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What you need
These instructions show how to prepare a crossed-beam photogate and its control box.
You'll need to solder solder components and wires for this project. We'll provide guidelines for getting good solder joints, but we recommend that you have previous experience soldering on a PC board. With soldering, you can't make changes easily like you can with a breadboard. If you solder something in the wrong place, repair can be time-consuming.
Having the right tools will make the job easier. You'll need to provide your own. Here's what we recommend.
15-30 W soldering iron (with a new or pointed tip) and solder
Wire stripper (photo below)
A small diagonal cutter (photo below) makes it easy to trim the legs of the components after you solder them to the PCB, but other kinds of snipping tools such as scissors may work.
Needle-nose pliers (photo below) make it easier to handle small components, especially if you have big fingers.
A heat sink (photos below) protects heat-sensitive components while soldering.
A desoldering tool (photos below) helps in clearing solder from a hole. The cylindrical type works better than the bulb.
A magnifying glass is used to inspect solder joints.
A lighter or matches to shrink heat-shrink tubing
Wire cutters and
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.
For the project box
Drill motor and bits to drill holes in the project box lid. Bit sizes are 3/32", 1/8", 1/4", 5/16", and 3/8". (For metric equivalents in millimeters, multiply by 25.4.) You can substitute a 1/8" bit for the 3/32" one.
Hammer, punch (or nail), small round file
Wrenches or sockets to tighten components onto the project box
Aids to troubleshooting
If you need to troubleshoot problems with the circuit, it will help to have at least some basic skills in building and testing electrical circuits. These involve such things as testing for polarity and continuity, for examining solder connections, and for measuring voltages. In addition to such knowledge and exerience, having a continuity checker or multimeter and clip wires may be helpful.
Click here for a detailed, illustrated list of all the parts you'll need. You can use this list to identify the parts and make sure you have them all.
If you like to have a circuit schematic handy while you work, print this pdf document for reference. While not essential, the document may be helpful for people who have experience in reading schematics.
Soldering components to the PCB
If you don't have your soldering iron heated up, do that now, because you'll be soldering before long. You'll be doing some detailed soldering work, so an iron with a good tip will make it easier. In this part of the assembly, you'll solder to the PC board, shown in Figure 1 below. You'll insert components and wires from the front side of the board and solder them to the back side, which is copper-plated. Since the holes of the board aren't numbered, we've superimposed numbers on the front view of the board in Figure 2 and on the back view in Figure 3. Orient the board with the word RadioShack at the bottom as shown in the photos. This is important since the board isn't symmetric when flipped vertically.
Position the 8-pin socket as shown in Figure 4 on the front of the board. Note that the notch in one end of the socket is at the bottom. Note also the lines which a and b, which are intended to point out the lack of top-bottom symmetry of the board.
Figure 1. Front and back views of PC board
Figure 2. Front of PC board, annotated
Figure 3. Back of PC board, annotated
Figure 4. Board with 8-pin socket positioned
Turn the board over and crimp the pins of the socket to hold it in place as shown in Figure 5 below. Again, note that the work RadioShack is at the bottom of the board. (Sorry for belaboring this point; we just want to make sure that you get started out right.)
Solder the pins now. Completely fill each hole but be careful not to bridge the solder between holes. If you do, shake any excess solder off the tip of the iron and draw it along the board through the bridge to remove it. Several tries may be required. If this doesn't work, you may need to use a desoldering tool. The completed solder job is shown in Figure 6.
Two more components will be added to the board; these are one of the 100-ohm resistors (shown to the right) and the transistor. Figure 7 shows the positions of these components on the board. Solder the resistor first. Bend the legs down and push them through holes 1 and 2. (Refer to Figures 2 and 3 as needed for hole numbering.) Solder the legs to the back of the board and clip them off.
Next orient the transistor in holes 3, 4, and 5 as shown in Figure 7. Note that the flat side of the component faces the outside of the board. Push the transistor in so that only about 1/8" of the legs is showing from the front side of the board. (If the legs extend too far above the front side, it will be easier to accidentally bend back the transistor and break a leg.) When you solder the legs on the back side, solder quickly to avoid overheating the transistor. If you have a heat sink, clipping that to a leg will help absorb heat. The completed soldering job is shown in Figure 8.
Figure 5. Socket pins are shown crimped from the back
Figure 6. Soldering of the socket completed
Figure 7. Jumper wire ready to insert into R5
Figure 8. Completed soldering job from the back
Figure 9. PS2501 optocoupler (note location of the dot at lower right)
Figure 10. Optocoupler inserted into the socket
You won't be soldering to the PC board again until after you've completed the project box. However, you can insert the PS2501 optocoupler now. This component is shown in Figure 9 above. (If the numbering on your optocoupler isn't identical to that shown in the photo, don't worry. What's important is the designation 2501.) Note the location of the dot in the lower right-hand corner of the chip. This is used to obtain the correct orientation.
Seat the pins of the optocoupler firmly into the socket as shown in Figure 10, again with the dot at lower right. You can set the PC board aside now and go on to assemble the project box lid.
