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Opto Wobble

The Opto Duet with an LFO on its pin 5s, a CV input and a power starve · By Onion Madder · Mess O' Pedals

The Opto Duet is two 555s coupled by nothing but light: a slow engine ticks, its LED flashes, and the flash bends the voice. This puts my Dual-Wave LFO on the same board and sends its output into pin 5 of either 555, through an on-off-on toggle and nothing else. Pin 5 is where a 555 keeps its reference voltage, so the LFO wobbles the engine's tempo or the voice's pitch, whichever the toggle picks, and the LFO's own rate and blend knobs shape it. A CV jack goes into the engine's pin 5 as well, through 47K, and a 2K pot in the supply starves the whole board. In this box the Duet's two photocell eyes are knobs, tempo and pitch, and the vactrol is not fitted.

The Opto Wobble layout drawn as a stripboard diagram, titled Opto Wobble, 23 by 18 stripboard. Eighteen orange copper strips lettered a to r run left to right with twenty-three columns numbered above them. The top nine strips hold the Opto Duet: two pale rectangles marked IC1 NE555 and IC2 NE555 side by side, a diode D1 on the top strip, resistors R1 to R4, pink round electrolytics and small green ceramic capacitors, with dark blue links, one running diagonally under IC1. Strip j is nearly empty, cut once, with a pad marked CV at its left end and a resistor R10 standing from it up to IC1's pin 5 strip. The bottom eight strips hold the Dual-Wave LFO: a pale rectangle marked IC3 NE5532, resistors R6 to R9, capacitors C10 to C12 and more links. Three long links carry ground and the supply between the two halves. Pads are labeled around the edges: plus 12V, GND, LDR_T1, LDR_T2, P5_1, SPK_B, LED_K, LED_A, CV and VR2_3 on the left, VCC, LDR_V1, LDR_P1, LDR_V2, LDR_P2, P5_2, LED2_K, LED2_A, SPK_A, VR2_1, RATE_LED, VR1_3 and VR1_W on the right. Beside the board the panel parts are drawn with a wire to every pad: a power toggle, a 2K starve pot between the toggle and the plus 12V pad, wired on its wiper and third lug, a DC jack, a quarter-inch line out and a 3.5 mm CV in jack, a red power LED with an off-board 4K7 resistor, pots marked 25K and 100K for tempo and pitch, a rate pot, a blend pot, and an on-off-on toggle wired from the blend pot's wiper to the P5_1 and P5_2 pads. 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 and the three chips as dashed outlines with pin 1 boxed. A parts list follows, then a block headed OFF THE BOARD listing each panel part and where each of its legs goes.
The board as the layout file draws it. The Opto Duet is on strips a to i and the Dual-Wave LFO on k to r, each exactly as its own page has it. Strip j between them carries the CV input and a bridge for the supply. The lower drawing is the solder side, mirrored - the side you actually work on.

What you'll need

tap to expand / collapse

Everything from both boards, plus one resistor, the wobble toggle and a starve pot. The part numbers below are this board's; the LFO's parts are renumbered from its own page so nothing collides.

  • 2× NE555 timer, 8-pin DIP (IC1, IC2) - spot it: NE555 printed on top, notch marks pin 1
  • 1× NE5532 dual op-amp, 8-pin DIP (IC3) - the LFO. Spot it: 5532 in the part number, a notch at one end
  • 1× 1N5817 Schottky (D1) - spot it: black body, gray band. Reverse-polarity protection, and it protects the LFO too: it runs from the same rail
  • 2× 47K (yellow-violet-orange) - R6, the LFO's rate floor, and R10, which carries the CV input onto IC1's pin 5
  • 1× 100K (brown-black-yellow) (R9) - the LFO comparator's feedback. It stands on end, across two holes
  • 2× 10K (brown-black-orange) (R7, R8) - the LFO's half-rail divider
  • 2× 1K (brown-black-red) (R1, R3) - one per 555 timing chain
  • 2× 100R (brown-black-brown) (R2, R4) - the output resistors
  • 1× 1 µF film (C10) - the LFO's timing cap. Film, not ceramic
  • 1× 10 µF electrolytic (C1) - the engine's timing cap
  • 1× 47 µF electrolytic (C11) - holds the LFO's half rail still
  • 3× 100 µF electrolytic (C3, C6, C7) - two output couplers and one across the rails. Spot it: the stripe marks the negative leg
  • 4× 100n ceramic (104) (C4, C8, C9, C12) - the voice's timing cap and a decoupling cap beside each chip
  • 2× 10n ceramic (103) (C2, C5) - the pin 5 caps, one per 555. They stay: the LFO and the CV go in beside them
  • Stripboard, 18 strips × 23 holes or larger, and wire for seventeen links. J15, J16 and J17 are the ones that join the two halves: J15 the grounds, J16 and J17 the supply, by way of strip j

