John Sokol's Blog: Laser Cut

Showing posts with label Laser Cut. Show all posts

Monday, June 01, 2026

Laser cut , Snake Bot -

This is a 2 Dimensional Snake Robot that has flexible couplings between each joint so It can rest flat on the surface. There are 8 segments, made using 40mm x 40mm x 20mm metal gear High Torque servos

The is constructed from laser cut 4.7mm MDF (Medium Density Fiberboard) , press fit passive Skateboard Bearings 608zz 8mm x 22mm x 7mm. The same type used in fidget spinners.
The design is done in OpenSCAD and was inspired by Japanese woodworking styles, without screws, glue or fasteners with the exception of the RC Servo Hub.

https://github.com/johnsokol/OpenSCAD-misc-projects/tree/master/RCservo-snake

Laser cut pattern file in OpenSCAD.

The parts press fit over the servo. The Bottom left piece just press fits over the servo output and the only screws are the 3mm screws that hold part on to the servo. and the Ball Bearings just press fit over the end of the t on that part. heat shrink or tape should be placed on the ball bearing to increase friction with the surface.

When fed a sine wave it can propel across a surface with nothing driving the wheels directly.

https://www.youtube.com/watch?v=9scTZW8f14I

https://www.youtube.com/watch?v=cNn-HVFAm0c

Movement

Image from: Locomotion Efficiency Optimization of Biologically Inspired Snake Robots: Eleni Kelasidi, Mansoureh Jesmani , Kristin Y. Pettersen and Jan Tommy Gravdahl

Friday, May 29, 2026

Building Neon Logic Gates: A Practical Maker’s Guide

Companion to the pneumatic computer essay. This one is the how-to. If the pneumatic post asked “could a kid watch a computer think?”, this one is the answer for adults willing to handle a few hundred volts. The result: a clear-glass-and-acrylic stack with orange glowing cells that act as 1-bit memory, wired through a PCB underneath into anything from a single visible bit to a four-bit counter. Aesthetic of 1950s computer; build cost of a long weekend.

⚠️ This project uses 150–180 V DC at low current. It’s not as dangerous as mains AC at the same voltage, but it can hurt you and in unusual circumstances kill you. Section 11 covers what you need to do to keep it safe. Read it before you build anything. If you are not already comfortable building HV circuits with current limiting and bleeder resistors, build the all-acrylic pneumatic version instead — it’s the same pedagogy and you can’t electrocute yourself with air.

1. What you’re building

A single cell is a small glass-and-acrylic sandwich about the size of a postage stamp, holding 5-15 torr of neon, with two or three metal electrodes passing through the glass into the chamber. Apply ~90 V across the main electrodes, pulse a third “trigger” electrode, and the gas ignites into a visible orange glow. The cell stays lit until you drop the supply voltage below the sustaining threshold of ~65 V. One cell is a 1-bit memory you can see.

A row of eight cells, mounted on a PCB that handles the supply routing and the trigger pulses, is a visible 8-bit register. You load a number into it by sequentially pulsing the trigger inputs. You read the number off the front by which cells are glowing.

A few rows of cells wired with appropriate cross-couplings is a visible counter or, with more care, a visible adder. The final device is a small wooden-and-glass box about the size of a hardcover book, sitting on a table at the front of a classroom or maker space, glowing softly orange while it counts.

The whole thing runs on a wall transformer, draws under 10 watts, and sits silently. Compared to the pneumatic version: no moving parts, no compressor noise, faster operation (milliseconds per bit, not half-seconds), and the aesthetic is straight out of 2001.

2. How the cell works, in 500 words

Low-pressure neon has a useful property called the Townsend breakdown that makes it behave as a natural bistable element.

Below a critical voltage (the ignition voltage, typically 80-100 V for a 5-10 mm electrode gap at 5-15 torr neon), the gas in the chamber is electrically neutral and effectively an insulator. No current flows. The chamber is dark.

Above the ignition voltage, an avalanche of ionisation starts: a stray free electron is accelerated, collides with a neon atom, liberates more electrons and ions, and the cascade runs away until the chamber is full of plasma. Current flows freely. The chamber glows orange (specifically at 585.2, 614.3, and 640.2 nm — the strong visible neon lines).

Critically, once ignited, the voltage required to sustain the discharge is much lower than the ignition voltage — typically 60-70 V. The cell exhibits hysteresis: it stays lit until the supply voltage drops below the sustaining threshold, at which point the plasma deionises and the cell goes dark.

This hysteresis is what makes the cell a 1-bit memory. Sit the supply voltage at ~75 V — between ignition and sustain — and the cell is stable in either state. If you push the supply momentarily above 90 V (or, equivalently, momentarily lower the effective ignition threshold by injecting electrons via a third electrode), the cell ignites and stays lit at 75 V. If you push the supply momentarily below 60 V, the cell goes dark and stays dark at 75 V. The cell remembers.

The third electrode — the trigger — is the practical knob you control. A small voltage pulse on the trigger (typically 20-30 V on top of the cathode reference) creates a small auxiliary discharge that seeds the main chamber with enough ions to lower the ignition threshold below the running supply voltage. The main discharge fires. After the trigger pulse ends, the main discharge sustains itself.

The whole cycle is fast: ignition takes microseconds, sustained operation is steady, and extinction (when you drop the supply) takes hundreds of microseconds while the plasma recombines.

