Dead-Bug Cicada
A gated four-oscillator CD4093 synth, point-to-point · By Onion Madder · Mess O' Pedals
The Dead-Bug Choir drones. The Cicada pulses. The CD4093 is a quad 2-input NAND Schmitt trigger - four logic gates that each become a square-wave oscillator with one resistor and one cap, exactly like the 40106. The difference is the second input: it's an enable. Hold it high and the voice sings; pull it low and the voice goes silent. Wire one voice's output into another's enable and they start switching each other on and off — stutters, rhythms, and the pulsing insect-chorus that gives this build its name. Built dead-bug, of course: legs flattened out flat and every part soldered straight to them. No breadboard, no PCB.
This is the Choir's rhythmic cousin. Same rules, same vape-pack power, same dead-bug method as the Dead-Bug Screamer — and it makes a perfect gate source for the rest of the series. If you've built the Choir, you already know 90% of this; the new part is the enable pin, and that's the whole point.
⚠ Before you start tap to expand / collapse
Hot iron, obviously. 350°C of don't-grab-that. Ventilate, don't breathe the flux smoke, park it in its stand every time.
CMOS has one hard rule: never leave an input floating. On the 4093 that means both inputs of every gate. A gate you don't use at all: tie both its inputs to ground. A gate you do use: the timing input is handled by the RC network, but the enable input still needs to go somewhere — to V+, to a switch, or to a modulation source. Never just leave it hanging. Floating CMOS inputs draw current, buzz, and can latch up.
Mind polarity on the electrolytic output cap and your power source - backwards electrolytics bulge or pop.
If you power it from a salvaged lithium pack, the usual cell cautions apply - see Do You Want to Survive the Vapocalypse?
How it works, in one breath
A Schmitt-trigger gate flips its output with hysteresis - it snaps high and low at two different thresholds instead of one. Hang a resistor from a gate's output back to one of its inputs, and a capacitor from that input to ground, and the cap endlessly charges and discharges between the two thresholds: a relaxation oscillator, a square wave, no crystal. The resistor sets the pitch (bigger = slower), the cap sets the range (bigger = lower). So far, identical to the 40106. But a NAND gate has two inputs, and its output only goes low when both inputs are high. Use the first input for the RC oscillator; the second input is a gate. High = the oscillator runs. Low = the output is jammed high and the voice stops dead. Four gates, four voices, and four enable pins you can play, switch, or wire to each other.
What you'll need
New to the parts? Each line notes how to spot it - the printed marking, the package, or the polarity clue.
- 1× CD4093 (a.k.a. CD4093B / HEF4093 / MC14093) quad 2-input NAND Schmitt trigger, 14-pin DIP - spot it: 4093 printed on top; a notch marks pin 1. Do not confuse it with the 4011 (plain quad NAND, no Schmitt - it won't oscillate cleanly).
- 4× 100K–1M pots for playable pitch (with a ~1K series resistor each so they never hit zero ohms). Spot it: pots are marked B = linear taper (use these), value after.
- 4× Ceramic capacitors, one value per voice - run the ladder: 101 (100pF, highest whistle), 102, 103, 104, up to 105/106 (1 µF+, slow ticking). Four voices want four values; the spread is what stops it sounding like one fat buzz. Spot it: the printed code is the value - 103 = 10nF, and they don't have to match each other to work.
- 1× 0.1 µF ceramic - supply decoupling (printed 104).
- 4× Mixing resistors, ~10K, one per voice - bands brown-black-orange, or meter them; non-polar.
- 4× Mixing capacitors, one per voice - 1 µF non-polarised (NP / bipolar). Audio swings both ways, so a cap that doesn't care about direction is the safe choice. Spot it: NP caps are usually printed NP or BP on the body.
- 1× 10 µF electrolytic - output DC-blocker. Spot it: a little can; the stripe marks the minus leg, longer leg is +.
- Optional: 2–4 patch points, switches, or body-contact bolts for the enable inputs - this is where the gating lives (see Step 6 and Going further).
- 1× Output: a small speaker + an LM386 amp for real volume (that's the Dead-Bug Bullfrog), OR a jack to an amplifier - see Step 7.
- 1× SPST toggle for on/off - a 2-lug switch.
- 1× Power source - at least two vape cells in series (7.4V; three gives you 11.1V), or a 9V battery.
- 1× LED + a series resistor (~1K–10K) as a power indicator. Spot it: the longer leg is + (anode); the flat spot on the rim marks the negative side.
- 1× Enclosure preferably grounded; if not, copper tape for grounding.