The template is sized to fit within the underside of the project box lid (that is, on the interior side of the box). Position the template inside the lid as shown in Figure 11 below. Then use a nail or punch to mark the positions of the centers of the holes to be drilled.
Remove the template and drill the holes. We recommend drilling small pilot holes first, for example, 3/32" or 1/8". The plastic has a tendency to grab the bit, so hold the plastic securely. We've found that spade bits work best for drilling the 1/4", 5/16", and 3/8" holes. The hole size is most critical for the LED holder. If the hole is slightly small, it will take a lot of force to snap the LEDs into the holder. If the hole is slightly large, the LED will wobble in holder; however, this won't affect its function. Figure 12 shows th inside of the box lid after the holes are drilled, while Figure 13 shows the reverse view from the outside of the lid.
Figure 11. Template placed in underside of project box lid
Figure 12. Inside of project box lid after holes drilled
Figure 13. Outside of project box lid after holes drilled
Adding the components to the project box lid
We recommend adding the LED and its mount first, since people tend to have difficulty getting the holders to snap into place. The LED mount has two parts, which we will call the collar and the ring. (See photo to the right.) The collar is first slipped into a hole on the project box from the top side toward the interior of the box and snapped into place. Then the LED is pushed up into the collar from below. Turn the LED so that the shorter leg is oriented as shown Figure 14. It's important to get the LED to snap into the collar. It may take extra force to push it the last bit of the way. You'll know it's in when it snaps. If you don't push the LED in all the way, it will be loose in the collar. Note in Figure 14 how the bottom of the red case sits down inside the collar. Also see Figure 15, which shows the LED from the top of the box. Note how far it extends above the collar. If you find it too difficult to snap the LED into the holder, remove the LED and the collar and file the hole slightly. The tightness of fit of the LED in the collar is very sensitive to the size of the hole.
Once you have the LED snapped in, push the ring over the collar from below. See Figure 16.
Figure 14. LED inserted into collar from below
Figure 15. LED seated in the collar
Figure 16. Completed LED holder assembly from below
Three of the components that will be added to the project box lid are jacks. We provide the photos below to help in orienting and wiring the lugs of the jacks correctly. Figures 17-19 show the 3.5mm stereo jack, the 0.25-in stereo jack, and the AC adapter jack respectively. The numbering on the lugs is the same as will be used later in the instructions.
Figure 17. 3.5mm stereo panel jack with lugs numbered
Figure 18. 0.25-in stereo panel jack with lugs numbered
Figure 19. AC adapter panel jack with lugs numbered
For the remaining components, we don't provide step-by-step photos. The completed assembly of all components on the project box lid is shown in Figures 20 and 21 from below and above, respectively. Refer to the Parts List for help in identifying components. In Figure 20, the lugs of the three jacks are numbered the same as in Figures 17-19 above. Orient these jacks the same as shown in Figure 20.
Remove the nut washer from the 1/4" stereo jack, insert the jack through the box lid from below, replace the washer, and screw the nut on.
Remove the nut from the 3.5mm stereo jack, insert the jack through the box lid from below, and screw the nut back on. Needle-nose pliers can be helpful in tightening the small, round nut.
Remove the nut and washer from the AC adapter jack, insert the jack through the box lid from below, replace the washer, and screw the nut back on.
The two switches have a retaining ring with a key tab, a washer, and a nut. Remove all three and then insert the switch from below. Orient the slot on the threads to be on the same side as the 3/32" key hole. Then slip the retaining ring on so that the key tab slips into the 3/32" hole. Slip on the washer and nut and tighten.
The positions of the two potentiometers (10 kΩ and 100 kΩ) are
indicated in Figure 20. The 100-kΩ
pot is to the right. The potentiometers have key tabs on the body of the pot. Remove the washer and nut from a pot, slip it in from below and orient it so that the key tab passes up through the 1/8 " key hole. Then put on the washer and nut and tighten.
Now you can put the knobs on the pots. Turn all the pots counterclockwise as far as they will go. Orient the pointers so that they point as shown in Figure 22 and tighten the set screws. Figure 23 shows the knobs mounted and turned to their halfway settings. The labels have also been added.
Besides the four mounting holes on the outside corners of the lid, there are four other holes that haven't been used yet. These holes are for standoffs that will support the PC board. You'll mount the standoffs later so that they won't get in the way while you're adding wires to the lid.
Figure 20. Components mounted on project box lid (view of underside)
Figure 21. Components mounted on project box lid (view of the top)
Some of the components on the box lid will be hardwired to each other. Make connections for now but don't do any soldering yet.
There are two resistors that are connected between components on the lid. These are shown in Figure 24. The 220-Ω resistor (color code red-red-brown) is connected from lug 3 of the AC adapter jack to lug 3 of the 0.25-in stereo jack. The second 100-Ω resistor (brown-black-brown) is connected from the left lug of the 100-kΩ pot to the center lug of the 10-kΩ pot. Figure 25 shows a close up of the resistor wiring.