For logic, the trigger structure gives you natural building blocks:

Bit storage — one cell with a single trigger is a 1-bit memory.
OR gate — two triggers on the same cell, either one fires it.
Set-Reset latch — one cell for the set, one for the reset, cross-coupled supply lines.
Sequential logic — multiple cells wired to fire in sequence via shared trigger lines and capacitive coupling.

Combinational AND/XOR are awkward in pure neon logic and were historically done with diodes in front of the trigger inputs. You’ll do the same.

3. Bill of materials

For a build that produces one working cell plus the supply infrastructure to drive it, expect to spend about $400-500 in materials and $200-400 in tools (most of which you can borrow or already have).

Materials for one cell

Item	Spec	Source	Cost
Cast acrylic sheet	3 mm clear	Local plastics supplier, TAP Plastics, Home Depot	$5/sheet (12×12”)
Float glass	3 mm clear, ≥4×4”	Glass shop scraps, eBay, hobby store	$3/piece
Stainless steel pin stock	1 mm dia × 50 mm	McMaster 90145A115	$0.50/pin (need 3)
Buna-N O-ring sheet	1.5 mm thick	McMaster 8634K12	$25/sheet (enough for 50 cells)
Torr Seal epoxy	Two-part	Edwards Vacuum, Kurt J. Lesker	$45/kit
Apiezon Q wax	Vacuum sealing	Kurt J. Lesker, eBay	$25/stick

Gas-handling and electrical infrastructure

Item	Spec	Source	Cost
Single-stage rotary vane pump	≥10 L/min, can hit 10⁻² torr	eBay used (Edwards E2M5, Welch 1400)	$150-300
Neon lecture bottle	99.99%, 1.7 L at 1700 psi	Praxair, Airgas, AGSI	$80-120
Pressure regulator for neon	Dual-stage, 0-30 psi outlet	Praxair, eBay	$80 used
Pirani gauge + readout	Range 10⁻³ to 10² torr	eBay used (Granville-Phillips 275)	$80
Vacuum tubing	6 mm OD nylon or 1/4” copper	Hardware store	$30
Compression fittings	1/4” Swagelok-compatible	McMaster, eBay	$40
Needle valve	1/4” Swagelok	McMaster 4901K23	$35
HV DC supply	0-250 V adjustable, 50 mA, with current limit	XP Power AHV28-P50 or build from voltage doubler	$80 module, $30 DIY
HV trigger pulse driver	One MOSFET HV pulse generator per cell	DIY from CD4093 + IRF830	$5/cell
HV current-limit resistors	100 kΩ, 5 W wirewound	Mouser, Digi-Key	$3 each
Bleeder resistor	470 kΩ, 5 W	Mouser	$2
FR-4 PCB	Custom, two-layer	JLCPCB, OSHPark	$5 for 10 boards

Tools

Tool	What for	Cost if buying
CO₂ laser cutter, ≥40 W	Cutting acrylic and gaskets	Hacker space access
Drill press + 1 mm + 3 mm bits	Through-holes in glass for electrode pass-throughs	Hacker space access
Glass-drilling diamond bits	If drilling glass yourself	$20 set
Toaster oven	Bakeout at 60-70 °C	$40
Small vacuum desiccator or bell jar	Bakeout chamber	$50 used
Multimeter that handles 250 V DC	Voltage testing	Already have
100 MΩ HV probe (or 10:1 divider)	Measuring supply voltage on a meter	$40
Soldering iron, basic electronics tools	Wiring	Already have

The biggest single-item cost is the rotary vane pump. You can avoid the pump entirely by going to a vintage TIG-welder distributor and asking for a used pump from a refrigeration shop — they’re $50-100 and work fine for this pressure range. Or borrow from a university physics lab.

4. Designing the cell

The cell geometry is constrained by three things:

Paschen’s law sets the relationship between pressure, electrode gap, and ignition voltage. At ~10 torr neon, the ignition voltage minimum is around 80 V at a gap of ~5 mm. Smaller or larger gaps raise the ignition voltage.
Visibility wants the glow region as large as possible, so a bigger chamber is better aesthetically.
Power dissipation wants the chamber small. At 75 V sustained and 2 mA, each cell dissipates 150 mW continuously. A four-bit register with all bits lit dissipates 0.6 W, plus another ~10 W in the HV supply and current-limit resistors.

The cell I recommend for a first build:

Chamber size: 15 mm × 15 mm × 2 mm deep
Electrode gap: main electrodes 8 mm apart, both 1 mm pins
Trigger electrode: 1 mm pin, located 3 mm from the cathode and 3 mm offset from the main discharge axis
Glass plates: 25 mm × 25 mm × 3 mm
Acrylic spacer: 25 mm × 25 mm × 2 mm, with the chamber cut out and a 1 mm wide × 1 mm deep O-ring groove cut around the chamber perimeter

At ~10 torr neon, this geometry gives an ignition voltage around 90 V and sustains at 65 V. The glow fills the chamber visibly — bright enough to read by in a darkened room, soft enough that you don’t need to dim the room lights to see it.

For the laser cutter, draw the acrylic spacer as a single 25 mm square with:

A 15 mm × 15 mm cutout for the chamber
A continuous 1 mm × 1 mm groove around the chamber for the O-ring
Three 1.5 mm holes for the electrode pins (slightly oversized to allow alignment slop)

Cut at standard acrylic settings (50% power, 30 mm/s on a 60 W laser). The O-ring groove is cut as a separate engrave step at 20% power, single pass — this gives you a slot the O-ring can sit in.

