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Assembly and Operating Instructions for Kits


Instructions for Building the Crossed-Beam Sensor Control Box (CBS-E)


Assembly instructions for other kits


Photo Gate 28


These instructions are for building the control box for the Crossed-Beam Sensor Control Box. Instructions for building the photogate frame are here. The 3.5mm plug, 2 IR LEDs, and 2 IR phototransistors are used in the frame assembly.




For individual photos of the parts supplied in the kit, see the Parts List.


If you would like to refer to a circuit schematic while you work, download one here.


In these instructions, click on any image for a larger view. (In order to open the image in a new tab or window, right click and select open in new tab or window.)



Tools that you'll need


Having the right tools will make the job easier. You'll need to provide your own. Here's what we recommend.

  1. 15-30 W soldering iron and solder
  2. Wire stripper (photo below)

  3. A small diagonal cutter (photo below) makes it easy to trim stray wires, but other kinds of snipping tools such as scissors may work.

  4. Needle-nose pliers (photo below) make it easier to handle wires, especially if you have big fingers.

  5. A magnifying glass is useful to inspect solder joints.

  6. A lighter or matches to shrink heat-shrink tubing, if the project requires it.

  7. A drill motor and these drill bit sizes: 3/32, 1/8, 1/4, 5/16 in

  8. Rubber or contact cement to affix the label to the project box lid

  9. Hammer and a pointed instrument such as a large needle or a punch
  10. Stencil or razor knife and a straightedge
  11. Hole punch such as that used for punching paper for binders
  12. Mild solvent such as denatured or rubbing alcohol and soft, clean cloth
Wire strippers diagonal cutter needle-nose pliers
Wire stripper Diagonal cutter Needle-nose pliers


Be sure to solder in a well-ventilated area. Keep the tip of your soldering iron clean by wiping it against a wet sponge. Once the tip is clean, touch a bit of solder to the tip to tin it and improve heat conductivity. Inspect your solder joints to see if the solder flowed well to make good electrical contact. If it looks like the solder formed a bead, that's likely a bad joint and will not conduct. Reheat to flow the solder.


Adding Components to the PCB

Step 1. Adding the resistors to the PCB


Photo 1a: This photo shows the upper, printed side of the PCB for the project.


Photos 1b-d: Start with the 1k resistor (see photo to the right). The resistance value is indicated by the sequence of band color, brown-black-red in this case. Note that the 1k-ohm resistorresistors aren't polar and can be placed in either of the two possible orientations. Bend the legs at right angles to the body of the resistor and insert into the location labeled R1 on the PCB as shown in Photo 1b. Push the resistor flush with the board as shown in Photo 1c. Solder the legs on the opposite side and clip off the excess leg lengths. See Photo 1d.


Photos 1e,f: Solder the remaining resistors in the locations listed below. Views from above and below the board are shown in Photos 1e and 1f.

  • 1k (brown-black-red) in R2
  • 680 (blue-gray-brown) in R3 and R4
  • 470 (yellow-violet-brown) in R5
  • 220 (red-red-brown) in R6


Photo 1a Photo 1b Photo 1c
Photo 1d Photo 1e Photo 1f


Step 2. Adding the IC socket to the PCB


Photo 2a: Place the 8-pin socket on the board as shwon in Photo 2a. Note that the notch on the socket corresponds with the notch on the PCB. Make sure that all 8 pins slip through the corresponding holes on the PCB.


Photo 2b: On the under side of the board, crimp the pins toward each other as shown in Photo 2b. Then solder the pins, being careful not to bridge solder between pins. If this happens run the tip of the soldering iron between the pins to remove the bridge. This may take several tries.


Photo 2c: The completed soldering job is shown in Photo 2c.

Photo 2a Photo 2b Photo 2c


Step 3. Adding the capacitor, transistor, and fuse holder to the PCB


Photo 3a: Select the 0.1-uf capacitor (labeled 104M) as shown in Photo 3a. Push the legs of the capacitor into the holes for C1 and solder on the opposite side. Clip the protruding legs.


Photo 3b: Insert the transistor into location Q1 with the flat side of the component corresponding with the flat side printed on the board. Push the transistor down to seat it firmly. Then solder the three legs on the opposite side. Avoid overheating the SCR as damage could be caused to the component. If you take too long soldering on a particular leg, wait for the SCR to cool before moving onto to another leg. Be careful to avoid bridging the solder between legs. Clip the protruding legs when finished.