Off the board

  • 1× 25K pot - TEMPO, where the Duet has its tempo eye. Wired as a variable resistor, lug 1 and wiper; lug 3 stays empty
  • 1× 100K pot - PITCH, where the Duet has its pitch eye. Lug 1 and wiper; lug 3 stays empty
  • No vactrol. The Opto Duet's LED and photocell are not in this build, so LED_A, LED_K, LDR_V1 and LDR_P2 stay empty
  • 1× 100K linear pot (B100K) - the LFO rate. Wiper and lug 3; lug 1 stays empty
  • 1× 1K linear pot (B1K) - the LFO blend, all three lugs. Its wiper is what feeds the wobble: triangle at one end of the knob, square at the other
  • 1× on-off-on toggle - LFO: sends the wobble to the engine, nowhere, or the voice. Its common is the blend wiper and its two throws are the two pin 5 pads, with nothing in series
  • 1× 2K pot - STARVE, in the supply after the power toggle. 12V on the wiper, lug 3 to the board, lug 1 empty
  • 1× 1/4″ jack - the line out, in place of the speaker
  • 1× 3.5 mm jack - CV in
  • 1× LED and a 4K7 resistor (yellow-violet-red) - the power light, from VCC to ground
  • 1× DC jack, 12V, center-negative, and an SPST toggle for power in the positive lead
  • An upright 125B - the drill templates are in the rail

The layout file is the guide

Like every layout on this shelf, the build document for this one is the file itself: ↓ Download Opto_Wobble.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, all from the same file. No coordinates are written out on this page on purpose: the editor is the viewer, and a number typed twice is a number that can drift.

Two boards, hole for hole. The file was built from the two shipped layout files, not redrawn, and it only exists because every net of both came through intact - the generator refuses to write it otherwise. The editor's four warnings on it are the Opto Duet's own: C1 is a squeeze beside IC1, and R2 and R4 stand on end.

The netlist

Quoted verbatim from the layout file, and emitted from the copper rather than typed beside it. The lines worth finding are IC1_CTRL and IC2_CTRL, the two pin 5 strips, and TRIANGLE and SQUARE, the two ends of the blend knob that feeds the wobble.

The off-board wiring

PadGoes to
+12V / GNDThe supply, 12V center-negative, through the power toggle, then the STARVE pot - the toggle onto its wiper, lug 3 to +12V, lug 1 empty - and D1 on the board takes it from there. The LFO runs from the same rail, so the starve drags everything down together. GND also takes the CV jack's sleeve and the power LED's cathode
VCCThe 4K7, then the power LED's anode
LDR_T1 / LDR_T2TEMPO, the 25K pot, lug 1 and wiper, where the tempo eye would go
LDR_P1 / LDR_V2PITCH, the 100K pot, lug 1 and wiper, where the pitch eye would go
LDR_V1 / LDR_P2Nothing. On the Opto Duet these take the vactrol's photocell, in parallel with PITCH; this build has no photocell. LDR_V1 and LDR_P1 are one strip and LDR_V2 and LDR_P2 the other, so PITCH already has both strips
LED_A / LED_KNothing, for now. On the Opto Duet this is the LED that flashes with the engine, sealed to that photocell
SPK_A / SPK_BThe line out: tip to SPK_A, sleeve to SPK_B
VR2_1 / VR2_3The blend pot, lug 1 and lug 3. Its wiper goes to the common of the on-off-on toggle
P5_1 / P5_2The wobble. One throw of the toggle lands on P5_1, IC1's pin 5, and the other on P5_2, IC2's pin 5. Center is off. Nothing sits between the blend wiper and the pin
CVThe CV in jack's tip. R10, 47K, is on the board
VR1_3 / VR1_WThe rate pot, lug 3 and wiper
LED2_A / LED2_K / RATE_LEDNothing in this box. They are the Opto Duet's indicator and the LFO's rate LED, left on the board for anyone who wants them back