5. Bakeout

This is the step you do before assembly. The purpose is to drive adsorbed water and volatiles out of the acrylic so the cell holds its gas composition for weeks instead of hours.

Procedure:

Cut all your acrylic pieces fresh — don’t bake them, cut them, then leave them in air. The fresh-cut surface re-adsorbs water within minutes. Cut just before bakeout.
Place the pieces in a small vacuum desiccator inside a toaster oven.
Connect the desiccator to the rotary pump via a vacuum hose.
Turn on the pump first. Let it pump down to <0.1 torr. Then turn on the toaster oven and set it to 60-65 °C. Do not exceed 70 °C — cast acrylic softens above this temperature and your O-ring grooves will distort.
Let it bake at temperature under continuous pumping for 48 hours minimum, 72 hours preferred. The outgassing rate drops by about a factor of ten every 24 hours during this period.
Cool to room temperature under vacuum. Vent the desiccator with dry nitrogen or filtered argon slowly — fast venting can re-deposit moisture from any condensation on cold surfaces.
Use the parts within a few hours of bakeout. Don’t leave them in open air overnight before assembly.

A small humidity-indicator card inside the desiccator is a useful sanity check: it should remain blue throughout the bakeout.

The same bakeout applies to the O-ring sheet (if it’s Buna-N — Viton is fine without bakeout) and to anything else organic that will be inside the sealed volume.

6. Assembly

You’ll need: baked acrylic spacer, two pieces of glass, three stainless pins, O-ring stock, Torr Seal, and a small flat surface clean enough to work on.

Drill the glass. The bottom glass piece needs three 1.5 mm holes for the electrode pins, located to match the spacer’s pin holes. Use a diamond bit at low RPM with water cooling. Take your time; cracked glass is the most common assembly failure.
Cut the O-ring. Cut a length of Buna-N stock to fit the groove in the acrylic spacer. Join the ends with a single drop of cyanoacrylate (super glue) to form a closed loop. The O-ring should sit slightly proud of the groove surface when placed in.
Set the electrodes. Push the three 1 mm stainless pins through the holes in the bottom glass, leaving about 3 mm protruding into what will be the chamber and about 10 mm protruding below for external connection. Seal each pin to the glass with a small dab of Torr Seal mixed per the package instructions. Cure 24 hours. This is your single hardest seal — take care.
Build the stack. - Bottom glass (with pins) face up - Acrylic spacer (with O-ring in groove) - Top glass - Sandwich the whole thing between two metal plates (aluminum is fine) with nylon bolts at the four corners. - Tighten the bolts evenly until the O-ring is compressed to about 50% of its uncompressed thickness. Do not over-tighten — you’ll crack the glass.
Mount the gas connection. The top glass needs one more small hole, off to the side of the chamber, where the gas-fill tube enters. Pre-glue a short length of 3 mm OD glass or metal tubing into this hole with Torr Seal before final assembly. The tube extends ~10 mm above the top glass for connection to the gas manifold.
Check the seal. Connect the cell’s fill tube via flexible tubing to the rotary pump. Pump down. Watch the Pirani gauge — it should read down to ~10⁻² torr within a minute or two. If it plateaus at higher pressure, you have a leak. The leak is almost always at one of the electrode pins or at the O-ring (which means it wasn’t compressed evenly).
Fill with neon. Once the cell pumps down cleanly, close the pump valve, open the neon supply via the needle valve, and let the cell pressure rise to 10 torr. Close the needle valve. Disconnect from the gas manifold by pinching off the fill tube with a small metal clamp (the tube can be permanently sealed later by heating and crimping, or by gluing the clamp on).

Total assembly time per cell: ~3 hours including epoxy cure (which you do overnight). With practice you can build five cells in parallel in roughly the same time.

7. The HV supply

You need a DC supply that delivers 0-180 V at up to 50 mA, with current limit and a bleeder resistor.

The cheap path: a 0-30 V variable DC bench supply driving a 6:1 voltage doubler made from off-the-shelf parts. Two 1 µF/400 V electrolytic capacitors, two 1N4007 diodes, configured as a Cockcroft- Walton ladder. Input 24 V AC (from a small line transformer), output ~150 V DC. Add a 100 kΩ series resistor for current limit and a 470 kΩ bleeder across the output.

The cleaner path: buy an XP Power AHV series module or equivalent. They cost ~$80 and give you a clean 0-250 V variable output with built-in current limit.

Either way, always include the bleeder resistor. When you turn the supply off, the capacitors retain charge for many minutes otherwise. The bleeder drains them within 10 seconds.

For the trigger circuit, you need a 30 V pulse, ~10 ms duration, floating reference, into the trigger electrode. Easiest implementation: a CD4093 Schmitt-trigger NAND gate driving an IRF830 MOSFET that switches a small 30 V supply onto the trigger pin via a 10 kΩ resistor. The MOSFET is rated for 500 V, so even if the cell shorts to its trigger you don’t kill the driver. One driver per trigger input.

Lay all of this out on a single board underneath the cell array. Through-hole construction, generous spacing between high-voltage traces (≥3 mm), and a clear silkscreen showing which traces are at which voltage. Mount the supply on the underside of the wooden case, behind a barrier so nobody’s fingers can reach the live nodes.

8. First light

You’ve built one cell. You’ve built the supply. Time to make it glow.

