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Assembly and Operating Instructions for Kits


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Instructions for Building the Photo Gate 2i, PG2i


Assembly instructions for other kits


Sound Trigger 2



This trigger uses the same circuit as the SPG breadboard kit; however, the circuit is built on a pc board and housed in an enclosure for durability and convenience.


For individual photos of the parts below, see the Parts List.


Parts List

PC board

RCA jack


LED and holder

Small project box

400-V SCR (EC103D)
LM555 timer (8-pin socket optional)

3 470-Ω resistors
10-kΩ potentiometer (white knob)

0.01-uf capacitor

9-V battery holder

5-in green, red, blue, white, 7-in black hook up wire

Label for project box lid

Template for project box



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.)


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

    Wire cutters and stripper

    Small diagonal cutter

    diagonal cutter

    Needle-nose pliers

    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.


A drill motor and these drill bit sizes are also required: 3/32, 1/8, 1/4, 3/16, 1/4 in. Additional items needed for this project are a hot glue gun, rubber cement, and electrician's tape.


Building the photogate

Photo 1a

Step 1


Cut out the corners of the template and place in the bottom of the project box. Mark the locations of the three holes with a punch or nail. See Photo 1a.


Remove the template and drill the three holes the sizes indicated on the template. Photo 1b shows the completed drilling.


Photo 1b

Photo 2a

Photo 2b

Photo 2c

Step 2


There are several more holes to drill on different sides of the box. On the short side nearest the holes you've already drilled, drill a 1/4" hole in the center. (This hole will be for the RCA jack.) See Photo 2a.


On the other short side, drill three holes as shown in Photo 2b. The outer large holes are 3/16" each and are 1 3/16" apart, center-to-center. The small hole is 1/8" and is positioned midway between the outer holes. These holes will be for the interrupter leads.


On the long side shown in Photo 2c, drill an 1/8" hole. This hole will be for the battery holder leads.

Photo 3a

Photo 3b

Step 3


Refer to Photo 3a. Mount the RCA jack in the single hole that you drilled in the short side.


The pair of 1/4 - 3/32" holes are for the switch. The small hole is for the tab on the retaining ring.


The LED holder has two parts, the collar and the ring as shown to the right. Push the collar of the LED holder into the remaining 1/4" hole from the outside of the box. Then push the LED into the collar from the inside of the box. Substantial force may be needed to get the LED to snap into place. Finally, push the ring over the collar from the inside of the box.


The mounted components are shown in Photo 3b.

Photo 4a

Photo 4b

Photo 4c

Step 4


At this point, we move to wiring the PCB. A cut -out from a larger PCB is provided as in the Photo 4a. This photo shows the underside (copper clad) side of the PCB. Turn it over to position the 555 timer next.


Position the 555 timer as shown in Photo 4b. Note that the notch on one end is toward the bottom of the photo. In the event that you have an 8-pin socket, solder the socket in place of the timer. Then you can easily replace the timer if you need to do so at a later date.


Solder the 8 lugs of the timer on the back of the board, being careful not to create solder bridges between adjacent lugs. See Photo 4c for the completed soldering.



Photo 5

Step 5


There are three resistors, all with the same value of 470 ohms. Solder these in the locations shown. Note that for the resistor at far left, create an arch as shown. This will help keep one of the legs from touching a component that will be soldered later. After soldering the resistor, clip the protruding legs on the back of the board.

Photo 6

Step 6


Cut jumper wires and solder in the locations shown. Use the same colors shown in order that you don't end up with a shortage of colors later.

Photo 7

Step 7


There are three components left to solder, the SCR, the 10-k pot, and the 0.01-uf capacitor. Solder them in the locations shown in Photo 7. Note that the flat side of the SCR points to the left.


Turn the white pot to its halfway position. This shouldn't require further adjustment except for very critical applications.


Photo 8

Step 8


Cut 2-in lengths of green, red, and black wires and solder them in the locations shown in Photo 8. These will be used to connect to the interrrupter later.

Photo 9

Step 9


Now we return to the project box. Thread the wires of the battery holder through the 1/8" hole in the long side of the box. Use the sticky back hook and loop tape to stick the battery holder firmly to the side of the box. Tie an overhand know in the two wires inside the box in order to provide strain relief as shown in Photo 9.

Photo 10a

Photo 10b

Step 10


Cut 3-in sections of black, white, blue, and green wires and a 1.5-in section of red wire. Then do the following steps in order. Refer to Photo 10a.

