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Optical Tremolo Box 

Make] Projects 

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build, hack, tweak, share, discover,- 

Optical Tremolo Box 

Written By: Sean Michael Ragan 



1/8" drill bit (1) 

Computer and printer (1) 

Drill (1) 


Hacksaw (1) 

Lighter (1) 

Multimeter (1) 
from RadioShack. 

Pliers (1) 

Scissors (1) 

Screwdriver (1) 

Screwdriver (1) 

Soldering iron (1) 
from RadioShack. 

Unibit (1) 

aka step bit 

Wire cutter/stripper (1) 
from RadioShack. 

Wrench (1) 

Adhesive label (1) 

Project enclosure (1) 
from RadioShack. 

Rheostat (1) 
from RadioShack. 

Resistor. 15Q(1) 
from RadioShack. 

Solder (1) 

from RadioShack. 

Control Knob (2) 
from RadioShack. 

Hookup wire (1) 
from RadioShack. 

Potentiometer (1) 
from RadioShack. 

CdS Photoresistor (1) 
from RadioShack. 

LED holder (1) 
from RadioShack. 

Switch. DPDTM) 

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Optical Tremolo Box 
Wrench (1) 
• Wrench (1) 
Wrench (1) 
X-Acto knife (1) 

aka hobby knife 

from RadioShack. 

Adhesive tape, double-sided (1") 
from RadioShack. 


from RadioShack. 

Jack. 1/4" (2) 
from RadioShack. 

Motor (1) 

from RadioShack. 

Plastic tubing (1) 

/ used the ink tube from a BIC Soft Feel 

Retractable Ballpoint Pen. Medium Point. 

Wood screws (2) 

Grommet (1) 
from RadioShack. 

Transparency film (1) 

Terminal strip (1) 
from RadioShack. 

Universal mounting hub (1) 

Battery holder. 2xAA(1) 
from RadioShack. 

DC power plug (1) 
from RadioShack. 

Heat-shrink tubing, multicolor 
assortment (1) 
from RadioShack. 

Hook-and-loop fasteners, adhesive (1 


aka velcro dots 

Power supply (1) 
from RadioShack. 

AA Batteries (2) 
from RadioShack. 

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Optical Tremolo Box 

Jumbo self-stick cushion feet (3) 
from RadioShack. 

• Flexible LED flashlight (1) 


MAKE contributing editor Charles Piatt proposed a "Hypothetical Tremolo Wheel" in his 
article about online DIY guitar stomp-box communities (MAKE Volume 15, page 82, "Stomp 
Box Basics: Tremolo and Fuzz') . 

Well, it's hypothetical no more. I took Charles' cue and built this Optical Tremolo Box, which 
reads a patterned disk with a light sensor to create a warbling volume effect (tremolo) that 
you can custom-program with any pattern you like. 

How does tremolo work? So there's your electric guitar, and there's the amplifier it's plugged 
into, and there's the cable that runs between them. Open up that cable and you'll find 2 wires 
— one "ground" and one held at a positive voltage relative to "ground." The changing 
electrical potential between these 2 wires, over time, is what carries the sound signal. 

What happens if you short-circuit those wires, bridging them with a third wire? The sound 
goes away. The charge can find its way home now, via the short, without ever bothering to 
go all the way through your amplifier. And so it does. 

What if you bridge the 2 wires with a resistor instead? With a strong resistor, nothing 
happens — it's still easier for the charge to go through the amp. With a really weak resistor, 

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Optical Tremolo Box 

the sound cuts out. With a resistor in the middle range, the sound will be quieted, but not 
completely muted, as the charge divides itself between short and signal pathways. Use a 
variable resistor, and you have a crude volume control: turn the resistance way up, the 
sound will be loud; turn it way down, the sound will vanish. 

And here's where Charles had a clever idea: use a resistor that responds to light. Wave your 
hand in front of the photoresistor, and the volume will respond to the shadow of your hand. 
Mount a spinning disk with alternating clear and opaque bands in front of it, and the volume 
will follow the pattern on the disk, repeating as it spins. That's tremolo — a repeating 
variation in volume over time. 

Intrigued? Want to try building one? I thought you might. Let's get started. 

Step 1 — Drill the enclosure. 

• Download the drilling template (PDF) and print it out onto a full page adhesive-backed 
mailing label. 

• Cut out the 4 template sections, peel off the backing, and affix them to the front, top, and 
sides of the enclosure box. 

• Drill the holes where marked. Start each hole with a 1/8" brad-point bit, then switch to a 
unibit to drill the bigger holes out to finish diameter. 

• Peel off the templates and discard. A bit of rubbing alcohol can help if they don't come 
away cleanly. 

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Optical Tremolo Box 

Step 2 — Mount the rheostat. 

• This knob controls the motor speed. First, bend all 3 contacts on the rheostat down 90° so 
they're parallel to the shaft. Then turn the rheostat all the way "down" (counterclockwise). 