- Solid-core hookup wire, soldering iron, cutters, multimeter.
Quick reference - the whole build on one card
Pinout plus every connection at a glance. The steps below walk through it slowly; this is the bench card.
“A” and “B” are the two inputs, “Y” is the output. Notch marks pin 1. Keep the chip label-up and this map reads exactly as you see it. Watch the outputs: unlike the 40106's neat pairs, each 4093 output sits between the next gate's inputs — gate 2's output (pin 4) is right above gate 2's inputs, not after them.
Power (first)
- Pin 14 → V+ (3–15V; vape pack ideal)
- Pin 7 → ground
- 0.1 µF across pin 14 ↔ pin 7
- Every unused input → ground (both of an unused gate)
One voice (repeat per gate)
- Output → input A via R (pot + 1K series) - pitch
- Input A → ground via C - range
- Input B = enable → V+ to free-run, or to a gate source
- Output → to mixer
The four gates - in A / in B / out
- V1 → 1 / 2 / out 3
- V2 → 5 / 6 / out 4 (output above the inputs!)
- V3 → 8 / 9 / out 10
- V4 → 12 / 13 / out 11 (output above the inputs!)
Mix → out
- Each output → 10K → 1µF NP → shared mix bus
- Mix bus → output
- Mix bus → 10µF (+ to bus) → jack / LM386 → ground
Step 1 Presolder everything, then populate the panel
Do this before the chip ever comes out of its bag. Every joint you make on a loose part at the bench is a joint you don't have to make later, one-handed, inside a box, next to wires that are already in the way. Tin every leg and every wire end first - a tinned lug and a tinned wire join with a single touch of the iron.
Pots need one thing before they go in: snap the anti-rotation tab off unless your enclosure has the matching hole for it. Same move as in the ISD1820 build - grip it with needle-nose pliers and it breaks away cleanly. Then drill the panel and mount everything: four pots, the LED, the toggle, the output jack, and whatever enable switches or patch points you're adding.
Step 2 Flatten the legs — and leave it label-up
Bend all fourteen legs out flat, so they splay sideways instead of pointing down. Those flattened legs are your tie points; everything solders straight onto them.
Do not flip the chip over. Flipping the package mirrors the pinout, so every pin lands on the opposite side from the map. Leave it label-up, notch where you can see it, and the pinout above matches the chip in front of you exactly.
Step 3 Power rails - and the CMOS rule
Run a V+ bus and a ground bus, hook up the two power pins, and drop the decoupling cap across the chip. Then the rule that trips everybody up: every input you're not driving gets tied somewhere. A gate you skip entirely - tie both its inputs to ground. Leave the unused outputs alone; it's only floating inputs that misbehave.
| Connect | To | Why |
|---|---|---|
| Pin 14 | V+ | Supply positive (VDD) |
| Pin 7 | Ground | Supply negative (VSS) |
| 0.1 µF | Pin 14 → Pin 7 | Decoupling, right at the chip |
| Both inputs of any unused gate | Ground | No floating CMOS inputs - ever |
Step 4 Build one voice
Here's the whole oscillator, on gate 1. Resistor from the output (pin 3) back to input A (pin 1); cap from that input to ground; the output is your signal. Make the resistor a pot (plus a ~1K series resistor) and you've got a pitch knob. The second input, B (pin 2), is the enable - for now, tie it straight to V+ so the voice free-runs. Get this one squeaking before you build the rest.
| Connect | To | Part |
|---|---|---|
| Pin 3 (out) | Pin 1 (in A) | Pitch = pot + 1K series |
| Pin 1 (in A) | Ground | Range cap (e.g. 0.01 µF) |
| Pin 2 (in B, enable) | V+ | High = run (later: a gate source) |
| Pin 3 (out) | → to mixer (Step 5) | Signal tap |
Step 5 Add the rest of the swarm
Every voice is the same connections, just on a different gate. Here are all four, with the enable input tied to V+ for now (we'll cut it loose in Step 6). Give each a different cap for a wide spread, or the same cap with different pots for a tight, beating cluster. You don't have to build all four; two already gate each other into something alive.
| Voice | In A (R + C) | In B (enable) | Output | Suggested cap |
|---|---|---|---|---|
| 1 | Pin 1 | Pin 2 | Pin 3 | 0.01 µF - mid |
| 2 | Pin 5 | Pin 6 | Pin 4 | 0.1 µF - low growl |
| 3 | Pin 8 | Pin 9 | Pin 10 | 0.001 µF - high whistle |
| 4 | Pin 12 | Pin 13 | Pin 11 | 1 µF - slow tick / LFO |
Either input of a gate can be the timing input; the other is the enable. I use A for timing and B for enable throughout, just to keep it straight. And watch the output pins: on gates 2 and 4 the output is a lower number than the inputs.