Confirm the cell has the right gas pressure with the Pirani gauge.
Connect the cathode pin to the supply ground via a 1 kΩ resistor (this provides a small reference and limits any transient).
Connect the anode pin to the supply positive via a 100 kΩ series resistor.
Bring the supply up slowly from 0 V. Watch the cell.
At around 85-95 V, the cell will spontaneously ignite — a sudden orange glow filling the chamber. The supply current will jump to 1-2 mA.
Lower the supply slowly. The cell will continue glowing down to about 60-65 V, then suddenly extinguish.
Note both voltages. These are your cell’s actual ignition and sustaining voltages. They’ll vary a few volts cell-to-cell.

If it doesn’t ignite by 100 V: the gas pressure is too low or too high (Paschen curve has a minimum), or the gap is too small. If the pressure is way off, vent the cell and refill more carefully.

If it ignites way below 80 V: the gas is contaminated with moisture or air (so you have a higher proportion of nitrogen, which ionises more easily). Pump and refill.

If the glow is white/pink instead of orange: definitely contamination, probably nitrogen from a leak. Pump down, refill.

If it ignites but flickers: could be a leak (slow contamination changing breakdown voltage), or could be supply ripple. Add a 1 µF cap across the supply output.

Once you have stable orange glow with reproducible ignition/sustain voltages, set the supply at the midpoint (typically ~75 V) and verify the cell sits stable in either lit or dark state for at least 60 seconds. That’s your working bistable.

9. Triggering for memory operation

With the supply at 75 V (between ignition and sustain), you want to flip the cell between lit and dark with pulses.

To set (light up): pulse the trigger electrode to +30 V relative to the cathode for ~10 ms. The trigger creates a small auxiliary discharge in the chamber that locally lowers the ionisation threshold for the main gap. The main discharge fires. After the trigger pulse ends, the main discharge sustains itself.

To reset (extinguish): pulse the supply briefly below the sustaining voltage. The simplest way is to put a small MOSFET in series with the supply line that briefly drops it (a “kill switch”) for ~1 ms. The plasma deionises and the cell goes dark.

With the set and reset capabilities wired to two pushbuttons on the front panel, you have a manually-operated 1-bit memory you can flick on and off and watch. It’s already a satisfying object at this point. Put it on a shelf for a day and look at it from time to time. Verify it holds state for hours without drifting (it should).

10. Building out to a register

For an 8-bit register, you build 8 cells side-by-side on one PCB and wire them with shared supply, shared reset bus (kills all bits simultaneously when you want to clear), and individual trigger inputs (one per bit).

The PCB layout puts the cells in a 1×8 row, 30 mm apart, with the HV supply rail running along the top edge and the cathode-ground rail along the bottom. Each cell’s anode connects to the HV supply via a shared current-limit resistor (or, more conservatively, individual resistors so a failed cell doesn’t take out the supply).

Mount the trigger pulse drivers on the underside of the same PCB, one per cell. Bring the trigger inputs out to a header connector on the back. Drive them from a microcontroller — an Arduino Nano works fine — that sequences the trigger pulses according to whatever pattern you want to load.

An 8-bit binary counter is the simplest demonstration: the Arduino counts internally and on each tick re-loads the register with the new value, displayed as glowing bits. Counts up from 0 to 255 over four minutes if you tick once per second. Watching the bits propagate is the entire point.

A visible adder is more elaborate. You build two 4-bit input registers, the adder logic (NAND gates in solid-state, not in the neon — the neon is for the visible state), and a 4-bit output register. Set the inputs by flipping switches that pulse the trigger inputs. Press a “compute” button. The adder logic computes A+B and triggers the appropriate output bits. The output register shows the sum.

The neon cells are the display + state memory. The combinational logic in between is solid-state. This is exactly how the Harwell WITCH (1949) worked: dekatrons for storage and visible display, relays for the actual logic. You’re recreating that architecture at maker scale.

11. Safety

The non-negotiable rules.

Bleeder resistors on every capacitor in the supply. A 470 kΩ / 5 W resistor across the main output capacitor drains it to <30 V within 10 seconds of power-off. Without it, the supply caps can hold a hazardous charge for 30+ minutes.
Current limiting on every cell. The supply itself should have a hard current limit at ~50 mA. Each cell should also have a series resistor (100 kΩ or higher) so that even if a cell shorts, the steady-state current is limited to <2 mA.
Live conductors behind glass or behind a barrier. Nothing conductive at >40 V should be touchable from the outside. The front of the device is glass over the cells. The back of the device is a wooden panel or a perforated metal cover with the high-voltage routing inside.
One-handed operation rule. When the device is powered on, keep one hand in your pocket. If you must touch anything, only do so with one hand. This prevents current paths through your chest.
Verify dead before touching. Before working on the powered-off device, measure the supply rail with a multimeter to confirm <10 V. The bleeder resistor should make this automatic but verify.
Mains isolation. The HV supply should be galvanically isolated from the mains via a line-frequency transformer or an off-the-shelf isolated module. Don’t use a mains-rectified supply without isolation.
Plug-in warning label. A small label on the back of the device saying “180 V DC inside. Service by qualified personnel only. Allow 30 seconds after disconnect before opening.” Yes, the bleeders drain it in 10 seconds; the label says 30 to be conservative.
No kids inside. Children can interact with the front of the device — toggles, pushbuttons, watching the glow. They should never have access to the back. This means either screwed-shut panels (not snap-on covers a curious 8-year-old can pop off) or a key-locked back panel.