• We add a 15Q resistor, in series, to bias the range toward the slower speeds, which are 
more useful for tremolo effects. Solder one lead of the resistor to the rheostat center 
contact and the other to a 3" length of wire. 

• Use your multimeter to check the resistance between the center wire and each of the other 
2 rheostat contacts. One of them should read about 150 and the other about 400. Solder a 
4" length of wire to the 400 contact. 

• Cover the resistor and all soldered connections with heat-shrink tubing. Shrink it in place 
with a candle or cigarette lighter flame. 

• Remove the nut and washer from the rheostat, and insert it through the hole in the 
enclosure from inside. Fit the indexing tab into the alignment hole, put the washer over the 
shaft, and gently but firmly tighten the nut with a 12mm wrench. 

• Fit the knob onto the shaft and secure it in place by tightening the setscrew with a small 
flat-blade driver. 

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Optical Tremolo Box 

Step 3 — Mount the audio pot. 

• This knob controls the intensity of the tremolo effect. First, cut the shaft to the same length 
as the rheostat using a hacksaw. Then turn the pot all the way "down" (counterclockwise) 
and file a small flat surface on the side directly opposite the indexing tab. This will ensure 
that the the shaft always turns with the knob. 

• Again, identify the pot's "high" contact using the resistance setting on your multimeter. 
Solder a 4" length of wire to this contact. 

• Now, bend the remaining two contacts towards each other until they overlap, then solder 
them, together, to a second 4" length of wire. 

• Mount the pot in the enclosure, with its indexing tab in the alignment hole. Secure the shaft 
in place with its bundled flat washer and nut. This one takes an 1 1 mm wrench. 

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Optical Tremolo Box 

Step 4 — Add the photoresistor. 


• These cadmium sulfide (CdS) photoresistors come in packs of 5 with big, medium, 
and small sizes. We'll use one of the medium-sized resistors and an LED holder to 
mount it. 

• Remove the rubber insert from the LED holder. Guide the legs of the photoresistor through 
the holes in the insert until the resistor body is seated against the insert. 

• Cut two 4" lengths of green wire. Strip about 3/4" of the insulation off each end, twist the 
copper strands together tightly, and tin one end. Solder the tinned ends to the photoresistor 

• Slip a piece of 2mm heat-shrink tubing over each wire so it completely covers the solder 
joints and exposed photoresistor leads. Shrink it in place. 

• Push the insert back into the LED holder until the photoresistor is seated in the opening, as 
shown. Put the LED holder through the enclosure panel from the front, and tighten down 
the hex nut and split washer from behind using a 10mm wrench. 

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Optical Tremolo Box 

Step 5 — Mount the motor. 

• The hub we're using to mount the sweep disks has a 3mm arbor hole, but our motor's shaft 
is only 2mm across. A short section of ink tube from a ballpoint pen makes a perfect 
adapter. Just push the shaft of the motor into the tube and cut off the excess with a sharp 
hobby knife. 

• Solder a 3" length of wire to each of the motor's contacts. 

• Pass the motor shaft through the hole in the enclosure from the inside, then secure the 
motor in place with a #4x1/4" wood screw in each of the 2 mounting holes. 

• Don't overtighten the screws, or you risk stripping out the metal in the motor 


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Optical Tremolo Box 

Step 6 — Mount the switch. 

• Strip about 1" of insulation from one end of each of six 4" wires. Though the colors don't 
really matter, it's helpful to have 3 wires in each of 2 colors. I used red and black. 

• Tin the stripped end of each wire and wrap it around one of the 6 screw terminals on the 
switch. Give the wire a twist or two, tighten down the screw, and insulate the connection 
with heat-shrink tubing. One row of 3 contacts on the switch should be all "red" and the 
other 3 all "black," as shown. 

• To keep the switch from rotating in its mounting hole, apply 2 small pads of double-stick 
foam tape to the top — one on each side of the handle. 

• Peel the backing off the tape and insert the switch into the mounting hole from inside the 
enclosure. Align it as shown, then press the tape into place. Slip the label plate over the 
threaded shaft and then add the panel nut. Tighten gently with a 14mm wrench. 

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Optical Tremolo Box 

Step 7 — Add the power and phono jacks. 

• Attach 4" leads to the DC power jack and each of the 2 phono jacks. In each case, ground 
(black) goes to the outer or "case" contact. Solder and insulate the connections with heat- 
shrink tubing. 

• Insert the DC power jack through the mounting hole in the top panel, from outside the 
enclosure, and secure it inside with the bundled washer and panel nut. Tighten gently with 
a 14mm wrench. 

• Insert one of the phono jacks through one of the mounting holes in the side panel, from 
inside the enclosure, and secure outside with the bundled washer and panel nut. Repeat 
for the other jack. Tighten gently with a 12mm wrench. 

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Optical Tremolo Box 

Step 8 — Install the Flex-Light. 

• Remove the D-ring from the end of the Micro Flex-Light by using a pair of needle-nose 
pliers "in reverse": close the jaws, insert the nose into the ring, and then gently pull the 
handles apart to pry the ring open. Discard the ring or save it for another project. 

