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 D type Flip-Flop. This gives the circuit a memory function that allows the turnout to remain in its last position when the control switch is open.
If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: Alternating Output 556 Tortoise Driver
The first schematic is for the basic, Alternating Output Stall-Motor Switch Machine driver circuit. There are three complete Switch Machine drivers on each circuit board. No external parts are required.
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.
The following schematic is for a typical Stall-Motor switch machine driver circuit controlled by one normally open, push button switches.
When the INPUT button is closed the outputs of the 556 timers change polarities. When the INPUT button is closed again the outputs of the timers switch back.
If the RESET terminal is made LOW the driver will be forced to the NORMAL position with output pin 5 being HIGH and output pin 9 being LOW.
This cicuit does not have set starting position. The output can go to either state when power is applied to the circuit.
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 100uF supply capacitor that is common to all of the drivers.
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 terminals.
The RESET terminals of the drivers do not have terminal block positions but have solder pads for external connections.
For this circuit to work properly the voltage at the inputs must fall rapidly to zero volts. Any external control circuitry must be designed with this parameter in mind.
The OUTPUT of one timer is fed, through a 100K ohm resistor, to the THRESHOLD input of the other. This gives the circuit its FLIP-FLOP action and causes the outputs of opposite timers to be forced either HIGH or LOW.
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.
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.
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.
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 D 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 # |
| 3 | 556 IC TIMER DUAL 14-DIP | LM556CNFS-ND |
| 6 | 10K OHM 1/4W Resistor | 10KQBK-ND |
| 12 | 100K OHM 1/4W Resistor | 100KQBK-ND |
| 6 | 1.0uF 50V Biolar Miniature Aluminum Electrolytic | UPV1H010MDD-ND |
| 1 | 100uF 35V Miniature Aluminum Electrolytic | P5165-ND |
| 4 | 2 Position Terminal Block - 5mm | ED1601-ND |
| 1 | 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 3 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 of the photo.
The board has been commercially made and is tinned.
Option 1 - Circuit Boards Only
Stall Motor Driver Circuit Boards Only:
12.50 dollars US each.
Plus postage.
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Option 2 - Assembled Circuit Boards
Stall Motor Driver circuit boards with all parts:
23.00 dollars US each.
Plus postage.
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Option 3 - Unassembled Kits
Stall Motor Driver circuit board with all parts:
21.00 dollars US each.
Plus postage.
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If you are interested in printed circuit boards please send an email to the following address: rpaisley4@cogeco.ca Subject: Alternating Output 556 Tortoise Driver
Your message will be answered as soon as possible.
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 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.
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: Alternating Output 556 Tortoise Driver
07 February, 2020