At 150-180 V DC with ~50 mA current limit, this device is in the “will hurt, very unlikely to kill” range for a healthy adult under dry conditions. With the safeguards above it’s roughly as dangerous as the inside of an open CRT TV from the 1990s, which is to say: respect it, but don’t be paralysed by it.

If your maker space’s safety policy has a hard cap at “low voltage DC only,” this project is over that line and you need to talk to your space’s leadership before starting.

12. Troubleshooting

Cell ignites at the wrong voltage. Almost always gas pressure or gas purity. Pump down to <10⁻² torr (verify with the Pirani gauge), hold for 10 minutes to let outgassing equilibrate, refill with fresh neon. If pressure is the problem, you’re hitting the wrong part of the Paschen curve — go up or down 5 torr and re-test.

Cell ignites OK initially but extinguishes after a few seconds. Current limit is too aggressive. Drop the series resistor from 100 kΩ to 50 kΩ.

Cell ignites and won’t extinguish. Current limit is too loose, or the supply voltage is too high. The sustaining voltage is fundamental to the cell — if your supply is above it, the cell stays lit. Drop the supply 10 V.

Glow is purple/pink instead of orange. Air leak. Nitrogen contamination glows pink. Find the leak by squirting isopropyl alcohol on suspect seams while pumping — IPA’s vapour pressure changes the Pirani reading at the leak point.

Glow is bright orange but flickers. Supply ripple. Add filtering caps to the supply (1 µF / 400 V across the output, plus a small 100 nF ceramic for high-frequency noise).

Glow is dim and uneven. Either pressure is too high (most likely) or the chamber is too deep relative to the electrode gap. Re-pump and re-fill at lower pressure (try 5 torr).

Trigger doesn’t fire the cell reliably. Trigger pulse amplitude too low or duration too short. Increase to 50 V / 50 ms and retest. If still unreliable, the trigger electrode is too far from the main discharge path — re-make the cell with the trigger closer to the cathode (within 3 mm).

Cells holding state become unreliable after a few weeks. Outgassing. Re-pump and refill. If happening within days, you have a slow leak — find it via the IPA technique.

13. Going further

Things you can build on top of the basic cell:

A multi-decade counter. Use commercial dekatrons (eBay) as the counter elements; use your custom cells as the visible “active” bit indicator showing which decade is at non-zero. A 4-decade counter counts to 9999, fits in a hardcover-book-sized case, and runs indefinitely from a wall transformer.

A self-clocking ring oscillator. Two cross-coupled cells with an RC delay between them will oscillate at a frequency set by the RC time constant. Build a chain of these as a visible clock generator, each cell flashing in turn at a rate the kid can see.

A visible shift register. N cells in a row, with each cell’s trigger connected to the previous cell’s anode via a capacitor. A pulse on the first trigger propagates down the register at a rate set by the capacitor values. Watch a single lit bit walk down the row.

A pneumatic-and-neon hybrid. Connect the output of the pneumatic adder to the trigger inputs of an 8-bit neon register. The mechanical computer feeds its answer into the visible electronic display. Two substrates, one demonstration. Worth the extra weekend.

A teaching kit. Once you have a stable cell design, document it as a kit and put it on Tindie or Crowd Supply. The market is small but loyal — Nixie-tube clock builders are a real subculture and they will find this and love it. Open-source the PCB and laser-cut files on GitHub; build a small business selling the consumables (electrodes, gas cylinders, O-ring stock).

Coda

The neon logic gate is one of the few projects where the way the device looks while operating is also exactly how it operates. A transistor is just a black SOIC package and you have to take the datasheet’s word for it. A neon cell is a chamber where you can literally see the plasma. The pedagogy and the aesthetic are the same thing.

The 1950s knew this. The Harwell WITCH, on display now at the National Museum of Computing at Bletchley Park, has been running since 1951 and is still pulled out for demonstrations. Visitors don’t remember the relays; they remember the dekatrons, glowing as they count.

What you’ll build is in that lineage. Smaller, simpler, less ambitious, but the same idea. Computation you can see.

If you build one, send me a photo.

1. Why a visible computer matters

A kid born in 2026 will grow up surrounded by computation, none of which is visible. The phone is a slab. The laptop is a slab. The hearing aid, the doorbell, the elevator, the car, the toaster — all slabs, all opaque. Computers are not, to a modern child, things that do anything visible. They’re things that show things on screens about what they’ve already done.

I think this is bad for intuitions. Computation isn’t magic. It’s a sequence of small mechanical decisions: this bit, then this bit, then this bit, with logic governing each step. When the decisions happen at a billion times per second inside a slab, you cannot watch them, and the abstraction layer between “I press a key” and “a character appears” feels like sorcery.

But computation runs at any speed you build it for. If you slow it down to one decision per second, in a medium you can see, kids understand it. I have watched this happen with Lego Mindstorms, with the Phillips Hydraulic Computer demos at LSE, with the Antikythera replicas at the museum in Athens, with the Z3 reconstruction at the Deutsches Museum. The reaction is the same: when a child can see a machine deciding things, they get it. The mystery evaporates. They don’t think it’s stupid; they think it’s cool. The slowness is the feature.

A pneumatic computer in clear acrylic is the maker-space version of that pedagogy. You can build one with a $5,000 laser cutter and $300 worth of materials, in maybe four weekends, and afterwards it sits in the front of a classroom or the kids’ room of a maker space and does one job, well, forever: it adds numbers slowly enough that you can see it think.

