The circuits on this page are switch machine drivers that can be used to operate 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 supply to the switch machine motors.
The 556 timers 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.
The switch machine used to develop these circuits is the Circuitron Tortoise™ slow motion type switch machine (Model number 800-6000). Other makes of stall motor switch machine should also work with this type of circuit.
The first schematic is for the typical Stall-Motor switch machine driver circuit. The NORMAL and REVERSE switches cause the TRIGGER and THRESHOLD pins of their respective timers to be connected to the circuit common.
WARNING - If the polarity of the power supply for this circuit is reversed or the circuit is connected to an AC or DCC source this circuit will be damaged. The maximum supply voltage for this circuit is 15 Volts.
A a capacitor, typically 4.7uF, 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 provides noise filtering for the inputs to reduce the chances of false triggering by electrical noise in the connected wiring.
This capacitor can be left out of the circuit but the switch machines may not return to the same position that power is applied to the circuit.
When the NORMAL button is pushed pin 9 of the 556 is HIGH and pin 5 is LOW. The throw arm of the switch machine will move to the number 8 terminal side of the switch machines circuitboard.
When the REVERSE button is pushed pin 9 of the 556 is LOW and pin 5 is HIGH. The throw arm of the switch machine will move to the number 1 terminal side of the switch machines circuitboard.
If both the NORMAL and REVERSE buttons are pushed at the same time a disallowed state will result. Both pins 5 and 9 will go to a HIGH condition and the motor will not move.
The LED that is on depends on the direction of current flow through the switch machine motor. In this circuit Green is on for NORMAL and Red for REVERSE.
The circuit can also be controlled by a single input as shown in the following diagram. This may be useful if noise is a problem as only the input with a capacitor is used to control the circuit. Also, the control wiring can be simplified if needed.
The 2.2K resistor in the push button circuit prevents an accidental 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 circuit can also be used to control relays. These might be used to control frog or track polarities.
In the next schematic, one set of the switch machine's built in contacts, a 10uF capacitor and two 10K resistors 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 park over a switch and then 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 the timers to be forced alternately 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 rated maximum current the 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 need a regulated power supply but it should be well filtered.
Optoisolators could be added to the output of the circuit to control signal systems or solid state relay circuits.
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 any areas that may create electrical noise.
If multiple locations are operated from a common power supply, a 100uF capacitor should be placed near the circuitboards 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.
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 as OpAmps such as the LM324 or LM358 were found to be too unstable if the inputs are allowed to float such as when both push buttons are open.
The circuits on this page do not make use of the DISCHARGE terminals for the timers as these are not available on the available printed circuitboard.
The next diagram shows the full circuit of the PCB, including the small filter capacitor that is common to all of the drivers.
| QTY | DESCRIPTION | PART NUMBER |
| 4 | 556 IC TIMER DUAL 14-DIP | IC 1, 2, 3, 4 |
| 8 | 10K OHM 1/4W Resistor | R1, 2, 3, 4, 5, 6, 7, 8 |
| 5 | 4.7uF 50V Miniature Aluminum Electrolytic | C1, 2, 3, 4, 5 |
| 6 | 2 Position Terminal Block - 5mm | - |
| 2 | 3 Position Terminal Block - 5mm | - |
| OR | ||
| 18 | T42-1 Push-In Terminals | - |
The DigiKey part number for the 556 Timer is LM556CN-ND.
The DigiKey part number for the 2 position terminal block is ED1601-ND.
The DigiKey part number for the 3 position terminal block is ED1602-ND.
The DigiKey part number for the T42-1 Terminals is V1069-ND.
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 circuitboard can be used with Diode Matrix systems. Shown below is a basic 5 track yard controlled by a matrix.
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.
This circuit is a variation of the Single Input Driver Circuit shown above. The circuit would be used to control crossing gates that are operated by stall type motors.
The terminals numbers shown on the drawing correspond with those on the printed circuitboard for the Railway Grade Crossing circuit at this site.
This circuit allows a single push button switch to control the position of the turnout. For multiple locations buttons can be added in parallel to the first.
The circuitboard could also be used as part of a Stall Motor Reverse Loop Conrol Circuit. See the second diagram.
When installing the components on the circuitboard 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 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.
07 September, 2009