Opto Duet

Two 555 theremins that play each other, on one stripboard · By Onion Madder · Mess O' Pedals

I was inspired to create this after building two of my 555 theremins and wiring their LDRs against each other to see what would happen. This layout combines two copies on a single board that easily fits inside a 125B enclosure, with room for a 9V battery if you prefer it. although the circuit wants at least 6v to run properly. Using a battery over time will leave you with a pile of half-dead cells. This build takes approximately 45 minutes.

The Opto Duet layout drawn as a stripboard diagram, titled Opto Duet, 23 x 9 stripboard. Nine orange copper strips lettered a to i run left to right, with twenty-three columns numbered above them. Two pale squares marked IC1 NE555 and IC2 NE555 sit one per half of the board. Gray X marks stand in vertical lines where the strips are cut, splitting the two halves apart and separating each chip's pin rows. Dark blue link wires stand vertically, one running diagonally under each chip. R1 and R3 1k and R2 and R4 100R are outlined rectangles, C2 and C5 10n and C4, C8 and C9 100n small outlined boxes, and C1, C3, C6 and C7 pink circles. A diode marked D1 lies along the top strip at the far left, across a cut. Holes carrying wires off the board are labeled with fanned leader lines: plus 12V, GND, LDR_T1, LDR_T2, SPK_B, LED_K and LED_A down the left, and VCC, LDR_V1, LDR_P1, LDR_V2, LDR_P2, LED2_K, LED2_A and SPK_A on the right. Beside the board the panel parts are drawn with a wire to every pad: a power toggle, a DC jack, the tempo eye, the speaker and the LED sealed to the cell on the left, the vactrol cell, the pitch eye and the indicator LED on the right. A parts list runs underneath, and a credit line reads Made with Copper Bottom. Below it the same board is drawn a second time from the solder side, mirrored, with the column numbers running right to left, every solder point as a dark dot, the cuts as X marks, each chip as a dashed outline with pin 1 boxed and the off-board pads labeled on the margin they now sit nearest. — The board as the layout file draws it. The cuts down the middle of each chip and the line down column 12 are what hold the two halves apart. D1, top left, is the reverse-polarity diode. The lower drawing is the solder side, mirrored - the side you actually work on.

The finished stripboard held over the open enclosure, solder side toward the camera: rows of holes joined by bright blobs of solder along nine copper strips, with the column numbers printed down the green edge of the board. Below it the open purple enclosure already has its toggle switch and quarter-inch jack mounted, with orange, black, blue, yellow and white wires looping out of them. — The same board, solder side up, over the box it is about to go into. The power switch and the output jack are already mounted. There is no DC jack on this one - it runs on the 9V battery that goes in on top of the board.

It is built and it works. The first attempt died at 90% when a copper strip lifted off a cheap board mid-solder. The second went together on the same cheap board with the iron turned down, and it came up on the first power-up: it sings from a center-negative 12V DC wall wart. The second build died when I accidentally plugged it into a center-positive supply, which is why I added the rectifier diode, D1 on the board now. The third build is up and running: third time really is the charm.

The Opto Duet v2 board on a dark workbench mat, not yet boxed. A narrow strip of brown stripboard at the lower left carries two black eight-pin chips, small blue resistors, yellow ceramic capacitors, black electrolytic cans and a small green LED, with a bundle of yellow, orange, black and blue wires leaving its left end. Two potentiometers with bare knurled shafts sit above it on green and blue wires, and at the right a quarter-inch jack is wired on orange and black leads, with a purple cable plugged into it through a chrome right-angle plug. — The v2 board, built and working. The two pots are mine and are not on the published drawing, and this one plays through a jack where the drawing has a speaker.