2. The 2019 note in context

I scribbled the note down because I had three things converging at once.

I had a laser cutter handy. Laser cutters do one thing brilliantly: they cut precise 2D shapes out of flat material. Stacking those 2D shapes turns them into 3D channels for whatever fluid you want to push through. The features can be small (well under a millimetre), the materials are cheap (acrylic at a few dollars a sheet), and the cycle from “design change” to “new test piece” is an afternoon.

I had been reading old computing history. The Phillips Hydraulic Computer (1949, LSE) modelled the UK economy with water flowing through transparent perspex tanks. Konrad Zuse’s relay computers clicked their way through addition in the 1940s. There was a whole era when computers were visibly mechanical things, and the abstractions we now take for granted — bit, register, gate — were literally pieces of metal you could touch.

And I had been thinking about kids. Specifically, about how kids encounter the word “computer” before they have any model of what one does, and how the gap between “computer” the magical box and “computation” the precise mechanical sequence is one of the worst educational gaps in modern STEM. Whatever closes that gap is a public good.

The note was the intersection. Laser cutter plus old fluidic-logic history plus pedagogy of visible computation equals: clear-plastic adder. Six years later I still think it’s the right answer.

3. Fluidic logic is a real thing, and it used to be a serious field

The thing I half-knew in 2019 and fully understood by writing this essay: people built actual working computers out of air in the 1960s. Not toys. Not demos. Industrially-supported, multi-thousand-gate, production hardware.

The seed event is Raymond “Billy” Horton’s 1960 fluidic amplifier, developed at the U.S. Army’s Harry Diamond Laboratories in Washington, D.C. Horton showed that a small jet of air, deflected by the Coanda effect, could behave as a bistable element — a flip-flop with no moving parts. The paper landed in Control Engineering and the field exploded. Within five years there were dedicated conferences, commercial fabrication houses (Bowles Engineering being the biggest), multiple competing gate geometries, and Air Force programs funding the development of entirely-fluidic computers as candidates for high-radiation, high-vibration, high-EMP environments where electronics couldn’t survive.

The flagship build was Norden Systems’ MOD-A, around 1966 — a general-purpose digital computer made entirely of air channels. It worked. It was slow (kilohertz, not gigahertz) and big (room-sized, not chip-sized) but it computed, in the same way a contemporary electronic mainframe computed, with no electrical components anywhere.

Then ICs got cheaper. By 1975 a TTL flip-flop cost a few cents; a fluidic flip-flop cost a few dollars and was a thousand times slower. The economics collapsed. The Air Force funding dried up. Bowles Engineering eventually went under. The conferences shrank. “Fluidic logic” became a forgotten word.

There was a quiet survival in two specialist domains: gas-turbine controllers (where the working fluid is already there) and high-rad environments. But the general-purpose fluidic-logic research programme died on schedule, around the time of the first 8-bit microprocessor.

What brought it back, decades later, is soft robotics. Around 2015 George Whitesides’s group at Harvard demonstrated soft robots whose control logic was implemented in pneumatic gates inside the robot’s own body — no electronics, no batteries, just compressed air driving a small fluidic computer that decided how the robot would move. Daniel Preston’s group at Rice and Rob Wood’s group at Harvard extended this. The application is robots that can operate inside the body during surgery, or in vibration-sensitive environments, or untethered in places where batteries fail. It’s a real and active field again.

For a kids’-education pneumatic computer, none of those advanced techniques matter. The 1960s-vintage slide valves, scaled up to human-visible size and cut from clear acrylic, are exactly the right choice.

4. How a fluidic gate works, mechanically

There are two families of fluidic gates and you choose based on what matters to you.

The Coanda-effect gate has no moving parts. You shape a chamber so a jet of air naturally attaches to one of two walls. A small sideways control jet can detach it and shove it to the other wall, where it stays. Output flips. It’s elegant, it’s fast, and it’s fussy: the chamber geometry has to be right to a few percent, the pressures have to be tuned, and watching it work requires a smoke generator because the air is invisible.

The slide-valve gate uses a small moving piece — a spool, a piston, a membrane — that physically shifts position when pressure changes on one side versus the other. The spool either connects or disconnects two channels, depending on which way it’s sitting. It’s slower, mechanically more visible, and much easier to design and debug because the logic is literally the visible mechanics. You can watch the spool move.

For a clear-acrylic kids’ build, the slide-valve approach is the obviously right one. The bits are moving things. You can see them. A flip-flop is two small pieces of plastic, one in each “remembered” position. A NAND gate is a piece of plastic that slides one way when both inputs are pressurised and another way when either isn’t. The mechanism is the lesson.

For the indicators on the output, I’d use little vertical glass or clear-plastic tubes with a coloured ball inside. When the bit is 1, air pressure lifts the ball to the top of the tube. When the bit is 0, the ball sits at the bottom. The output is four (or eight) coloured balls arranged in a row, and the result of the addition is a binary number you read by which balls are up. With a small reference card (“0001 = 1, 0010 = 2, …”), a kid translates the binary into a decimal answer in their head. That little act of decoding is itself pedagogy — they’re decoding binary, which is what the machine is.

5. Laser-cut layered construction

This is the practical move that makes the project achievable at hobby gauge.