Photos 3c,d: The last component to solder to the board is the fuse holder. It'll take a bit of force to snap the 2 legs into place. Once in place, solder the legs. Photo 3c shows the board with all components in place with the exception of the ICs and fuse, which will be added as the final step. The completed under side is shown in Photo 3d.

Photo 3a Photo 3b Photo 3c Photo 3d


Preparing the Project Box

Step 4. Drilling the lid and the bottom of the box


Note that Photos 4a,b show the veiw from the under side of the project box lid.


Photo 4a: Cut out the drilling template and place it in the underside of the project box lid. Using a hammer and a sharp, pointed instrument such as a large needle or a punch, mark the centers of the holes on the template.


Photo 4b: Remove the template. Drill 3/32-in starter holes at each of the locations that you marked. Then drill the starter holes to the sizes indicated on the template. Photo 4b shows the under side of the lid with all holes drilled.

Photo 4a Photo 4b


Step 5. Adding the label


Photo 5a: Trim around the black border of the lid label. A stencil or razor knife with a straightedge will help to get straight cuts, but sharp scissors will also serve.


Photo 5b: Pull the backing off one of the laminate sheets, and lay the sheet, sticky side up, on a table. Carefully place the lid label, label side down, onto the sticky side of the laminate sheet as shown in Photo 5b. In order to avoid getting air bubbles, apply the shorter edge of the label first and gradually push it down onto the laminate with a finger. Remove the backing from the other laminate sheet and carefully apply it, sticky side down, to create a sandwich of the two laminate sheets with the label in the middle. Use the technique described above to avoid getting air bubbles. When the laminate is in place, rub a finger over the laminate and around the edges of the label to ensure a good seal. Finally, use a stencil or razor knife or sharp scissors to trim the excess laminate from around the label.


Photos 5c,d: Use a 1/4-in hole punch to punch out the 1/4-in and 5/16-in holes on the label as shown in Photo 5c. If you don't have a hole punch, the point of a sharp knife can be used to carve out the holes. Don't worry if some edges are ragged. These will be covered by the components later. The label with holes punched is shown in Photo 5d. The 1/8-in holes will be punched later.


Photos 5e-g: Now it's time to glue the label to the top of the lid. First cut the 2-in section of plastic tubing into two 1-in pieces. For glue, use rubber cement or other repositionable adhesive. Spread the adhesive over both the lid (as shown in Photo 5e) and the under side of the label (not shown). Place the label in position on the lid, and align it using the two pieces of plastic tubing. Push one piece of tubing through the Power (On-Off) hole and the other through the Shutter hole as shown in Photo 5f. Flatten the label by rubbing a finger over it and hold in position for a minute or so to insure adhesion. Then carefully remove the plastic tubing to avoid shifting the position of the label. Any glue that extends beyond the edge of the label can be rubbed off with a finger. If there is glue smeared on the label that can't be rubbed off, use a mild solvent such as denatured or rubbing alcohol and a soft, clean cloth to clean the label. Finally, place the label under a stack of books or other weight to press it for a few hours. When completed, the lid with label should look like Photo 5g.


Photo 5h: Use a knife to expand the punched holes for the Fine and Coarse Delay pots and the 9V In jack to 5/16 in. Then check the under side of the lid to see if portions of the label overlap any of the 1/4-in holes. If so, use a stencil knife, razor, or knife blade to trim the label back to the boundaries of the hole. This will make it easier to insert components later.


Photos 5i,j: Use a razor blade or sharp knife to cut a cross in each of the four 1/8-in holes. Then use a pencil to widen the holes. See Photos 5i and 5j. It's not necessary to clear the holes of paper.

Photo 5a Photo 5b Photo 5c Photo 5d
Photo 5e Photo 5f Photo 5g Photo 5h
Photo 5i Photo 5j    


Adding Components to the Lid of the Project Box

Step 6. Adding the jacks and switch


Photos 6,a,b: Photo 6a shows the under side of the box lid. Mount the two black, 3.5mm jacks in the locations shown. Note the orientation of the side tabs of the jacks. Photo 6b shows the upper side of the box lid with the 3.5mm jacks mounted.


Photos 6c-e: The switch will be mounted next. In Photo 6c showing a switch, notice the ring with the tab pointing toward the base of the switch. When you mount the switch on the lid of the project box, point the tab the opposite direction. Then when you insert the switch through the 1/4-in hole on the under side of the lid, the tab will slip into the 3/32-in hole, serving to prevent the switch from turning. Photo 6d shows the underside of the lid with the switch mounted, and Photo 6e shows the top side.