Step 6 Now make them gate each other
This is the move the 40106 can't make. Instead of tying an enable input (B) to V+, drive it from something that goes up and down:
- A slow voice gating a fast one. Take the output of your 1 µF voice (a slow tick, below hearing) and run it — through a ~100K resistor to be safe — into a fast voice's enable input. The fast voice now switches on and off in time with the slow one: rhythmic pulsing, the cicada chirp.
- A switch on the enable. An SPDT toggle that flips the enable between V+ (run) and ground (mute) gives you a per-voice on/off you can play by hand.
- Body contacts. Bring an enable input out to two bolts with a big pull-up resistor (~1M to V+). Bridge them with a wet finger and your skin drags the gate around.
Chain a few of these and the four voices start playing each other - which is the entire reason to reach for a 4093 instead of another 40106.
Bench note - untested variation tap to expand / collapse
Driving an enable input straight from another gate's output (no resistor) works because they're both logic-level CMOS, but I like a ~100K in series as insurance and because it lets the timing cap on the driven voice bleed the transition a little, which softens the chop. I have not A/B'd the two ways on the bench yet - once I do, the verdict lands here.
Step 7 Mix them down, and get it out of the box
You can't just tie the outputs together - they'd fight. Give each voice a ~10K resistor then a 1 µF NP cap into a shared mix bus (resistor first, cap second). Then run the bus to the output. Like the Choir, there's no volume pot on this build - the level is set by the amp you plug into.
| Connect | To | Part |
|---|---|---|
| Each voice output | 10K resistor → 1 µF NP cap | resistor first, then cap |
| All caps | Mix bus | the shared node |
| Mix bus | 10 µF (+ to bus) → output | DC blocker |
| Output | LM386 + speaker, or jack to an amp | logic outputs are quiet on their own |
Those little logic outputs can't drive a speaker on their own - straight into 8 Ω you'll get a whisper. Feed the mix bus into the Dead-Bug Bullfrog (an LM386 amp) for real volume, or out a jack into an amp or pedal.
Step 8 Power up
Wire your SPST toggle in the V+ line, flip it, and start sweeping pitch knobs with every enable tied high - you should hear four independent tones. Then start patching enables and listen for the chop coming in. Swapping caps mid-build is the fastest way to find a range you like.
Voltage note tap to expand / collapse
The CD4093 is CMOS and runs happily anywhere from about 3V to 15V. More volts shifts the pitches up and stiffens the output. At least two vape cells in series (7.4V - three gives you 11.1V) or a 9V battery.
Step 9 If it won't sing
- A voice is dead silent? First check its enable input (B) is actually high - a floating or grounded enable means the output is jammed high and the voice can't run. Then check the feedback resistor really goes output→input A, and the cap goes input A→ground.
- Mixed up which pin is the output? On gates 2 and 4 the output is the lower pin number (4 and 11), sitting above the inputs. Easy to swap.
- Whole chip buzzy or hot? A floating input. Every input of every gate goes somewhere - timing network, enable source, or ground.
- No sound at all? Confirm pin 14 at V+ and pin 7 at ground, and that you didn't skip the 0.1 µF decoupling cap.
- Gating sounds like clicks, not rhythm? That's the enable switching too fast, or a fast voice gating a slow one instead of the other way round. Slow voice gates fast voice.
Going further: make them misbehave
Four gated voices is a lot of instrument. To push it:
- Cross-modulation, like the Choir. Beyond gating, run one voice's output through a resistor (~100K) into another voice's timing input (A) for pitch-warping FM on top of the on/off gating.
- Feed the series. A gated 4093 voice is a great clock for the rest of the trio - run one into a CD4024 to divide it into sub-octaves and rhythms, or into a CD4046's CV input. And it pairs naturally with the Choir: let the Cicada's slow voice gate the Choir's drones on and off.
- Two chips, eight voices. The 4093 is cheap. A second chip doubles the swarm and gives you enough enables to build real patterns.
Build one voice, tie its enable high, get it squeaking - then cut the enable loose and let the swarm start arguing. That argument is the instrument. Show me what you make.
The 4093 gated oscillator is folk knowledge of the DIY-synth / Lunetta world - the same NAND-Schmitt building block behind countless noise boxes. This write-up, wiring, and words are my own. Build it, bend it, make it yours.