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Dragonfly VCO

A CD4046 phase-locked loop used as an oscillator, drawn in Copper Bottom and built · By Onion Madder · Mess O' Pedals

The Dead-Bug Dragonfly on copper. A CD4046 is a phase-locked loop - a chip built to make one signal follow another - and almost every synth build ignores the following half and uses it as a plain, very well-behaved voltage-controlled oscillator. This one keeps both. There is a pitch knob for playing it, and there is an input and a mode switch for letting the loop chase whatever you feed it.

The Dragonfly VCO layout drawn as a stripboard diagram, titled Dragonfly VCO, 14 by 9 stripboard. Nine orange copper strips lettered a to i run left to right with fourteen columns numbered above them. A pale rectangle marked IC1 CD4046 sits left of center, its top end on strip a and its bottom end on strip h, with strip i running under it as the ground strip. Left of the chip, R5 stands down the left edge, a pink circle is the electrolytic C5, a small outlined box between strips f and g is the timing cap C1, and R1 stands up from strip i at the bottom left. Right of the chip, C2 stands between strips a and b, C4 lies along strip c over a cut, R3, R2 and R4 stand across the middle strips and C3 stands between strips h and i. Four dark blue link wires: two short ones and a longer one left of the chip, and a long one down the right side from strip b to strip i. Gray X marks show the cut strips, including a line of them down the middle of the chip. Holes carrying wires off the board are labeled in the margins: VOL W, VOL T, OUT T, GND, PITCH G, RANGE L, OUT S, VOL G and IN S down the left; V plus, PITCH V, PITCH W, RANGE W, IN T, MODE 1 and MODE 2 down the right. Around the board the panel parts are drawn with a colored wire to each hole: three pots, two toggle switches, two quarter-inch jacks and a DC jack. A parts list and an off-the-board table run underneath and a credit line reads Made with Copper Bottom. Below the front view 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, the 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. Every part, link, cut and off-board pad in one picture. The file itself is below. The lower drawing is the solder side, mirrored - the side you actually work on.
The 9 by 14 board wired up loose on a dark gray desk mat, not boxed. The small piece of stripboard sits at the right of center, its strips lettered B to I along one edge, with a black chip across the middle, a few beige and blue resistors, three yellow ceramic capacitors, a small electrolytic capacitor standing at the top edge and a green wire link. Blue, yellow and green wires run from the board to three pots with bare shafts: two marked B100K at the left, one above the other, and a third, brass-bodied, lying at the bottom. An orange and a black wire run from the top of the board up to a panel DC jack with a black power cable plugged in, and more black, yellow and blue wires leave the board to the right, out of the frame. There is no toggle switch in the picture.
Mine, built and working on this layout, still on flying leads: the three pots and the DC jack. The mode toggle and the power toggle are left off.

What you'll need

tap to expand / collapse
  • 1× CD4046 phase-locked loop, 16-pin DIP - spot it: CD4046 printed on top, notch marks pin 1
  • 1× 10n ceramic (printed 103) - the VCO's timing cap, and the thing that sets its range. Bigger means lower
  • 3× 100n ceramic (104) - supply decoupling and two in the control path
  • 1× 10 µF electrolytic - the output DC blocker
  • 2× 1K (brown-black-red), 1× 10K (brown-black-orange), 2× 100K (brown-black-yellow)
  • 1× stripboard, 9 strips × 14 holes or larger, and wire for four links

Off the board

  • 3× 100K pots, off-board - pitch, range and volume. Spot it: B100K is the linear one
  • 1× SPDT switch for the mode, and two jacks - one in, one out
  • 1× 9V supply. A 4046 runs from 3V to 18V
  • 1× 9V battery snap or a DC jack
  • On my box, not in the file: a 100K pot in the supply leg as a starve, a toggle for power, a UV LED across strips 13 and 14 of the built board (the PITCH_W and RANGE_L strips), and a second toggle for the octave-drop bend described further down. Two 3.5 mm jacks in place of quarter-inch, which is what the drill template is cut for
  • An SPST toggle for power, in the positive supply lead between the DC jack or battery and the board's supply pad. Not in the layout file - it is off the board, and every build on this shelf gets one

The layout file is the guide

Like the 555 theremin and the Opto Duet, the build document for this one is the file itself: ↓ Download Dragonfly_VCO.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.

The netlist

Quoted verbatim from the layout file. It was written from the board, so checking the board against it can only pass - what it is for is carrying the circuit somewhere else, and telling you what each strip is supposed to be doing while you stare at it.

The off-board wiring

Sixteen pads leave the board.