The finished board on a dark desk mat, photographed from the solder side: a long narrow strip of copper-clad board, its rows of holes joined by bright blobs of solder, with flying leads in orange, black, blue, yellow and white leaving the top edge. A red mini toggle switch sits at the upper left on yellow and orange wires, with a violet sheen across its body, and next to it an ultraviolet LED burns so brightly it flares white in the photo. A teal heat-shrink sleeve swallows one pair of leads, and two small cream photocells stand on their own black wires. A large black round speaker lies to the right of the board. — Built and running. The solder side is up here; the chips are underneath. The red toggle is the power switch, off-board in the positive supply leg, so it is not in the layout file.

Hear it: boxed up and running into the Quacker, an envelope filter, rather than straight into the amp. This is the second one I built, in a glow-in-the-dark box, so it is not the purple enclosure in the photos further down. Copper Bottom is open on the layout behind it. Halfway through I pick the box up and turn the eyes toward the room light. 19 seconds, sound on. Yours will not sound like this - the photocells you happen to grab move it a long way, and that is expected.

What you'll need

tap to expand / collapse

2× NE555 timer, 8-pin DIP - spot it: NE555 printed on top, notch marks pin 1
1× stripboard, 9 strips × 23 holes or larger - a cheap board is fine, but turn your iron down.
2× 1K resistor (brown-black-red) - one per timing chain
2× 100R resistor (brown-black-brown) - the output resistors
1× 10µF electrolytic - the engine's timing cap, the part that turns a scream into a tick.
3× 100n ceramic (printed 104) - the voice's timing cap, and C8 and C9, one across the supply beside each 555
2× 10n ceramic (printed 103) - control-pin decoupling, one each
3× 100µF electrolytic - two output couplers, one across the rails
1× 1N5817 Schottky (D1) - spot it: black body, gray band. On the board, in the +12V lead, band toward the chips. Keeps your board from getting fried like mine

Off the board

3× photocell (LDR) - the wiggly-line disc. Same type as the theremin guides. Any will work, but which three you grab sets how the finished box sounds
2× LED, any color - both run bare, no resistor.
1× small speaker and a power supply - a 12V DC wall wart, center-negative. A 555 chip will run from 4.5V, but this box wants 6V and up: below that the LEDs stop flashing, because they light off the swing through the coupling caps. A fresh 9V battery runs it; a tired one takes the LEDs out first and looks exactly like a broken build.
Heat-shrink big enough to swallow the LED and one photocell face to face - that is the whole vactrol
1× - a toggle switch for the positive supply leg, so the box has an off.
1× DC jack, center-negative to match the wall wart

The layout file is the guide

Like the 555 theremin layout, the build document for this one is the file itself: ↓ Download Opto_Duet.json - import it into Copper Bottom and the editor gives you the board picture, the placement walkthrough, the electrical checks and a printable build sheet.

There is also a one-chip version on an NE556 - the same circuit redrawn, checked in Copper Bottom and not yet built.

The off-board wiring

Fourteen pads leave the board. Everything with a face goes where its job says:

Pad	Goes to
+12V / GND	The supply, one for both oscillators. Mine is a 12V DC wall wart, center-negative. The positive leg goes through an on/off toggle to the +12V pad; D1 on the board takes it from there. A 9V battery snap works too
VCC	Nothing. A test point on the protected side of D1, and it tells Copper Bottom's checker that strip is a supply
SPK_A / SPK_B	Speaker hot / return. Both boards mix at SPK_A through their own 100Rs - no extra mixing resistors, see below
LED2_A / LED2_K	The indicator. Same wiring as the first LED but on the voice's half - across the strip between C6 and R4, cathode to the ground rail - so it flashes with the voice. Nothing is sealed against this one; it is there to be looked at
LED_A / LED_K	The bare LED - anode to LED_A, cathode to LED_K. It seals face to face with the vactrol cell inside heat-shrink
LDR_V1 / LDR_P1 LDR_V2 / LDR_P2	Four pads, two nodes. LDR_V1 and LDR_P1 are the same strip; so are LDR_V2 and LDR_P2. Two photocells bridge those two nodes, which is what puts them in parallel: the vactrol cell, sealed in heat-shrink with the LED, and the pitch eye, facing out of the box for room light and your hands. Take any upper pad and any lower pad for each cell - on my board the vactrol sits on LDR_V1 and the pitch eye on LDR_V2
LDR_T1 / LDR_T2	The tempo eye, one cell across the engine's two timing strips. It faces out of the box too: room light and your hands set the tick rate

