB - Assembled Example

Option 1

  Stall Motor Driver circuit boards only:

  8.00 dollars US each for 1 or 2 circuit boards.

  7.50 dollars US for each additional circuit board.

  Plus postage.


Option 2

  Stall Motor Driver circuit boards with all parts:

  17.00 dollars US each for 1 or 2 circuit boards.

  16.50 dollars US for each additional circuit board.

  Plus postage.


Option 3

  Stall Motor Driver circuit boards with all parts except the terminal blocks:

  12.00 dollars US each for 1 or 2 circuit boards.

  11.50 dollars US for each additional circuit board.

  Plus postage.



  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: 556 Tortoise Switch Machine Driver

  Your message will be answered as soon as possible.

Driver Circuit Block Diagrams

  The next diagram shows the PCB circuit in block form. This diagram could be used as connection diagram to aid in wiring the drivers.

  The next diagram shows the optional use of the RESET terminal.

  The RESET terminal of the drivers does not have a terminal block but has two solder pads for external connections.

Additional Circuits


Basic Diode Matrix

  The circuit board can be used with Diode Matrix systems. Shown below is a basic 5 track yard controlled by a matrix system.

Diode Matrix Circuit

  Each diode carries about 1 milliamp, so as long there is enough power for the motors, very complex routings can be created.

  Also see the Stall-Motor Diode Matrix Circuits or the Stall-Motor Router Circuits pages at this site.

A Users Diode Matrix For An Interlocking

Photos used with permission.

Users Diode Matrix Diagram For Up To 24 Switches

Users Photo Of The Interlocking In The Diagram Above


Diode Matrix With Track Indicator LEDs

  This circuit has LEDs that indicat which track is selected. The LEDs are connected to the output of the 556 switch machine drivers. This circuit uses more diodes than the basic matrix circuit shown above.

Diode Matrix Circuit With Track Indicator LEDs

  The circuit uses a full diode matrix. This causes the switch machines that are not used to access a particular track to move to their NORMAL position when any other track is selected. In other words; All of the switch machines are commanded to move when any button is pushed, even if the machine does not actually move.

  This means that trains cannot be sitting on turnouts above the a selected track when a change occurs.

Tortoise™ Switch Machine Terminal Connections

circuit board Assembly

  When installing the components on the circuit board start with parts with the lowest height and work up to the tallest parts. For example starting with the diodes then resistors, IC's, transistors, capacitors and terminal blocks.

Recommended Tools


Even More Circuits

3 Way Switch Controller

  The following circuits are designed to control a 3 Way turnout system using push buttons.

  The first circuit can control separate turnouts where the points do not interfere with each other.

  The second circuit has time delays built in that allow integrated points to move without interfering with each other. The delays are alternating allowing one set of points to move before the other in opposite directions.

  These circuits can be controlled through a matrix by placing diodes where the push buttons are in the circuit and then connecting the cathodes of the diodes to a matrix system.

Rotary Switch Controls

  The following circuit is a matrix like circuit that allows a rotary switch to control a ladder track arrangement. Other "One of X" output devices could be used in place of the rotary switch.

Light Controlled - Alternating Output

Cascaded Delay Circuits

Cascade Delay Circuits Using 556 Slow-Motion Switch Machine Drivers

High Current Switch Machines

  The 556 Timer Stall-Motor Switch Machine Driver circuit can drive switch machines that use high current motors such as the Fulgurex type machines, however, these motors operate near the current capacity of the 556 IC's.

  If Stall-Motor Switch Machine Driver is used to drive this type motor then sockets should be used for the IC's to allow easy change out of the 556 timers ICs if needed.

High Current Switch Machine Motor Driver Circuit (Fulgurex)

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Please Read Before Using These Circuit Ideas

  The explanations for the circuits on these pages cannot hope to cover every situation on every layout. For this reason be prepared to do some experimenting to get the results you want. This is especially true of circuits such as the "Across Track Infrared Detection" circuits and any other circuit that relies on other than direct electronic inputs, such as switches.

  If you use any of these circuit ideas, ask your parts supplier for a copy of the manufacturers data sheets for any components that you have not used before. These sheets contain a wealth of data and circuit design information that no electronic or print article could approach and will save time and perhaps damage to the components themselves. These data sheets can often be found on the web site of the device manufacturers.

  Although the circuits are functional the pages are not meant to be full descriptions of each circuit but rather as guides for adapting them for use by others. If you have any questions or comments please send them to the email address on the Circuit Index page.


  If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: 556 Tortoise Switch Machine Driver

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02 May, 2019