Instruction Manual for the TPG Kit

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Assembly Instructions for the Transistor Photogate on a Breadboard (TPG-BB)

last update: 07/30/2008 11:51 AM -0400

These instructions may be used in combination with the instructions for building a delay unit in order to provide a photogate with a selectable delay. For instructions on building the delay unit, go here.

Parts List

The following parts are included with the kit. (If you purchased the TPG in combination with a delay unit, then a single set of wires was provided with the combination.)

Infrared phototransistor (PT)
Infrared emitter (LED)

2N2222 transistor (or PN2222A)
400-V SCRs (EC103D)

Resistors
1 470-Ω(yellow-violet-brown)
1 10-kΩ (brown-black-orange)

100-kΩ potentiometer (gray knob)

Wires
3-ft of 2-conductor cable
3-ft of 3-conductor cable
Hookup wire
9-V battery cable^*

Breadboard

^*A fresh 9-V battery is required but not included with the kit. You'll also need a wire cutting and stripping tool such as the one shown to the right. If you do any soldering, you'll need a soldering iron, solder, and a heat sink.

Click for larger view

Click on the thumbnails below in order to view full-size images of the breadboard with the components that have been added in each step.

Using the Breadboard

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The breadboard offers an easy way to build electrical circuits without soldering. The 2"x3" breadboard provided with your kit contains an array of holes where wires and components are to be inserted. The holes in the center portion of the breadboard are identifiable by row (vertical in the photos) and column (horizontal). There are two sets of 30 rows numbered by 5's, and each set of rows has 5 columns labeled a-e and f-j. The 5 holes on each row are electrically connected to each other (but not across the center channel), so any components inserted into the same row would be connected just as if they had been soldered. However, the components can be removed and replaced with other components at any time, without the hassle of unsoldering and resoldering parts.

On either side of the breadboard are two columns marked by blue and red lines. The 25 holes in each column are electrically connected, but the columns aren't electrically connected to each other. The outermost column marked with the red line at the top will be used for all +9 V connections, while the outermost column marked with the blue line at the bottom will used for all ground (negative) connections.

Assembling the Photogate

Note that the photographs show a delay unit already built on the right side of the board. The photogate may be used with or without the delay unit. However, the 9-V battery cable is required for the operation of either kit. This is the cable coming in from the left with the red and black leads. The column of 25 holes to which the red wire is connected will be termed the positive column, while the column to which the black wire is connected will be termed the negative column. While wiring the circuit, be sure to have the battery disconnected from the battery cable.

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Step 1: Adding the Potentiometer

The 100-kΩ potentiometer (gray knob) 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 7 and 9 on Column j, and the rear leg over the positive 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.

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Step 2: Adding the SCR and Transistor
The silicon-controlled rectifier (labeled EC-103D) is the output of the photogate circuit. Hold the SCR as in the diagram to the right in order to identify the leads. Putting in this SCR is easy since all three leads go in consecutive rows along Column c. Put the cathode into Row 8 on Column c. The gate will then go into Row 9, and the anode into Row 10 of that column.

SCR pin diagram

A = anode (+)
G = gate
C = cathode (-)

The transistor looks identical to the SCR but is labeled PN2222A (or 2N2222A). Its three leads go in consecutive rows along Column g. To identify the leads of the transistor, hold it as in the diagram to the right. Put the emitter into Row 8 on Column g. The gate will then go into Row 9, and the collector into Row 10 of that column.

Transistor pin diagram

E = emitter (-)
B = base
C = collector (+)

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Step 3: Adding the Resistors

Locate the brown-black-orange resistor (10 kΩ). Insert one end into Row 9, Column d, and the other end into Row 8, Column f. Next, find the yellow-violet-brown (470 Ω) resistor. Insert one end into Row 3, Column g. The other end should reach over to the nearest hole in the positive column.

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Step 4: Adding the Wires

Two short wires are needed for this step. Strip about 1/4" of insulation off each end. One wire should join Row 8, Column b to the nearby negative column. The other should join Row 10, Column h to the nearby positive column.

Photogate connections shown with 3-conductor cable; click to view

Note that for some versions of this kit, the colors of the wires in the 3-conductor cable are red, black, and green. In that case, simply replace the word white with green in the instructions.

Step 5: Connecting the LED and phototransistor

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.

To begin wiring, use the gray 3-conductor cable. The 4 legs of the PT and LED 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.

Wrap the red wire around the longer leg of the LED, and the white (or green) wire around the shorter leg of the PT. Next prepare a jumper wire that will go from the shorter leg of the LED to the longer 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 black wire of the 3-conductor cable to either one of the legs onto which you wrapped the jumper wire.

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

Black to the negative column
white (or green) to Row 9, Column h
Red to Row 3, Column f

You may want to wait to solder the leads until you've tested the circuit in step 8. When you do get around to soldering, here are some tips.

Note about soldering: When you connect a wire to a leg of the PT or LED, first wrap the wire tightly around the leg several times. Then 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.

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Step 6: Connecting the Photogate Trigger to a Delay Unit
If you're using a delay unit, add a wire from Row 10, Column a, the output of the photogate trigger, to Row 18, Column h, the input of the delay unit. This will allow the photogate trigger to trip the delay unit.

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Step 7a: Connecting the Photogate Trigger to a Flash Unit
If you're using a delay unit with the photogate, skip to step 7b. If you're connecting the output of the photogate directly to a flash unit, use the instructions below.

Prepare an output cable to your flash unit as follows.

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.)

Once the cable is ready, insert the red wire of the cable into Row 10, Column a and the black wire to the negative column.

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Step 7b: Connecting the Delay Unit to a Flash Unit
If you've constructed a delay unit, then you've already prepared an output cable for your flash. The photograph to the left shows the output cable connected to Output 2 of the delay unit.

Step 8: Operating the Circuit

Connect a 9-V battery to the battery clip. Lay the PT and LED down on a table a few inches apart, pointing at each other. You may wish to tape them in place so the components can't shift positions. Run your finger between the PT and LED quickly in order to break the photogate beam. (If you pass your finger through too slowly, the photogate may not respond.)

If your flash cable is connected directly to the photogate or to Output 1 of the delay unit, you should notice an immediate discharge of your flash unit. If your flash cable is connected to Output 2 of the delay unit, you may notice a short delay before discharge, depending on the setting of your delay circuit. If you don't get a discharge, one possible reason is that the sensitivity isn't adjusted correctly.

Adjusting the sensitivity: Turn the 100-kΩ potentiometer in one direction or the other until the flash discharges spontaneously. Then back up the dial just before the point of spontaneous discharge.

If you change the distance between the PT and LED 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.

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