Photos 6f,g: The power jack will be added next. Note the numbering of the lugs in Photo 6f. When you mount the jack, orient the lugs as shown in Photo 6g. The upper side of the lid with the power jack mounted is shown in Photo 6h.

Photo 6a Photo 6b Photo 6c Photo 6d
Photo 6e Photo 6f Photo 6g Photo 6h


Step 7. Adding the LEDs and pots


LED holderPhotos 7a,b: The LED holder has two parts shown to the right. The first step in mounting an LED is to snap the collar of the LED holder into the hole from the upper side of the lid. See Photo 7a, which shows a collar in he process of being snapped into place in the Trigger hole. Snap a collar into this location and another into the Power hole. Photo 7b shows both collars snapped into place.


Photos 7c,d: From the under side of the lid, insert each LED into its collar as shown in Photo 7c. Note that one leg of the LED is longer than the other. Orient the LEDs so that the longer leg is the one nearer the outside of the lid. Push each LED into the collar until the LED snaps into place. This may require quite a bit of force, depending on how tight the fit is. You can use a small, blunt instrument to push on the base of the LED until you hear it snap into place. With the LEDs fully seated, the view from the side will look like Photo 7d.


Photo 7e: The final part of the LED assembly is to place the ring over the collar on the underside and push it into place as shown in Photo 7e.


Photos 7f-h The final components to mount are the variable resistors, also known as potentiometers or pots. Note that there are 2 washers under the nut. Remove the nut and one of the washers. Note also that there's a metal tab beside the shaft. We recommend snipping off about one-third of the tab so that it doesn't create an indentation in the lid label when mounted. See Photo 7f. Mount the pots as shown in Photo 7g, being sure to mount the 10K and 100K pots in the indicated locations. These are different values and are not interchangeable. On the top side of lid, shown in Photo 7h, place the remaining washer and the nut on the shaft and tighten.


Photo 7i. Before adding the knobs, turn the shafts of the pots all the way counterclockwise. This is the 0 position. Loosen the set screw on each knob and mount on the shafts with the white indicator mark pointing to zero. Tighten in place.

Photo 7a Photo 7b Photo 7c Photo 7d
Trimming the height of a tab
Photo 7e Photo 7f Photo 7g Photo 7h
Photo 7h      


Wiring the Project Box Lid

Step 8. Wiring the component-to-component connections


Refer to this graphic to identify pins and part IDs in wiring the lid. Don't solder connections until instructed to do so.


Photos 8a: Bend the shorter leg of the trigger LED (LED2) over and bring it up through lug 1 of the gate jack, J3. Bend the leg back on itself to hold it in place.


Photos 8b: Cut two sections of green wire, 1 in and 1.5 in. Strip each end back by 1/4 in. Make these connections:

  • 1-in section: Lug 2 of SW1 to lug 2 of J1
  • 1.5-in section: Lug 1 of RV2 to lug 2 of RV1

Photo 8c: Cut two sections of red wire: 1 in and 1.5 in. Strip the longer section back by 0.5 in on one end. Wrap the bare wire around a nail as shown in Photo 8c. Strip all the other ends of the two wires back by 1/4 in.


Photos 8d,e: Drop the spiral winding that you created in the previous step down over the longer leg of the power LED (LED1) as shown in Photo 8d. Connect the other end of this wire to lug 2 of RV2. Then connect the shorter of the two red wires between lug 2 of RV2 and lug 3 of the power jack, J1. You can solder connections now or wait until later. If you solder now, solder only those connections circled in yellow in Photo 8d.

Photo 8a Photo 8b Photo 8c
Photo 8d Photo 8e  


Connecting the Jumper Wires

Step 9. Connecting the jumper wires to the lid and the PCB


The wires that you'll add in this step are connected to components on the lid on one end only. The other end will be connected to the PCB. Continue to refer to this graphic to identify pins and part IDs.


Except as indicated below, you may now solder wires as you connect them below or wait to solder until finished with all connections. When you solder a leg of an LED, solder near the base of the LED and then clip off the excess leg length. Also, make sure that the legs of each of the LEDs are well separated in order to avoid shorts.