D1 is the whole of the reverse-polarity protection. It sits on the top strip across a cut, between the +12V pad and everything else, so a backwards plug meets a reverse-biased diode and nothing on the board sees it. C8 and C9 beside the two chips keep each 555's supply quiet. A Schottky is the part to use for D1 rather than a 1N400x: it drops about 0.3V instead of 0.7V, and this box already wants at least 6V.

That pad doubling is the parallel wiring, and parallel is load-bearing: the lit cell wins, so each engine flash yanks the pitch up and releases it. In series they would politely average and the chirp would die. It also means the pad names are a convenience, not an assignment - nothing electrical changes if your vactrol lands on the pads labeled for the pitch eye.

The three cells you grab will change how this thing sounds, and that is not a fault. A photocell is barely a spec: the common 5mm types look identical in the bin and run roughly ten to one apart in resistance, and in a 555 the cell is the timing resistor. So two boxes off the same pile genuinely are two different instruments. The tempo eye sets how fast the engine ticks. The vactrol cell and the pitch eye set how hard each flash yanks the voice - they are in parallel, so what matters is the ratio between them rather than either one alone. And a cell's recovery from light is slow and varies, which is roughly the difference between a click and a chirp. Mine click, warble or chirp from build to build for exactly this reason. Which of the three causes dominates is a read of the circuit, not something I have measured.
If yours sounds wrong rather than merely different, swap the two voice cells for each other before deciding one is bad - they are the same two nodes, so it costs nothing to try. Past that, a meter sorts a pile fast: read each cell in room light, under your thumb, and under a phone torch. Then uncover it and watch how long the number takes to climb back. That last reading is the click-versus-chirp one, and it is the one nobody ever takes.

The trick: the LED lives behind the cap

This is the discovery the whole box turns on, found by moving one bare LED strip by strip until the loop came alive. Put the LED on the engine's output strip and it just glows steady - twice over. Bare, it clamps the output at its own ~2V forward drop; and even resistored, a DC-coupled LED reports a level, and a feedback loop fed on a level finds a comfortable equilibrium and parks there.

Move it to the far side of the 100µF output cap - the strip between the coupling cap and the output resistor - and both problems vanish at once. The cap limits what a bare LED can draw, and a cap only conducts while the voltage moves, so on that strip the LED reports change. A loop fed on change has no resting point: if it ever sits still the LED starves, the photocell drifts, and the sitting-still is over. It has to hunt forever, at the tempo of the photocell's slow memory. That is why every classic opto-feedback synth AC-couples its lamp driver - and why this one was rediscovered at a bench with a lifted eyebrow instead of a datasheet.

If the LEDs fade, dim, or come back after a rest and then die again, look at your supply before you look at the board. They flash off the swing through the coupling cap, that swing tracks the rail, and the ultraviolet LEDs in mine want about 3.2 to 3.4V of it before they light at all - more than a red or a green would - while the 555s keep oscillating happily down to 4.5V. So the LEDs go dark first and the box carries on making noise, which reads as a wiring fault and is not one. Mine did exactly this on a dying 9V battery: rest it, it works for a minute, then fades. Back on the 12V wall wart it is fine.

Boxing it up

Mine lives in a 125B, horizontal: the two eyes and the indicator LED on the face, the power switch and a 1/4" output jack on the top edge. The two speaker pads feed that jack instead of a speaker - tip to SPK_A, sleeve to SPK_B - so the box plays through an amp.

Six holes, and none of them are fussy: 5 mm for each photocell and for the LED, 6.5 mm (1/4") for the toggle bushing, 9.5 mm (3/8") for the jack, and 8 mm if you want a barrel jack for the supply rather than passing the wall wart lead through a grommet. Wire that barrel jack center-negative. Keep every hole at least 12 mm in from an edge: the corner bosses and the lid lip live in there.

