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.
|
