The FromMeyer Board

  History
I belong to two model railroad organizations.  In both cases, we employ slow speed switch machinesto throw the position of a track switch.  Usually, these switch machines employ low current, low voltage motors (i.e. Hankscraft, Tortoise, etc.) that are geared down to get the necessary torque.  The result is a  positive switch actuation that is quiet, reliable, and more like the real thing  (the cheap switch machines use fast-acting solenoids, which like to stick after time).   Now, there are a number ways you can control the motors.  You can run them with a simple toggle switch or switches.  The problem with this system is that the switches may constrain the action of  controlling multiple switch machines with one switch.  The next option is to use relay logic and pushbuttons with diodes that fan out to the relays.  Well, relays can get expensive, are noisy, and can reset easily if there is a blip in the power.  The design that my friend Galen, and myself came up with was a solid state solution to the relay logic...essentially a direct replacement, that didn't suffer from the mechanical limitations.


  Circuit Description

Click here to view the schematic

The circuit basically employs two integrated circuits for its design.  The first IC, CD4013 is a dual D-flip flop which is used to retain the memory of the pushbutton actions.  In this design, we are not using the synchronous clocked portion of the IC design, but rather the asynchronous set/reset portion of the IC.  We chose this IC over a simple set/reset latching IC because of its availability and cost.  By applying a high signal to either the set or reset pins, the outputs Q and Q bar go high or low accordingly. 

The second IC, a LM324 is a quad operational amplifier.  Its sole purpose is to drive the final motor control.   Fortunately, the motors are low current in design, and two amplifiers, one on each side of the motor are capable of driving it directly.  Q and Q bar drive inputs on two of the amplifiers, which the other input of the amplifier tied to a voltage reference established by the two 3.3k resistors.

The remaining circuit components consist of four 3.3k Ohm pull down resistors across the inputs of the CD4013, a 470µF capacitor which provides the CD4013 with some memory retention when the power is interrupted, even for longer periods of time, and a blocking diode to prevent the capacitor from being discharged by the rest of the circuit.

Note that this circuit will drive two switch machines.  If a crossover control is designed, the inputs can be bridged so that both motors behave identically to one another.  Additionally, push buttons can be fanned out using diodes to control multiple motors in various configurations, such as for a yard lead.  Also, if desired, LED indicators can be driven from the CD4013 (leaving the full current capacity of the LM324 available) to indicate switch position.  Be sure to use a reasonable resistor value to limit current draw.  A 1k Ohm should be sufficient at 12 volts.



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©2004  Ray Meyer, N9PBY