A fluidic gate is mostly geometry. The interesting features are channels and chambers, both two-dimensional in cross-section. You can fabricate a complete gate by:

Designing the layout as a stack of 2D layers in Inkscape, Solvespace, or Onshape. Each layer is one slice through the device. Some layers are channel layers (with the channels cut out). Some are cover layers (with just port holes). Some are spacer layers.
Cutting each layer out of clear acrylic on the laser cutter. A 3 mm cast-acrylic sheet cuts in about 30 mm/s at 60 W. A layer with a couple of dozen gate features cuts in 20-30 seconds.
Stacking the layers in order, with a thin film of acrylic solvent (dichloromethane, a.k.a. “weld-on”) between them.
Pressing the stack under weight overnight while the solvent chemically welds the layers into a single block.
Drilling input/output ports through the top sheet and attaching pneumatic tubing with barbed fittings.

Channel features down to about 100 μm are routine on a hobby laser cutter. With care and a focused beam you can hit 25 μm. Hobby fluidic gates work fine at channel widths of 0.5 to 2 mm — luxuriously large, high yield on the first cut.

The aesthetic bonus is that the finished stack is transparent. You can see every channel, every chamber, every slide valve. A small LED panel underneath illuminates the whole device from below, and the channels glow when air flows through them (more so if you fog the air with a trace of theatrical haze, but even dry air shows up because of the slight density difference between fast-moving and still air). The thing looks like a piece of glass with a circuit diagram trapped inside it, and the circuit moves.

6. Can you build an adder?

This was the empirical question the 2019 note ended on. The answer is yes, and walking through it is the way to see whether the project is actually feasible at the home-laser-cutter gauge.

NAND is universal. If you have a NAND gate you can build any digital logic. So we’ll build a NAND and then use it everywhere.

A NAND gate in slide-valve fluidics is, mechanically, two small spools in series with a pull-up channel from the air supply. Default state: output is pressurised (= 1) because the pull-up channel feeds it. When both inputs are pressurised, both spools shift to a position that opens a vent path from the output to atmosphere, so the output goes to 0. When either input is at atmospheric pressure, its spool stays in the default position, keeping the vent closed, so the output stays at 1. Standard NAND truth table:

A	B	NAND(A,B)
0	0	1
0	1	1
1	0	1
1	1	0

About 2 cm × 1 cm of acrylic real estate per gate, four ports, two moving spools.

A half-adder from NANDs is two outputs: SUM = A XOR B and CARRY = A AND B. Built entirely from NAND gates that’s 5-6 gates total if you share the inner subexpression, 8 if you don’t.

A full adder is two half-adders plus an OR gate (for the carry-out), and OR is three NANDs. Total: about 12-15 NAND gates per bit of addition.

A 4-bit ripple-carry adder is four full adders chained, so about 50-60 NAND gates total.

At 2 cm × 1 cm per gate plus some routing space, the whole adder fits comfortably on a piece of acrylic about 20 cm × 30 cm. That’s the size of a typical laser-cutter bed. Six or eight 3 mm sheets stacked = about 25 mm thick. The whole device is the size of a hardcover book.

Speed: about half a second per addition. Slide-valve fluidics at hobby scale switches in 10-50 ms per gate, and the carry has to ripple through four full adders, so end-to-end maybe 200-500 ms. The kid can watch the result settle. The slowness is the entire point.

Latches and flip-flops are similar: an SR latch is two cross-coupled NAND gates (so 4 spools), a D flip-flop is a few more. With latches you can build registers, and with registers you could build a small accumulator that adds successive inputs over time. The 4-bit incrementer (counts up by 1 each time you push a button) is the right starting point: simpler than a general adder and just as educational.

7. The educational design

This is where the kid-pedagogy details matter, and where the project goes from “interesting maker project” to “actually-pedagogical artifact.”

Big inputs. Four big plastic toggle switches across the front, labeled bit 0 through bit 3. The kid sets the input by physically clicking them up and down. Tactile, deliberate, the input is a thing they did.

Big indicator balls. Four (or eight, if you want to show input and output) tall clear tubes across the top of the device, with brightly-coloured ping-pong-ball-sized indicators inside. High = 1, low = 0. The kid sees four balls do their thing every time.

A small reference card. “0000 = 0. 0001 = 1. 0010 = 2…” laminated and Velcro’d to the side of the device. The kid translates the binary output into a decimal answer. The reference card is part of the learning.

Visible carry. The wiring between full-adder cells should be visibly distinct — different channel colour, brighter LED behind it, something — so that when the carry ripples from bit 0 to bit 1 to bit 2 to bit 3 over the course of half a second, the kid can see the ripple happening. The carry is the thing that makes addition interesting (it’s why 9 + 1 = 10, why kids learn carrying in second grade), and being able to point at the carry physically moving through the machine is the moment the pedagogy lands.

A reset button. Big, red, satisfying. Vents the entire device. The balls fall back to zero. The kid does it again.

No electronics anywhere visible. Yes, you might use a small microcontroller to drive solenoids on the input toggles (so the device can demo itself in a loop unattended), but the microcontroller lives inside the base, and from the kid’s perspective the whole computer is the transparent block. The point is that a thing made of nothing but air and plastic does math.

An information sign. A small placard near the device with a two-paragraph “what is this” — enough that a teacher or parent can read it and explain. The placard should say what NAND means, what addition in binary looks like, and that the entire computer in your phone is just an enormous and very fast version of this. The intellectual jump from “this slow plastic adder” to “the phone in your pocket” is the headline lesson.