Built, and heard

Built on one board, exactly as the file draws it, and it works. Building it is where the wiring got simpler: the LFO goes through the toggle straight into pin 5, the dual pot that the two-board version had is gone, the vactrol is gone - the photocell and, for now, its LED - and there is a 2K starve pot in the supply. The starve sounds glitchy and cool.

The Opto Wobble board held in one hand over a workbench: a single piece of orange stripboard with column numbers printed along its top edge and row letters down its right, two black 8-pin chips near the top, one at the left and one at the right half hidden under wires, and a third at the bottom center. Blue and yellow wire links arc over the board, with three black and two blue electrolytic capacitors, yellow ceramic capacitors, a small black diode at the top left and blue-banded resistors between them. Bundles of blue, yellow, green, orange, black, red and white wires leave the board's edges to a row of pots and jacks lying on the bench behind it.
Mine: the one-board build, held over the bench with its pots and jacks behind it, wired the way this page describes.
Hear it: the earlier two-board box, standing up with the lid on, turned by hand. 8 seconds, sound on.

How it works, in one breath

A 555 decides when its timing cap is full by comparing it against a reference, and pin 5 is that reference. It normally sits at two thirds of the supply on an internal divider, with a 10n cap to hold it steady. Push pin 5 up and the cap has further to charge, so the chip slows down; pull it down and it speeds up. So the LFO on pin 5 of the engine wobbles the tempo of the flashes, and on pin 5 of the voice it wobbles the pitch. There is nothing between the blend wiper and the pin, so the LFO does not nudge the reference, it sets it: how far the wobble goes is the blend knob's swing, and how fast is the rate knob.

The starve pot is a resistor in series with the supply, so winding it up drops the voltage every chip on the board sees at once. What that does to the sound is in Built, and heard.

The CV input is the same idea from outside the box. R10, 47K, keeps whatever you plug in from overpowering pin 5, so a CV bends the engine's tempo rather than grabbing it. The LFO itself is the Dual-Wave LFO exactly as its own page describes it, running from the Duet's protected rail.

Things to try

None of these are on my board. The first one was, on the two-board version; the rest are the obvious knobs.

  • A depth knob. A dual 50K pot, one gang between each throw of the toggle and its pin 5 pad, wired as a variable resistor, is how the two-board version was wired: turned up it is a gentle sway, at the bottom it is this page. A 10K to 22K resistor in series with each gang would keep it a sway all the way down.
  • The eyes back. Photocells on the LDR_T and LDR_P pads instead of the TEMPO and PITCH pots, the Opto Duet's way, and the room light plays it again.
  • The vactrol back. An LED on LED_A and LED_K, bare, and a photocell on LDR_V1 and LDR_P2 sealed to it, and the engine's flash bends the voice the way it does on the Opto Duet.
  • CV into the voice. A second 47K from a second jack onto P5_2 does for the pitch what R10 does for the tempo.
  • Clock it from the Spider Sequencer into the CV input, and each step moves the engine.

Two of my own layouts on one board: the Opto Duet, whose engine-and-voice arrangement is mine, and the Dual-Wave LFO, whose triangle-and-square pair came from the Experimentalists Anonymous archive. The wobble wiring, the layout and the words are Onion Madder's, drawn in Copper Bottom.