Step 1: Adding the 555 Timer

The 555 timer is an 8-pin IC that also has a notch and circle identifying Pin 1.  Orient the IC so that the notch faces the left side of the breadboard. Now find Row 4 and look across to where it meets Column e. Place Pin 1 there. Pin 8 should easily fit in Row 4, Column f. Press the IC firmly down in place; again, it should be seated across the center division of the breadboard.

Step 2: Adding the Potentiometer

The potentiometer allows you to adjust the sensitivity of your photogate. It has three legs, two in the front and one in the rear. Place the two front legs over Rows 8 and 10 on Column a, and the rear leg over the nearest hole on the nearby negative column. The front legs should be facing the center of the breadboard, while the rear leg faces the outside of the breadboard. Press the legs in firmly as far as they will go, but avoid bending them.

Step 3: Adding the SCR

The silicon-controlled rectifier is the output of the photogate circuit.  Putting in this SCR is easy since all three leads go in consecutive rows along Column e. Hold the SCR as in the diagram to the right in order to identify the leads. Put the cathode into Row 11 on Column e. The gate will then go into Row 12, and the anode into Row 13 of that column.

Step 4: Adding the Capacitor

Locate the capacitor labeled 103. This has a value of 0.01 µF. Insert one lead of this capacitor into Row 7, Column i, and the other lead into Row 9 of the same column.

You may wish to trim the leads of the capacitor so that it sits closer to the breadboard.  This will reduce the chance that the leads of two components accidentally touch each other and create a short.  When you add the resistors in the next step, you may wish to trim their leads also.

Step 5: Adding the Resistors

Locate the brown-black-orange resistor (10 kΩ).  Insert one end into Row 6, Column c, and the other end into Row 12 of the same column.  Next, find the yellow-purple-brown (470 Ω) resistor.  Insert one end into Row 2, Column g. The other end should reach over to the nearest hole in the negative column.

Step 6: Adding the Wires

Now you'll connect all your electronic components together. Each wire only needs to be 2 inches in length or less. You can estimate how much you'll need to bridge across two holes before cutting, although it's always better to have longer wires than ones that are too short.

Strip about 1/4" of insulation off each end. The list below will tell you which rows and columns your wire ends should fit into. The longest wires are listed first so if you happen to cut a piece that's too short, you'll be able to use it later.

Step 7a: Connecting the LED and phototransistor (or interrupter)

The photogate has a light-emitting and a light-sensing component.  The former is a light-emitting diode (LED), which emits an infrared beam.  The sensing component is an infrared phototransistor (PT). When the beam is broken by an object, the blockage causes the voltage to rise across the PT, which gates the SCR at the output of the circuit.

For the SPG1 kit, the PT and the LED are the individual components shown to the right. The LED is the component with a blue case, and the PT has a clear case. (In an earlier version of this kit, the PT and LED both had clear cases. If you have this version, note that the LED has longer legs than the PT.) For both components, one leg is shorter than the other. The shorter leg is positive on the PT, while on the LED, the longer leg is positive. The wiring instructions given later in this section will ensure that the correct polarity is maintained.

For the SPG2 kit, the PT and LED are housed in the two posts of the single component, termed an interrupter, shown to the right.

To begin wiring, use the gray 3-conductor cable. The 4 legs of the PT and LED (or interrupter) will be soldered to these three conductors.

Strip 1" of insulation from each of the conductors on one end of the cable. The PT and LED will be attached to this 1" end. Now strip ½" of insulation from each conductor on the other end of the cable. This ½" end will connect with the breadboard. Strip an additional 1" of the gray outer shielding from the ½" end so the individual conductors can reach to their destinations.

If using individual PT and LED components, make the following connections:

Wrap the black wire around the shorter leg of the LED.
Wrap the white (or green) wire around the longer leg of the PT.

Next prepare a jumper wire that will go from the longer leg of the LED to the shorter leg of the PT.  The length of this jumper will depend on how far apart you want to separate the PT and LED for your photography.  Strip the wire back about an inch on each end wrap it onto the legs of the components.

Now wrap the red wire of the 3-conductor cable to either one of the legs onto which you wrapped the jumper wire.

For best connections, solder the red, black, and white (or green) wires to the legs onto which they are wrapped. Before soldering each leg, clip a heat sink (a metallic alligator clip will work for this) to the leg just below the plastic case. This will prevent the component from heating excessively during soldering. Before starting to solder, make sure you're working in a well-ventilated area in order to avoid inhaling the solder fumes. A fan to blow the fumes away from you will help. Prepare the tip of the soldering iron by holding the solder to it so that solder can melt and flow over the tip. This will improve heat conductivity.  Touch the solder on the leg to which you're soldering the wire.  Hold the flat of the soldering iron tip on the leg but not directly on the solder. As soon as the leg is hot enough, the solder will flow. Move the solder around so as to melt solder into the wire and onto the leg along the length of the leg.

The symbolsand  on the top view of the interrupter to the right refer to the LED and phototransistor (PT) respectively. The numbers refer to the legs on the underside (not shown).

Solder the black wire to leg 2.
Solder the white (or green) wire to leg 3.
Solder a short jumper wire between legs 1 and 4.
Solder the red wire to the jumper between legs 1and 4.

If you have an interrupter with short legs, you won't be able to wrap wires around the legs. Just hold the wire to the leg and tack solder the two parts together. Since you won't be able to use a heat sink, minimize the amount of time that the soldering iron is in contact with the leg.

Whether using individual PT/LED or an interrupter, do the following:

Connect the free ends of the 3-conductor cable to these holes:

Black to Row 2, Column j
white (or green) to Row 6, Column j
Red to the positive column

Step 7b. Connecting the flash unit

You may already have an output cable for your flash if you prepared one for the delay circuit. If not, use the following instructions.

The 3 feet of 2-conductor is used to connect the output of the photogate trigger to the PC cord of a flash unit. From one end of the 2-conductor cable, strip 1" of the gray insulation, being careful not to cut the insulation on the red and black wires.  Then strip 1/2" of insulation from each of the red and black wires.  These will connect to the breadboard.  Next, strip 2" of the gray insulation from the other end of the cable.  Strip each of the individual wires back 1".  These will connect to the PC cord.  One way to make this connection is to cut the socket off the end of the PC cable, strip the insulation on the individual PC wires back by 1", splice the red wire of the gray cable to the positive wire of the PC cable, and splice the black wire of the gray cable to the negative wire of the PC cord.  (The positive wire of the PC cord is usually the wire that goes to the center pin of the PC socket.  For more information on connecting to a PC cord, see this page: http://hiviz.com/tools/triggers/makeown.htm#connect.)

In order to connect the flash unit to the output of the photogate, connect the red wire to Row 13, Column b, and the black wire to the negative column.

If you're connecting the flash to a delay unit,

For the delayed output (Output 2) of the delay circuit, connect the red wire to Row 18, Column b and the black wire to the negative column. (This is the connection shown in the photograph.)

If you change the distance between the PT and LED (if using the individual components) or if the orientation of either component changes slightly, you may need to readjust the sensitivity.  The maximum separation is about 6 inches. The larger the separation, the more care you need to take in aligning the components.