PadGoes to
V+ / GNDThe supply. PITCH_V and PITCH_G are the same two rails brought out again so the pitch pot has its own ends to sit across without a second wire back to the board
PITCH_V / PITCH_W / PITCH_GThe pitch pot, all three lugs - a divider this time, not a variable resistor, because pin 9 wants a control voltage rather than a resistance. Wiper to PITCH_W
RANGE_W / RANGE_LThe range pot, wiper and one lug. This one sets how far the pitch knob can sweep - the 4046 calls it R1
MODE_1 / MODE_2The mode switch. MODE_2 is the VCO's control-voltage node; MODE_1 comes off the phase comparator. Join them and the chip starts trying to lock onto whatever is at the input instead of sitting where the knob put it
IN_T / IN_SInput jack, tip and sleeve - what the loop chases in the second mode
VOL_T / VOL_W / VOL_GVolume pot, all three lugs: a divider, so it can reach silence
OUT_T / OUT_SOutput jack, tip and sleeve

Built, and heard

Built on the bench in September 2026, straight off the first drawing of this board - 17 strips by 20 holes, kept here as Dragonfly_VCO_v1.json - and it works. The 9 by 14 file above is the same circuit on less copper, every net checked against the built board, and I have built that one too, in October 2026: it sounds exactly like it is supposed to. I left the mode toggle and the power toggle off that board, so on it the mode switch and the input's chase are untried. The first board also sounds incredible - that is the word I used the moment it came up, and I am not taking it back. The 4046 tracks the way the dead-bug version promised it would, and with something on the input jack and the mode switch thrown it goes hunting the way the paragraph below says. Not boxed yet; the drill template further down is still a proposal.

The finished board on a dark desk, wired but not boxed. A small rectangle of orange-brown stripboard sits in the middle with a 16-pin chip, a handful of resistors and small yellow capacitors, and blue wire links. Three potentiometers with bare shafts lie above it on blue and yellow wires, a small blue toggle switch lies to the left on green wires, and a green 9V battery sits to the right on red and black leads. Two quarter-inch jacks on white wires lie at the bottom left: one has a blue cable plugged in that runs up to a small lilac practice amp with its power light on, the other a yellow cable that runs down to the body of a pink electric guitar at the bottom of the frame. A pink keyboard sits behind the amp.
The first 17 by 20 layout, built and working, September 2026. Everything off the board is still on flying leads: three pots, the mode toggle, a 9V battery and two jacks, the blue cable out to the amp and the yellow one in from the guitar.

My box has more on it than the file does: a 100K starve pot in the supply leg, a power toggle, an LED, and a bend I found while poking at it - a toggle between the range pot's bare lug and pin 16, the lug the board does not use and the chip's supply pin. Throw it and the tone drops, about an octave. My reading of why, unmeasured: it puts the supply onto the range pot's track, so the pin-11 end is pulled up toward V+ and the current that sets the VCO's rate falls. Less current, lower pitch, and how far it falls depends on where the range knob sits.

The LED is a bend too, not an indicator. It is a UV LED across strips 13 and 14 of the built board - the pitch pot's wiper strip and the strip at the bottom of the range pot, just above its 1K floor - and it does two things at once: it flickers with the pitch knob, and it drops the tone as well, so the box never sits up in a thin whine. Same reading as the toggle: current from the pitch wiper through the LED follows the knob, which is the flicker, and what it dumps into the range pot's floor lifts that node and takes current away from the rate pin, which is the drop. It lights, so its anode must be on the pitch side, and it has no resistor in series - the pot track and the 1K floor are all that limits it. A pink LED in the same spot dropped it too far; a near-UV one needs more volts before it conducts, so it passes less current into that node and takes less off the pitch. None of this is in the layout file; the drill template further down is drawn for this box, so it has holes for all of it. On the smaller file above, those two strips are the ones carrying the PITCH_W and RANGE_L pads.

How it works, in one breath

The VCO half of a 4046 is one cap and two resistors. The cap between pins 6 and 7 sets the range, the resistor on pin 11 sets how wide the sweep is, and the voltage on pin 9 sets the pitch inside that. That is all a knob has to do here, and it is why this chip tracks so much more evenly than a 40106 does - it was designed to be steered.

The other half is a phase comparator: it looks at the VCO against whatever is on the signal input and puts out a voltage saying which is ahead. Feed that voltage back into pin 9 - which is what the mode switch does - and the oscillator stops obeying the knob and starts hunting for the input. On a bench that is a tracking oscillator. On a noisy input it is something much less obedient, which is the interesting setting.

Pin 15 is unconnected and should be. It is the chip's internal zener, unused in almost every build, and Copper Bottom will report it as an orphan every time you check this board. That one is expected. Pin 12 is left open too, which sets no floor on the lowest frequency the VCO will reach.

Don't eyeball the holes. There is a 1:1 drilling template for the 125B - the box drawn unfolded, so you wrap the sheet around it and every hole lands by folding instead of by measuring. Face plus four walls, true-size circles with a punch dot in the middle. The walls are blank; everything is on the face.
↓ Same box, DC and output on top (PDF)
The second sheet keeps every control and the CV input where this one has them and moves the DC jack and the output to the top wall, so those cables leave upward. Two ways to drill one box; pick one.
Drawn for the layout file's parts and nothing else: the three pots down the left, the mode toggle and the CV input on the right, the output and a DC jack along the bottom. My boxed build has more on its face - a supply starve, a power toggle, the DROP bend, an LED, and 3.5 mm jacks - and none of that is on this sheet; the parts list above says what those extras are.
Print at 100% / “Actual size”, never “Fit to page” - check the bar measures exactly 4″ before you touch a drill. The face is 66 by 121 mm but only 42 mm of it is drillable once the lid lip is clear.
Nobody has drilled this one yet. Where the holes sit is the house format applied to the parts; offer the sheet up to the box and check it against your own parts before you make a hole.

Circuit after the classic 4046 VCO, the same chip behind the Dead-Bug Dragonfly. Layout drawn by Onion Madder in Copper Bottom.