8. Why this hasn’t been built much

A handful of YouTube videos and Hackaday articles exist of hobbyist fluidic-logic projects. The Hackaday piece by Jenny List from 2018 is a fair survey. Most of these build a single gate or a 1-bit demonstrator and stop. Almost nobody has built a working multi-gate fluidic adder at hobby scale and posted about it.

I think the reasons are:

The application case is invisible. If your goal is “build a computer,” then a fluidic computer is a thousand times slower than the microcontroller in your phone, and roughly nobody who knows enough to build the fluidic computer is unconvinced of the microcontroller’s superiority. The “what is it for” question kills most attempts.
The pedagogical case is right there but undersold. If you reframe the goal as “build a thing kids can understand,” the slowness becomes a feature and the application case becomes “in the corner of a classroom or a maker space, for years, doing one job well.” Nobody seems to have led with this framing in public.
It’s a real project. A weekend isn’t enough. Four to six weekends is realistic, plus a couple of iteration cycles where gates don’t quite work. The maker who decides to do it has to commit to seeing it through. Most maker projects are afternoons.
Pneumatic stuff is loud. A workshop compressor isn’t suited to a classroom. The fix is to drive the demo with a small quiet diaphragm pump or a battery-powered miniature compressor running intermittently. Whitesides Lab’s soft-robotics work has good examples here.

None of these are unfixable. They just need someone to take the pedagogical framing seriously and put in the four weekends.

9. The plan

Concrete enough to start tomorrow.

MVP: a 4-bit incrementer. Takes a 4-bit input (set by four toggles), outputs input+1 (on four balls). About 50 fluidic gates. Carry ripples visibly from low bit to high bit. Reset button vents the device. Half a second per operation. A small placard explains.

Materials:

6 sheets of clear cast acrylic, 3 mm thick, roughly 30 × 30 cm. About $50.
A roll of 0.5 mm latex sheet for slide-valve seals. About $10.
Dichloromethane acrylic cement. About $15.
Pneumatic tubing (4 mm OD) and a bag of barbed fittings. About $30.
A bag of clear plastic ping-pong balls and clear acrylic tubing for the output indicators. About $20.
A small quiet diaphragm pump or a 12 V mini-compressor with a tank. About $80.
An LED panel for backlighting. About $40.
A laser-cut wooden base to mount it all in. About $30.

Total: about $275 in materials.

Tools:

A 60 W or 80 W CO₂ laser cutter (Hacker Dojo, TechShop, your friend’s garage, the local fab lab).
Inkscape or Solvespace for the layout. Solvespace is better because the constraint solver lets you parameterise channel widths once and recut quickly.
A drill press for the input/output ports.
Generic workshop tools (clamps, sandpaper, isopropyl alcohol for cleaning between weld steps).

Build cycle:

Weekend 1: Design and cut a single NAND gate. Test it on the bench with a hand pump. Iterate the channel geometry once or twice until it switches cleanly. Document the working geometry as a reusable cell.
Weekend 2: Tile the NAND cell into a full-adder block. Six NAND gates wired into half-adder + half-adder + OR. Test all 8 input combinations. Time it.
Weekend 3: Lay out and cut the 4-bit incrementer — four full-adder cells in a row, plus the input toggle interface and the output indicator manifold. Assemble.
Weekend 4: Build the wooden mounting base. Wire the toggles to the inputs. Tune the pump pressure (probably 5-10 psi). Add the LED panel. Add the reset button. Photograph the whole thing in action. Write up the build guide and put it on GitHub.

Optional, weekend 5-6: Add a small Arduino in the base that can demo the device unattended — cycle through inputs 0 through 15 every ten seconds so visitors who walk by can watch it count.

Open-source the whole thing. Solvespace files, BOM, build guide, photos. The point is that the second one of these costs four weekends; the hundredth one costs an afternoon because somebody has done the laser-cut layout work already.

The end state is a transparent acrylic block on a wooden base, about the size of a hardcover book, sitting on a table at the front of a classroom or in the kids’ corner of a maker space. Coloured balls across the top. Toggle switches across the front. Reset button on the side. A small placard. A child walks up, flips a few toggles, watches the bits ripple across the device, reads the answer, smiles.

That is what I had in mind in 2019. The note didn’t say so explicitly because it was a note to myself, but that’s the project.

10. Why this is worth a weekend

I’m going to be direct. The world has plenty of educational kits for kids that purport to teach computation. Most of them are LEDs on breadboards driven by an Arduino, where the Arduino is a black box doing the actual work and the LEDs are decoration. The lesson the kid walks away with is “I plugged things in and they lit up.” That isn’t computation pedagogy. That’s plumbing.

A pneumatic adder is different. There is no Arduino doing the work in secret. The plastic block is the computer. Every bit visible. Every gate visible. Carry visibly rippling. The kid who watches it has, in some real sense, watched a computer add. They can take that mental model with them into every screen they ever look at afterwards, and the screen will be less mysterious.

That’s worth the weekend.

It’s also, by the way, a beautiful object. Multi-layer clear acrylic with channels carved into it, illuminated from below, coloured balls bobbing on top, makes for a striking display piece. The aesthetic case is not nothing. Maker spaces compete for foot traffic and the “oh, what’s that” reaction is real. A pneumatic computer in the front window gets people to walk in.

So this is the design. The cost is $275 and four weekends. The output is a transparent device that adds binary numbers in plain sight at half a second per operation. The audience is every kid who walks past it for the next five years.

I’m going to build it. If you have a laser cutter and four weekends of your own to spare, you should too.

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