  1. Solder the red wire from the battery holder to the center lug of the switch. (Note that the black wire from the battery holder will be left unconnected for now.)
  2. Solder the black wire to the negative (side) lug of the RCA jack.
  3. Solder the blue wire to the postive (center) lug of the RCA jack.
  4. Strip half an inch of insulation from the 1.5-in red wire. Then wrap it around a brad or small nail as shown in Photo 10b. Remove the brad and slip the coiled wire over the longer (positive) leg of the LED. Slip the coil down near the base of the LED and solder in place.
  5. Connect the other end of the 1.5-in red wire to the bottom lug of the switch. Also connect the white wire to this lug. Then solder in place.
  6. Connect the green wire to the shorter (negative) leg of the LED in a manner similar to what you did for the positive leg.

Photo 11

Step 11


We move to soldering the interrupter next. This requires some very careful soldering due to the short and closely-spaced legs of the component. First, note the diagram to the right with a numbering scheme for the the legs. We'll refer to these numbers in the instructions that follow.


Now cut a 2-in length of white wire and strip off all the insulation. (In case you're wondering why we use white, it's because 2 extra inches of white are included in the kit for this purpose.)


You'll solder this bare wire between pins 1 and 4 of the interrupter. Create a small loop around each end to encircle the corresponding pin. When you solder, be careful to keep a distinct gap between adjacent pins. The completed soldering is shown in Photo 11.

Photo 12a

Step 12


Orient the PCB in the open project box as shown in Photo 12a and thread the green, red, and black wires from the PCB through the 3 holes in the side of the box. These three wires will connect to the interrupter. Don't connect them yet but note for future reference that the black wire will connect to pin 2, the green wire to pin 3, and the red wire to the jumper between pins 1 and 4. Orient the interrupter accordingly.


Now loop the red wire around the center of the jumper wire and solder it in place as shown in Photo 12b.

Photo 12b

Photo 13

Step 13


Here's the difficult part of the soldering. You'll need to solder the green wire to pin 3 and the black wire to pin 2. See Photo 13 for the orientation. One method is to create a small loop in the end of each wire to encircle the corresponding pin.Another method is to simply hold the wire parallel to the pin and solder it in place. Whatever method you choose, again be sure to maintain gaps between adjacent pins. You can bend them outward slightly if necessary.

Photo 14

Step 14


The completed soldering of the interrupter is shown in Photo 14. In order to avoid the possibility of acjacent pins contacting each other when the interrupter is connected to the box, we recommend using a sliver of electrician's tape in the gaps.

Photo 15

Step 15


Careful thread the wires back into the project box and position the interrupter as shown in Photo 15. Use plenty of hot glue to hold the interrupter in place. Just avoid getting hot glue near the ends of the 2 posts, as these are the locations of the light-emitting diode and the light sensor.

Photo 16

Step 16


Complete the soldering by connecting the 5 wires from the project box to the PCB. The connection points are indicated by arrows in Photo 16. Note that both black wires, the battery wire and the wire from the RCA jack, connect to the same row. This is the ground row of the circuit. The white wire connects to the row on the opposite side. This is the +9-V row.

Photo 17

Step 17


In order to prepare the label:

  1. Trim the label to the black border. We recommend using a stencil knife for fine, straight cuts.
  2. Peel the backing off of one of the sheets of laminate, and place the label onto the sticky side of the sheet. Press the label in place.
  3. Peel the baking off of the other sheet of laminate. Place the sticky side down onto the label. Press firmly in place, pushing any bubbles toward the sides.
  4. Trim the excess laminate from the perimeter of the label.

After laminating the label, use an adhesive such as contact (rubber) cement to glue the label to the box lid. Spread the adhesive onto both the lid and the back of the label, and then press the label in place. (Some adhesives require that you wait several minutes before pressing the two surfaces together.) If you're using contact cement, you don't have to worry about spreading the adhesive on the lid beyond the area that the label will cover. Just let the adhesive dry for a few minutes. Then you can roll the excess off with a finger.


In order to improve adhesion, place the box under a stack of books for several hours.


Using the Photo Gate 2i


In order to test for proper operation, insert a fresh battery in the battery holder. Turn the switch to On. The LED should light. Now run a finger through the interrupter. The LED should blink. In fact, as long as the interrupter is blocked, the LED will be off.


In order to use the photogate to observe or aid in the photography of high-speed events, connect a flash unit to the RCA jack. You may need to prepare a cable for this purpose. The cable must have a connector for your flash on one end and and RCA plug on the other end. If your flash doesn't have a connection jack but does have a hot shoe, you can use a hot shoe adapter such as the one shown here. Note that such an adapter also requires a standard PC cord.


Once the flash is connected and turned on, blocking the interrupter will discharge the flash immediately. If the event that you want to observe or photograph requires a time delay before the flash discharges, you will need to connect a delay unit the RCA jack and your flash to the output of the delay unit.


Assembly instructions for other kits




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