Check the plug before you power it, every single time. Pedal supplies are center-negative; most generic wall warts sold for everything else are center-positive, and they fit the same barrel jack perfectly. Backwards, the supply would go straight through both 555s and reverse-bias every electrolytic on the board. I swapped to a center-positive plug without looking and killed a working board with it, which is exactly what the 1N5817 in the positive leg is there to stop. With it in, a backwards plug does nothing at all - so if the box has gone silent right after you changed supplies, check the plug before you check your solder.

The board going into the enclosure with a strip of orange insulating tape stuck across its whole solder side and two blobs of white mounting putty pressed onto the tape. The purple box below already holds the jack and the switch, and a teal heat-shrink sleeve lies inside it. — Tape over the solder side, putty to hold it down. Bare copper against a metal box is a short waiting to happen.

Looking down into the open enclosure with everything inside: a yellow 9-volt battery lying across the top, a red toggle switch mounted through the left wall, the stripboard flat in the middle with its electrolytic capacitors standing up off it, the quarter-inch jack at the lower right, and orange, black, blue, yellow and white wires running between them. A teal heat-shrink sleeve sits along the right-hand wall. — Everything in. The teal heat-shrink on the right is the vactrol, the LED and its cell sealed face to face.

A red 9-volt alkaline battery on a black snap connector, with a blob of white putty stuck to its side, held above the open enclosure and about to be pressed down on top of the wired board. — A 9V fits on top with a blob of putty. Worth knowing: this is the supply that later sagged and took the LEDs out while the box kept playing.

The finished pedal held face up: a purple hammered-finish enclosure in landscape, with two photocells and a clear LED spaced in a row across the middle of the face, a silver toggle switch standing up from the top left edge and a quarter-inch jack at the top right. — Closed up. Tempo eye, indicator, pitch eye across the face; switch and output on the top edge, which is what the drill template draws.

Don't eyeball the holes. There are two 1:1 drilling templates, because there are two of these boxes and they do not stand the same way. Both are the enclosure drawn unfolded, so you wrap the sheet around it and every hole lands by folding instead of by measuring, with true-size circles and a punch dot in the middle.
↓ Horizontal 125B (PDF)
The box lying down, as in the photos above. Eyes and LED on the face; switch, DC and output on the top edge. Landscape sheet, because a horizontal 125B unfolds wider than a portrait page can print. The DC hole is dashed because it is optional.
↓ Vertical 125B (PDF)
The box stood up, as in the demo video, with everything on the face - I/O along the bottom, controls above. Portrait sheet, for the same reason in reverse. The face is only 42 mm wide once the lid lip is clear, so the three eyes share one row.
↓ Vertical 125B, jacks on top (PDF)
The same upright box with the DC and the output moved to the top wall, so the cables leave upward; the eyes, LED and switch stay where the sheet above puts them. Two ways to drill one box; pick one.
Print at 100% / “Actual size”, never “Fit to page” - check the bar measures exactly 4″ before you touch a drill.

Things to try

Retune the engine. The 10µF sets the tick rate - 4.7µF doubles the tempo, 22µF halves it. Same move as retuning a Hive voice.
Retune the voice. Its 100n is the stock theremin value; the breadboard guide's cap-swap logic applies unchanged.
If an LED ever dies young on the coupling node, ~100R inline should tame the edge pulses without killing the effect - untested, call it an experiment.
Aim the eyes. Both outside cells respond to angle as much as light - mounting them as a creature's eyes is not just decoration, it is the interface.
Add a jack. Line the bus out to an amp - and then bring the Hive's 10Ks back, because at that point they are correct again.

The 555 astable core is the same public circuit as my 555 Optical Theremin guides. The opto-coupled engine-and-voice arrangement - the LED behind the cap, the vactrol-plus-eye parallel pair, the chirp - is my own, found at the bench in August 2026. The layout, the file, and the words are mine. Build it, bend it, make it yours.