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 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 input push button switches are open.
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 drivers can also control signal LEDs and low power relays
The first schematic is for the basic Stall-Motor Switch Machine driver circuit. There are four Switch Machine drivers on each circuit board.
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 that 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 however 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.
When the NORMAL button is pushed, pin 5 of the 556 is will go HIGH and pin 9 will go LOW.
When the REVERSE button is pushed, pin 9 of the 556 is will go HIGH and pin 5 will go LOW.
If both the NORMAL and REVERSE buttons are pushed at the same time Both pins 5 and 9 will go to a HIGH condition and the motor will not move. (This will not harm the driver circuit or the Switch Machine's motor.)
If the RESET terminal is made LOW the driver will be forced to the NORMAL position and the push buttons will have no effect on the output.
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.
The next diagram shows the full circuit of the PCB, including a 4.7uF supply capacitor that is common to all of the drivers.
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.
In the ALTERNATING INPUT circuit the 4.7uF capacitor will have to removed from the NORMAL input of the 556 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.
The circuit can also be used to control relays that could be used to control frog or track polarities at reverse loops.
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.
For this circuit to work properly the voltage at the inputs must go below 1/3rd of the supply voltage. Any external control circuitry must be designed with this parameter in mind.
If both inputs are LOW at the same time, both of the outputs will be made HIGH.
As can be seen in the schematics, the OUTPUT of one timer is fed, through a 10K ohm resistor, to the TRIGGER and THRESHOLD inputs of the other. This gives the circuit its FLIP-FLOP action and causes the outputs of opposite timers to be forced HIGH or LOW.
The resistance of the feedback resistor is not critical and could be increased to as much as 100K ohms. A practical value range is between 10K and 33K ohm.
The maximum current that the outputs of the 556 timers can source or sink is 200 milliamps. These circuits could, in theory, control 10 or more Tortoise switch machines and additional LED's could be connected to the outputs.
These circuits do not require a regulated power supply but the voltage should be well filtered.
Optoisolators could be added to the output of the circuit to control signal systems or solid state relay circuits. Optoisolators could also be used at the inputs to the drivers.
The push button switches could be supplemented or replaced by train position sensing circuits or block occupancy detectors. This would allow the turnouts to be thrown automatically as a train approached or provide for operation of multiple turnouts by a single control device or signal system.
When routing the control wires for this type of circuit it is good practice to avoid wires that may create electrical noise.
If multiple locations are operated from a common power supply, a 100uF capacitor should be placed near the circuit boards at each station. The capacitor can be connected directly to the supply terminals of one of the boards.
The stalled current of the Circuitron Tortoise used in testing this circuit was found to be approximately 12 milliamps and the running current 3 milliamps. The power supply voltage was 12 volts and indicator LED's connected in series with the motor as shown in the schematic. Running and stall currents would be slightly higher without the indicator LED's.
The drivers can be operated at voltages as low as 5 volts.
More advanced circuits that make use of these basic circuits are shown on other pages. These include automatic reversing loop turnout controls and diode matrix systems for multiple turnout operation in yard throats.
It is not the purpose of this page to provide a detailed explanation of the logic involved in this circuit. If you would like more information on this subject please refer to the RS Flip-Flop Made With A LM556 Timer Chip page in the miscellaneous circuits section of this site.
The 556 timer was chosen for these circuits because OPAMPS such as the LM324 or LM358 were found to be too unstable if the inputs are allowed to float and have a much lower current capacity.
|QTY||DESCRIPTION||DigiKey Part #|
|4||556 IC TIMER DUAL 14-DIP||LM556CNFS-ND|
|8||10K OHM 1/4W Resistor||10KQBK-ND|
|5||4.7uF 50V Miniature Aluminum Electrolytic||P5177-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.
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.
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.
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.
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.
If you are interested in printed circuit boards please send an email to the following address: email@example.com
Your message will be answered as soon as possible.
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.
The circuit board can be used with Diode Matrix systems. Shown below is a basic 5 track yard controlled by a matrix system.
Each diode carries about 1 milliamp, so as long there is enough power for the motors, very complex routings can be created.
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.
The following circuit can be used to drive up to four switch machines in a sequence with a delay between them.
The circuit was designed to control four machines that operate the points of a double crossover so that the points moved at intervals but were controlled by one switch.
The 556 Timer Stall-Motor Switch Machine Driver circuit can directly drive switch machines that are operated by high current motors such as the Fulgurex type machines, however, these motor 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 if needed.
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.
02 January, 2013