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Optical Tremolo Box

Plug your guitar (or axe of choice) into light-programmable waveforms with this unique effects box!

  • By Sean Michael Ragan
  • Category: Art & Design, Music
  • Time Required: 5 hours
  • Difficulty: Moderate
Optical Tremolo Box

MAKE contributing editor Charles Platt 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, 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.

MAKE Volume 33 features our special Software for Makers section covering apps for circuit board design, 3D design and printing, microcontrollers, and programming for kids. Also, meet our new Arduino-powered Rovera robot and get started with Raspberry Pi. As usual, you’ll also find fascinating makers inside, like the maniacs on our cover, the hackers behind the popular Power Racing Series events at Maker Faire.

Try your hand at 22 great DIY projects, like the Optical Tremolo guitar effects box, "Panjolele" cake-pan ukelele, Wii Nunchuk Mouse, CNC joinery tricks, treat-dispensing cat scratching post, laser-cut flexing wooden books, sake brewing, growing incredibly hot “ghost chili” peppers, and much more.

On newsstands now, by subscription, or available in the Maker Shed

Buy now!

  • 1/8" drill bit
  • Computer and printer
  • Drill
  • File
  • Hacksaw
  • Lighter
  • Multimeter
  • Pliers
  • Scissors
  • Screwdriver
  • Soldering iron
  • Unibit
  • Wire cutter/stripper
  • Wrench
  • X-Acto knife

Steps

Step #1: Drill the enclosure.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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.

Step #2: Mount the rheostat.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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 15Ω 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 15Ω and the other about 40Ω. Solder a 4" length of wire to the 40Ω 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.

Step #3: Mount the audio pot.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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 11mm wrench.

Step #4: Add the photoresistor.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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 leads.
  • 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.

Step #5: Mount the motor.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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 #4×1/4" wood screw in each of the 2 mounting holes.
  • Don't overtighten the screws, or you risk stripping out the metal in the motor casing.

Step #6: Mount the switch.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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.

Step #7: Add the power and phono jacks.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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.

Step #8: Install the Flex-Light.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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 right.
  • 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.

Step #9: Wiring and assembly.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • 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 screw.
  • 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.

Step #10: Power it up.

NextOptical Tremolo Box
  • 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 2×AA 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 velcro).

Step #11: Make the sweep disks.

NextOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo BoxOptical Tremolo Box
  • Download the sweep disk art (PDF) and print it onto an 8.5"×11" transparency. If you want to design your own disk art, an editable SVG version of this file is available at makezine.com/go/tremolodisks.
  • 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.

Step #12: Rock your Optical Tremolo Box.

Optical Tremolo BoxOptical Tremolo BoxOptical 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 disk.
  • 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!

Conclusion

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

Here are a couple of ideas:

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

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18 Responses to Optical Tremolo Box

  1. ardian on said:

    Nice idea, but then who’s gonna play the guitar?

    Reply ↓
  2. Sean Michael Ragan on said:

    You’re absolutely right, Nick, but I can at least tell you why it’s not. Basically, I wimped out of including a pulse-width modulation (PWM) motor speed controller in this design and opted instead for a simple series rheostat in-line with the motor. You can only get away with regulating DC motor speed this way when the motor is very low-powered. If you were to do the same thing with a 9V motor the rheostat would get much too hot. Version 2 of the box will definitely include a proper PWM motor speed control circuit; here’s a simple one based on a 555 timer, for instance:

    http://www.dprg.org/tutorials/2005-11a/index.html

    Another nice feature of using a 9V motor would be that it’s easy to find them with mounting holes in the case that are actually threaded, so you could use machine screws instead of wood screws to mount one in the project box.

    Reply ↓
  3. Scott on said:

    David, i bet radio shack would have most of these items, if not all.

    scott

    Reply ↓
  4. Todd Birdsong on said:

    The terminal strip can get kinda sketchy when you start wiring things up. If you think you didn’t keep good track of what wire went to what terminal, you may have misconnected something. Check the switch wires first, then the motor wires.

    Reply ↓
  5. Todd Birdsong on said:

    I spent around $75 for eveything but solder and the iron.

    Reply ↓
  6. Todd Birdsong on said:

    I cut out six 3/16″ plexi discs today and mounted one to the motor shaft. The extra weight really makes a difference in terms of motor speed and centripetal force. If the disc is not placed exactly in the center of the shaft, the off-center rotation causes the entire device to vibrate and “walk” at its top speed. I might have to use something thinner, but with some rigidness for durability. (although it might be simpler to use a small case to protect the transperancy discs) Just another experiment…

    Reply ↓
  7. Todd Birdsong on said:

    Ok. I figured out my problem with the noise. I was using adhesive label material for the disc patterns and sticking them to 3/16 plexi. The adhesive material was too opaque for the LED to completely penetrate, causing the photoresistor to register the level of noise. I’ll stick with the transparency film for a clean sound.

    Reply ↓
  8. Allan Rosner on said:

    Did you figure out the wiring on your unit because i’m having the same problem. what did you do?
    Allan

    Reply ↓
  9. Sean Michael Ragan on said:

    Apologies, this was supposed to have been linked from the parts list, but it looks that link isn’t working correctly. The hub is from Pololu:

    http://www.pololu.com/catalog/product/1079

    Reply ↓
  10. Sean Michael Ragan on said:

    Yes, it looks like RS stopped carrying this LED flashlight shortly after this project published. It is available from Amazon, however, in both the silver color used in this build

    And a black model that I wish I’d known about first!

    Reply ↓
  11. Sean Michael Ragan on said:

    It worked for me just now. If you still have problems, e-mail me at sean@makezine.com and I can e-mail you a copy.

    Reply ↓
  12. Allan Rosner on said:

    Hi Dan did you ever got the motor to work? Im having to same problem. If so let me know
    Thanks Allan a_rosner@msn.com

    Reply ↓
    • Gavin on said:

      yeah i had the same problem too but i realized that somethimes the moter terminals get messed up and wont work if not in a certian position. try using tape to hold the terminal in the right position

      Reply ↓
  13. Sean Michael Ragan on said:

    Hey Allan,

    I know that you and some other folks are waiting on a schematic. It’s coming, I promise, and I appreciate your patience. We are in the process of getting our next special issue of MAKE wrapped up and sent off to the printers and I have just no time at the moment. Bear with me. It’s coming.

    In the meantime, you or anyone else who is having problems should feel free to e-mail me directly at sean@makezine.com. Pictures of your build will be helpful if you can provide them.

    Reply ↓
  14. Sean Michael Ragan on said:

    Yes, you can use a modded pot. The only important thing is that it be on the order of 25-50 Ohms max resistance.

    Reply ↓
  15. Sean Michael Ragan on said:

    Verify that the plastic disk is not scraping against the metal phototransistor holder when it rotates. I have observed that phenomenon in my prototype, and think it is due to static charge accumulating when the plastic rubs against the metal. The charge builds up, and periodically discharges into the audio line, resulting in the click. Let me know if that does not fix it.

    Reply ↓
  16. Sean Michael Ragan on said:

    No. But it would definitely be better if it did.

    Reply ↓
  17. Havok on said:

    What Soldering iron are you using at the “Mount the rheostat” picture ?

    Reply ↓

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