Circuitron SD-2 3 Position Semaphore Driver ~ 5520

Circuitron SD-2 3 Position Semaphore Driver ~ 5520

The CIRCUITRON SD-2 is designed to drive a CIRCUITRON Tortoise Switch
Machine to all three signaling positions when used in conjunction with a Semaphore signal. Linkage and
mounting hardware will need to be fabricated for each particular application and brand/scale of signal. The SD-2
requires a 10-18 volt DC power source for proper operation. With no connection to the [Y] or [R] pins on the
circuit board, the SD-2 will move the Tortoise to the “Green (Clear)” position. Connecting either [Y] or [R] to
GROUND or [-] will move the Tortoise to the “Yellow (Approach)” or “Red (Stop)” positions. These connections
can be made through a multi-pole switch or automatically with CIRCUITRON Block Detection Circuits.
MODIFYING THE TORTOISE: Refer to Figure 1. When the Tortoise is connected to the SD-2, you will note
that the center stopping position (for the “yellow” approach signal) is slightly off of center. This is due to the
internal switch configuration of the Tortoise and is unavoidable. The SD-2 will, however, consistently stop the
Tortoise in approximately the same center position going in either direction. It is recommended that a styrene
“stop” be glued to the Tortoise case across the arm opening to equalize the throw on either side of “center”.
You will need to connect the SD-2 and Tortoise on the bench following the wiring instructions below. With
power applied to the SD-2, connect a temporary jumper wire from the [-] power
supply terminal to the terminal marked [Y]. The Tortoise should move to the center
position and stop. Mark the centerline stopping position of the moving arm on the
case with a marker. Move the jumper wire to the [R] terminal. The Tortoise should
move to the “Stop” (Red) position. Again mark the centerline position on the case.
Quickly move the jumper wire from the [R] terminal to the [Y] terminal. The
Tortoise should again stop in the center position. If this position is slightly different
than the previously marked one, mark a new ‘center’ in between the two positions
noted. Measure the distance between the center stop mark and the “Red” stop
mark (X distance). Measure the same distance on the other side of the center mark
(Y distance) and mark the case for the “Green (Clear)” stopping position. Remove
power from the SD-2 and carefully apply pressure to the arm to slowly move it to
align with the “Green (Clear)” stopping position. This will be a short distance in
from the normal stop in the case opening. Glue a block of styrene square tube or
angle across the opening in the case to provide a new stop for the arm in the
“Green (Clear)” position. Make sure the glue is dry before proceeding.
MOUNTING THE TORTOISE: The Tortoise will need to be mounted horizontally
under the layout. A wood block or a pair of small steel angles may be used to attach the machine to the
underside of the layout. Linkage will have to be fabricated depending on the amount of throw required. The
Tomar semaphore only has about 1/8” total throw. Since it is likely that the Tortoise will be mounted some
inches away (for clearance), a new fulcrum will likely have to be fabricated also. One system that we have found
effective is to use a small steel angle bracket for the fulcrum and temporarily mount it just beyond the control
wire for the semaphore. Open up one of the pre-formed holes in the bracket with a 1/4” bit. Flatten a 12” piece
of 1/4” brass tube at one end and drill a clearance hole for the retaining screw on the Tortoise arm. Attach the
tube to the arm with the screw. Do not over-tighten. Place the end of the tube through the hole in the bracket
and keep the assembly just to the side of the semaphore control wire. Holding the Tortoise in place, gently move
the arm up and down through its complete motion. Note how much the tube moves at the point where it crosses
the control wire. Adjust the Tortoise position until you achieve about 1/8” throw at the wire. Mark the wire
location on the tube with a marker. Remove the Tortoise and tube and drill a small hole through the tube for the
wire to pass through. Mount the Tortoise after passing the wire through the hole in the tube and re-inserting the
tube in the bracket. Wire the Tortoise to the SD-2, apply power to the board and set the driver to the “approach”
aspect by connecting a jumper between [Y] and [-]. Manually adjust your semaphore to the 45 degree approach
position. Wedge a tiny sliver of wood or plastic into the hole in the tube so that the control wire is held firmly.
Using the SD-2 to power the Tortoise, run the semaphore to all 3 positions. If it wants to overrun the horizontal
and vertical positions, move the fulcrum bracket closer to the Tortoise a bit. You may need to re-adjust the 45
degree position with each move. After you are satisfied with the throw, firmly attach the fulcrum to the layout
and glue the wire into the hole with ACC.
WIRING: The SD-2 can be connected with .110″ x .032” solderless connectors (available from CIRCUITRON)
or by soldering leads directly to the terminals on the printed circuit board. If soldering, use a small pencil-type
iron and electronics-grade rosin core 60/40 solder (available from Radio Shack). Use only as much heat as
SEMAPHORE DRIVER- 3 POSITION
211 RocBaar Dr., Romeoville, IL 60446 SD-2
(815) 886-9010 FAX: (815) 886-9076
Electronics for Model Railroads
Figure 1
necessary to obtain a good joint and do not wiggle the terminal until the solder has cooled completely. A
section of CIRCUITRON’S PCMT can be used for simple, snap-in mounting of the circuit board or you may drill
holes in the mounting pads in the corners of the board and mount the SD-2 with screws and standoffs.