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Assembly and Operating Instructions for Kits


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Instructions for Building the Sound Trigger 2, ST2


Assembly instructions for other kits


Sound Trigger 2



This trigger uses the same circuit as the SK2 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

400-V SCR (EC103D)
PN2222A transistor
Piezoelectric disc

PC board

Small project box

1 5.1-kΩ (green-brown-red)
1 100-kΩ (brown-black-yellow)
2.5-kΩ potentiometer (red knob)

RCA jack

9-V battery holder with hook and loop tape

4-in each of yellow and black hook up wire

2-in of blue hook up wire

Label for project box lid



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 in. Additional items that may be useful for this project are a hot glue gun and rubber cement.


Building the sound trigger



Photo 1a

Step 1


Solder the 2 resistors (5.1k and 100k), the PN2222A transistor, and the EC103D SCR onto the PC board as shown. See Photo 1b for a better view of the placement of the legs of the transistor and SCR.


Photo 1b

Photo 2

Step 2


Add the 2.5k pot as shown and solder it in place. Then add yellow and black jumper wires. Note that the column of holes to far right will be the ground column, and the column to far left will be the +9V row.

Photo 3

Step 3


Drill three 3/32" holes on one of the small sides of the project box. These will be for inserting the pins of the piezo disc.

Photo 4

Step 4


Drill an 1/8" hole on one of the long sides of the box, positioning it as shown.

Photo 5

Step 5


On the short side of the box opposite the piezo disc, drill a 1/4" hole and mount the RCA jack.

Photo 6

Step 6


This shows the interior of box with components mounted before wiring.

Photo 7a

Step 7


Cut 2-in links of blue and black wire for the piezo disc leads. Blue will be for the positive pin, labeled M, and black will be for the negative pin labeled, G. The center pin will not be used. Remove the piezo disc from the box for soldering.


Soldering wire leads to the piezo disc is a little trickier than normal soldering. Here's a technique that works for these short pins. Melt a glob of solder onto the tip of the soldering iron. Then while holding a wire against a pin, touch the tip to the wire to deposit the solder. Let it cool and harden before letting go. If there's excess solder, you may need to trim it off with dikes in order to be able to pass the wire and pin through the holes in the side of the box.


Photo 7b shows a view from a different angle.


Photo 7b


Photo 8

Step 8


Solder black and yellow wires to the terminals of the RCA jack as shown.


In order to seat the piezo disc firmly against the box, use a generous amount of hot glue.

Photo 9

Step 9


Before connecting the PC board to the box, cut down the board to just the 5-row section with the components. Adjust the red pot to its halfway position. This is a sensitivity adjustment, and the halfway position is fine for most applications. In order to provide a more sensitive adjustment, see the section, Using the Sound Trigger, below.


Pass the red and black wires of the battery holder through the hole in the side of the box. In order to provide strain relief, tie an overhand knot in the wires on the inner side of the box.


Make these solder connections from the box wires to the PC board:


1. All black wires to the ground row of the PC board.

2. Red wire from the battery holder to the +9V column of the PC board.

3. Yellow wire from the center lug of the RCA jack and blue wire from the piezo disc to the holes shown on the photo.


Photo 10

Step 10


This shows the completed assembly looking inside the box. Carefully fold the wires into the box and push the PC board inside. Then screw on the lid. Use the hook and loop tape to stick the battery holder securely to the side of the project box.

Photo 11

Step 11


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 Sound Trigger


Before you can use the trigger, you'll need to prepare a cable to extend from the RCA jack to your flash unit. Once that's done, insert a 9-V battery in the holder. There's no on/off switch, so remove the battery when you're done. Even if you forget to remove the battery, the circuit draws very little current, and the battery runs down slowly.


In order to use the trigger, hold the microphone near the event that you want to photograph, for example, a balloon burst. The distance between the source of sound and the microphone will determine the time delay before the flash discharges. Figure about a foot for each millisecond of delay. Aim the flash at the subject. When the sound is produced, the flash will discharge. Note that the trigger is designed to respond to loud, sharp sounds like claps, bursts, explosions, etc.


Sensitivity adjustment: In the event that you want to get the highest sensitivity from the trigger, use the following procedure.

  1. Remove the lid of the box.
  2. Insert a battery and connect a flash unit. Check that the flash unit discharges with a snap of the fingers near the microphone.
  3. Using a jeweler's screwdriver, turn the red pot counterclockwise until the flash unit discharges sponteneously. Then back off clockwise a few degrees.* Verify that the trigger responds to a snap of the fingers, and reassemble.

*If the pot is turned too far counterclockwise, the trigger will continuously short the flash unit, preventing it from recycling. This is the reason for backing off the adjustment from the point of spontaneous discharge. Note also that this point may drift as the battery weakens.


Assembly instructions for other kits




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