You'll wire the components together next. Begin with the white wires. We use white to represent the ground connections. While any color could be used, if you use the same colors as in these instructions, you should have sufficient wire of each color to complete the assembly. The connections are shown in Figure 26 and listed in the table below. Here's a note about making connections to the LED. If you strip the wire back by half an inch and wrap it around a small nail or brad, then you can slip it easily onto the LED leg. See Figure 27.
lug 2 of AC adapter jack
center lug of left switch
right lug of left switch
shorter leg of LED
shorter leg of LED
lug 2 of 025-in stereo jack
Figure 24. Resistors connected on box lid
Figure 25. Close up of resistors
Figure 26. White (ground) wires connected on the box lid
Figure 27. Wire wrapped around nail
Figure 28. Connection of red wire
Figure 29. Connections of blue and yellow wires
Connect a red wire from lug 3 of the AC adapter jack (same as the 220-Ω resistor) to the left lug of the 10-kΩ pot as shown in Figure 28 above.
There are three more wires to connect between components on the lid. Two of them are blue and one is yellow. These are shown in Figure 29 and listed in the table below.
Here are some more tips about soldering in addition to those given previously.
Soldering to the legs of the LED: This component can be damaged by excessive heat; therefore, it's a good idea to clip a heat sink to the leg when soldering. This technique is shown in the photo to the right. Push the wrapped wires to be soldered down close to the red case of the LED. This is important, because the PCB will eventually be seated above the components on the board. It must be possible to trim down the legs of the LEDs so that they don't come in contact with the PCB.
Soldering the switches: Don't hold the soldering iron on the switch lugs too long, as the plastic can melt and break the internal contacts.
Soldering the 3.5mm jack: The lugs on these jacks bend and break easily. Go easy on them.
Soldering the 9VDC jack: Be very careful not to get too much solder on the lugs so that the solder drips down, particularly on the center lug. If the solder drips down, it can create a dead short between the positive and negative power terminals.
About cold solder joints: If you don't heat the metal before soldering the wire, the solder may not bond with it and you can get an open circuit. You can't necessarily tell by looking that you have a cold solder joint. The best approach is prevention by using good soldering techniques. Hold the tip of the soldering iron flat against the metal surface that you're soldering to. Touch the solder to the metal nearby rather than to the soldering iron. When the metal is hot enough, the solder will flow. Flow enough solder on the connection to fill the hole and cover the connection, but don't leave the soldering iron on the metal any longer than it takes to flow the solder. Examine the connection under a magnifying glass. If the solder beaded up, you may not have a good connection.
Figure 30 below shows the connections circled in green that can be soldered now. If a connection isn't circled, don't solder it yet, because additional wires need to be added to jump to the PCB.
Figure 30. Connections to be soldered
Figure 31. Connections of external wires
Figure 31 above shows 6 additional wires that will be use to connect the box lid components to the PC board. Before soldering the connections, we recommend cutting the wires to the lengths given in the table below. The lengths are sized to be long enough to reach the corresponding holes on the PCB but not so long as to add unnecessarily to the mass of wires that will have to be compressed when the PCB is finally bolted down.
Wires listed from left-to-right in reference to Fig. 31
Connection point on lid
right lug of left switch
longer leg of LED
left lug of 10-kΩ pot
center lug of middle switch
lug 3 of 3.5mm stereo jack
lug 1 of 0.25-in stereo jack
Now is a good time to add the standoffs. Use 4 of the 4-40 bolts to bolt the 4 standoffs to the box lid. Insert the bolts from the top of the box.
Lay the PC board next to the box lid as shown in Figure 32 below. Four of the wires are shown connected from the lid to the PC board. The connection points to the PC board are labeled with the same numbers as used in Figure 2 previously. Note that the green wire is the one from the LED.
Figure 33 shows all wires connected to the PC board. The remaining blue and green wires are connected to points 11 and 12.
The last part to connect is the battery holder. This is shown in Figure 34. The black wire of the battery holder is connected to lug 1 of the AC adapter jack, and the red wire is connected to hole 6 on the PC board. Figure 35 shows a closeup of the connection to the AC adapter jack.
Figure 32. Four connections from the box lid to the PC board
Figure 33. All wires connected to the PC board
Figure 34. Battery holder connected to the lid and PC board
Figure 35. Connection of battery holder to AC adapter jack
Figure 36. Placement of the battery holder in the box
Figure 37. Box lid with functions indicated
Figure 36 shows the placement of the battery holder in the box. Note how the battery holder is opposte the two pots. This placement will ensure that the lid will seat correctly on the box. Use the hook and loop strips to stick the battery holder down to the bottom of the project box. Also, gently compress the wires connecting the lid to the PC board and bolt the board to the standoffs.
Move the two switches on the box to the off position (to the left), insert a fresh battery into the holder, and seat the lid on the box. Make sure that no wires are pinched between the lid and the box. You may want to wait to screw down the lid until after you've tested the unit. Figure 37 shows the box lid with the functions of the jacks, pots, and switches indicated.
Then next step is to build a photogate frame. Click here.