• Slide the lapel clip fitting off the end of the lamp body. Discard the clip or save it for 
another project. 

• Fit the rubber grommet into the center-top hole in the front panel. It's easy; just squish it in 
until the lip of the hole is cleanly engaged with the groove in the grommet. A small flat- 
blade screwdriver may help with lifting and pushing the rubber, here and there, to get it just 

• Insert the bottom of the Micro Flex-Light into the grommet from above and push it into the 
enclosure. This will take a bit of force, but not much, and will hold the Flex-Light securely 
in place. You can adjust it up and down, or left and right, or take it out completely, for 
instance to replace the batteries. 

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Optical Tremolo Box 

Step 9 — Wiring and assembly. 

• Wire up the circuits using an 8-position terminal strip, as shown, cutting the leads to length 
and stripping the ends as you go. These "European" style terminal strips are nice because 
all you have to do to make a connection is insert the stripped wire end and tighten the 

• The circuit diagram is also available for download as a printable PDF . 

• For ease of access and assembly, the terminal strip is not mounted to the 
enclosure, and just hangs freely. By the time all 16 connections are made, it is 
quite secure and will not wobble around. 

• Put the plastic bottom of the enclosure in place and secure it with the 4 bundled screws. 

• Attach adhesive feet. TIP: Three feet are less likely to wobble, on an uneven 
surface, than 4. 


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Optical Tremolo Box 

Step 10 — Power it up. 

• To run the motor, 3V DC is 
supplied through the power jack, 
which takes a size M plug. You can 
use a 3V "wall wart" to run the unit 
from mains power. 

• You can also build a simple battery 
pack with a ready-made 2xAA 
battery holder, a size M coaxial 
plug, and a bit of heat-shrink 
tubing. Attach it to the outside of 
the enclosure with peel-and-stick 
hook-and-loop fasteners (aka 

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Optical Tremolo Box 

Step 11 — Make the sweep disks. 


• Download the sweep disk art (PDF) and print it onto an 8.5"x1 1" transparency. If you want 
to design your own disk art, an editable SVG version of this file is available at . 

• TIP: If you're using a laser printer, you may notice that the printed transparency 
acquires a slight curl from the heat. To flatten it, simply flip the printed film over and 
"print" a blank page on the opposite side. 

• Cut out each disk using sharp scissors. Then apply a 5/8" diameter velcro "dot" to the 
center of each disk. Use the softer, "loop" velcro on the sweep disks. 

• Apply a matching "hook" dot to the top of the aluminum mounting hub. 

• Slip the hub over the motor shaft. Make sure the hub is up enough along the shaft to clear 
the motor mounting screws when it rotates. Tighten the setscrew, using the Allen wrench 
that came with the hub, to secure it to the motor shaft. 

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Optical Tremolo Box 

Step 12 — Rock your Optical Tremolo Box. 

• Pick your favorite sweep disk and attach it to the hub by joining the velcro dots. 

• Plug your instrument or other sound source into one of the 2 phono jacks, and your 
amplifier or other sound receiver into the other. The tremolo circuit is symmetrical, so it 
doesn't matter which plug goes into which jack. 

• Plug in the battery pack or wall wart. Turn on the LED Flex-Light, then flip the switch in 
either direction. One will cause the disk to turn clockwise, and the other will cause it to 
turn counterclockwise, which may vary the effect depending on the pattern on your sweep 

• The knob on the right, above the switch, controls the motor rotation speed. At low 
positions, the motor may have a hard time getting started. Try turning the speed up a bit 
and then adjusting it back down to get the lower speeds. 

• The knob on the left controls the intensity of the tremolo effect. Turn it clockwise for more 
tremolo, counterclockwise for less. The brightness of the light on the photoresistor also 
affects the tremolo intensity, and though the high-intensity LED in the Micro Flex-Light 
provides plenty, you should experiment with other light sources. Especially the sun! 

We've shown you how to make your own Optical Tremolo Box. Now show us how to make it 

Here are a couple of ideas: 

1) Printable transparencies are a super handy way to make the sweep disks, and they work well 

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Optical Tremolo Box 

enough, but if you experiment a bit, you will probably find that the black areas on a printed 
transparency still let a bit of light through from the LED. Try covering the black parts of one of 
the disks with small pieces of electrical tape, which is truly opaque, and you'll be able to hear the 
difference: the tremolo effect gets stronger. What might be a better way to make the disks? 

2) Wiring a potentiometer in series with a DC motor is a simple, cheap, and easy way to control 
the motor's speed, but it is not ideal. Especially at the low speeds that produce better tremolo 
effects, a series potentiometer is inefficient. We can get away with it because so little power is 
required to spin the disk, but the circuit could certainly be improved. What might be a better way 
to control the motor's speed? 

We look forward to seeing what mods you come up with. 

This document was last generated on 201 2-1 1 -30 09:07:1 9 AM. 

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