Photo 9a: Cut the following sections of wire:

  • Blue: 1, 1.5, 1.5 in
  • Green: 2.5, 2.5 in
  • Yellow: 1, 1.5, 2.5 in
  • White: 1, 6 in
  • Red: 2.5 in

Strip one of the 1.5-in sections of blue wire and the 2.5-in section of yellow wire back by 0.5 in each on one end and 1/8 in on the other. Wrap each of the half-inch stripped ends around a nail as in Photo 8c above. Then strip all the remaining wires back by 1/4 in on one end and 1/8 in on the other. When you connect wires to the components on the lid, you'll use the ends with the greater stripped length. In a later step when you connect the wires to the PCB, you'll connect the 1/8-in stripped ends to the PCB.


Slip the spiral wrap of the blue wire down over the free leg of LED2 as shown in Photo 9a. Then make these connections to the 3.5mm jack, J3:

  • 1-in white wire to lug 1
  • 1-in blue wire to lug 2
  • 1.5-in green wire to lug 3

Photo 9b: Make the following connections to the 3.5mm jack J2. See Photo 9b.

  • 1.5-in green wire to lug 1
  • 1.5-in blue wire to lug 2
  • 1.5-in yellow wire to lug 3

Solder the connections that you made to LED 2 and jacks, J2 and J3.


Photo 9c: Before continuing with the wiring, you'll prepare the PCB for connections. Push the 4 1/4-in 4-40 bolts through the lid from the upper side and then screw on the four standoffs as shown in Photo 9c. Note how the wires with free ends have been repositioned (wires at the top pulled up and wires at the bottom pushed toward the center). This will make connections to the PCB easier.


Photos 9d-f: Position the PCB on the standoffs as shown in Photo 9d. Loosely bolt one corner in order to help hold the board steady as connections are made. Then make the following connections to the PCB, soldering as you go. You may trim the lengths of the wires as needed to make them fit easier under the board. Photos 9e and 9f are side views showing the connections for J2, J3, and LED2.

  • White wire from lug 1 of J3 to hole A on the PCB
  • Blue wire from lug 2 of J3 to hole C
  • Green wire from lug 3 of J3 to hole D
  • Blue wire from LED2 to hole E
  • Green wire from lug 3 of J2 to hole K
  • Blue wire from lug 2 of J2 to hole J
  • Yellow wire from lug 1 of J2 to hole I

Photo 9g: Earlier, you prepared a 2.5-in yellow wire by wrapping one end in a spiral. Slip this down over the shorter leg of LED2, and connect the other end to hole B on the PCB. Then connect the remaining yellow wire from lug 1 of RV1 to hole F.


Photo 9h: Connect the 6-in white wire from lug 1 of SW1 to hole M on the PCB and the 2.5-in red wire from lug 3 of power jack, J1, to hole G. Don't solder lug 3 of J1 yet, as there's one more wire to add in the next step.

Photo 9a Photo 9b Photo 9c Photo 9d
Photo 9e Photo 9f Photo 9g Photo 9h


Connecting the Battery Holder and Completing the Assembly

Step 10. Connecting the battery holder and completing the assembly


Photos 10a,b: Cut a 1.5-in section each of black and red wire. Strip each wire back half an inch on one end. Also strip the red and black wires of the battery holder by the same amount. See Photo 10a. Twist the black battery holder wire around the short, black section. Do likewise for the red wires as shown in Photo 10b. Then solder the connections.


Photo 10c: Cut two 1-in sections of the 3/32-in heat shrink tubing and slip over the soldered connections. Use a lighter or match to shrink the tubing. The purpose of these extensions is to make the wire ends less likely to break when the lid is removed for battery replacement.


Photo 10d: Connect the black wire of the battery holder to lug 1 of the power jack, J1, and the red wire to lug 3 of the same jack. You may now solder these connections and any others that you haven't already soldered.


Photo 10e: Bolt down the PCB firmly on all three corners. Then align the PS2501-2 optocoupler with the socket as shown in Photo 10e. Note the location of the white dot on the optocoupler. Once the pins are aligned, push down, applying as nearly equal pressure as possible on the surface of the IC to seat all pins.


Photo 10f: Position the battery holder in the base of the project box as shown in Photo 10f. Use the supplied hook and loop tape to secure the battery holder to the bottom of the box. Check that the power switch on the lid is turned off, and then insert a fresh 9-V battery into the holder.


Photo 10g: Place the lid on the box, being sure to seat it completely without any wires being pinched. Then the screw down the 4 corners of the box. This completes the assembly.

Photo 10a Photo 10b Photo 10c Photo 10d
Photo 10e Photo 10f Photo 10g  


Before you can test the control box, you'll need to build a photogate frame. See instructions here.


If you've already built the frame, go to the operating instructions.



Assembly instructions for other kits




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