Category Archives: Engineering

Uncovering the Silicon: μL914

At the 2018 Bay Area Maker Faire, our project Uncovering the Silicon showed off a number of simple and complex integrated circuits (with rather large feature size) under the microscope. We had a great time helping visitors look at the features and get a glimpse of what’s inside those black box integrated circuit packages. To take this to the next level for this year’s Maker Faire, we decided to try and close the loop; to take one simple integrated circuit and elucidate its workings well enough that visitors to our booth will be able to see every single component of the circuit, understand their function, and relate it to the macroscopic behavior of the chip. For this, we picked what turns out to be a rather obscure chip: the Fairchild μL914, which is a dual 2-input NOR gate. This chip belongs to the resistor–transistor logic (RTL) family.

uL914 IC closeup in circuit

Here’s what the chip looks like. It’s in a funny old “glob-top” can package with eight leads.

pinout

Here’s the pinout; there are two NOR gates in the chip, plus power and ground.

uL914 IC package circuit with switches and LEDs

Ken Shirriff built a circuit with the chip to demonstrate its functionality. When we push either of the two buttons for one of the gates, that LED will turn off.

Schematic

Here’s the schematic diagram, adapted from the original datasheet. If you look at the left side, if either of those inputs goes high, the transistor pulls the output low.

uL914 dual 2-input NOR gate die photo

John McMaster decapped a few of the chips and sent us a die photo. He made a video about the process — no small feat. We’ll be bringing one of these bare chips and a microscope (equipped with both eyepieces and a camera) to Maker Faire.

For the macroscopic scale, we approached visualizing this circuit from a couple of angles: the physical structure of the chip, and the electronic structure of the circuit.

uL914 die model render

Eric Schlaepfer used the die photo to model the structure of the chip in CAD.

uL914 PCB version

Simultaneously, Ken designed a printed circuit board version for use with discrete components that maintained the same structure as the IC.

Individual transistor acrylic model Individual transistor acrylic model

Working from Eric’s CAD model, we built a single NPN transistor model from layers of colored acrylic. If you lift it up, and look through the transparent middle layers, you can tell that the emitter (red) is embedded into the top of the base (yellow) and does not go all the down way through it. (Transistors like these are planar: The emitter is above the base, and the base is above the collector.)

The top layer of this little model has labels for the collector, emitter and base. It is removable so that the layers of the model can be more easily inspected.

The model of the chip die includes a transparent cover representing the oxide layer, and that supports the metal layer with the wire bond pads on the edges.

One of the reasons that this particular chip is educational to look at is that there are a few unused components on the die. There are two unused transistors: one of them is unconnected, and the other is shorted. There are also several unused resistors (resistors are the dogbone shapes). The unconnected and unused components are easier to see, and provide a visual example that is useful for understanding what the connected components look like under the metal layer.

It is also fun to imagine what other circuits could have been made with different connections.

We glued most of the layers together, but left the top two layers removable so that it is easier to see the internal structure when the top is removed.

(Aside: we left out most of the epitaxial pocket material, because even though we used transparent acrylic to represent it, the layers of the components are much more visible without it present.)

Acrylic chip model top with metal layer

There are cutouts in the oxide layer where the metal layer connects to the circuitry below.

Acrylic chip model with hot glue bond wire example

One of the most noticeable things you see when you look at this type of IC under the microscope is the bond wires. We’ve used silver glitter hot glue sticks to represent them.

bond wire closeup

The glob of melted glue represents where the wire is bonded to the pad.

Acrylic chip model with hot glue bond wires

When you look straight down on the model with its glitter bond wires, it looks very similar to what you’ll see in the microscope.

Acrylic model legend

To round things out for our acrylic model, we made a physical legend to make it easier to identify all of the parts of the model.

uL914 discrete version switch and LED circuit

Once Ken got his PCBs back from our friends at OSHPark, he built it up with the same example circuit.

discrete circuit closeup

The PCBs turned out beautifully, and it’s great to see the familiar discrete packages on the enlarged circuit. Ken has published the PCB design on Github.

We hope to see you at Maker Faire this weekend!

Bonus: Ken laid out some hypothetical alternate metal layers to use the same die to create different chips.

AxiDraw on MythBusters Jr

I’m super excited to see the new MythBusters Jr episode tonight, because I’m in it with the AxiDraw!


I had a fun time working with Valerie, Rachel, and Elijah, as well as the great crew.

Stroboscopic Ornaments

Jiří Zemánek, whose EggBot work we have featured before, sent in this wonderful video of stroboscopically animated Christmas ornaments.

The stroboscopic patterns are designed in MATLAB and drawn by EggBot Pro on colored glass Christmas ornaments. Motion of the balls is controlled by custom mechanism built using components from two Prusa i3 MK3 3D printers, like six stepper motors and two Rambo boards. On top of designing the patterns, which is Jiri’s hobby (when he is not busy with research) and building the whole contraption in a very short time, the team had to deal with issues including non-spherical ornaments, or how to use Rambo board to precisely control the velocity profiles.

We love to see how people make things, and Jiri did not disappoint, sharing process photos of making the rotation mechanisms.

With great help from his colleagues Martin, Krištof, and Filip they took Christmas ornaments to the next level and taught them to dance!

The final setup shot captures how they created such a beautiful video.

Merry Christmas to Jiri and the Advanced Algorithms for Control and Communications group! Thank you for sharing your project!

PCB Etching with AxiDraw

Patricio Gonzalez Vivo has been using AxiDraw for circuit board etching experiments.

He draws on copper clad boards with a Sharpie marker with the AxiDraw.

Drawing is followed by a chemical bath.

The marker is cleaned off of the remaining copper to reveal the design. It came out beautifully.

If you’ve used the AxiDraw for marker masking for etching, we’d love to hear about it!

AxiDraw Pantograph

Mike Jacobs added a pantograph to his AxiDraw to multiply its size range.

Not content with that, he doubled it.

Bigger!!! I extended the extension—now it’s a double pantograph!! The #axidraw #plotter draws ~8.5×12” out of the box. My pantograph increases that to 45×58”. The precision has gone hell and there’s some warping—

Continuing the saga,

Increasing unpredictability! I added a swiveling mini roller. Now I really have no clue what a finished piece will look like, and there’s a good chance that no two pieces will look alike.

It is always exciting to see what creative things people do with an AxiDraw!

Science Hack Day SF

I will be at Science Hack Day SF giving a lightning talk on Creative Off-Label Tool Use featuring some of the cool and unusual ways people are using AxiDraw and other tools we make. I’ll also have an AxiDraw in the hardware hacking area to play with.

Science Hack Day is October 27-28 and is free, so register now!

P.S. If you’re doing something interesting and science or research related with your AxiDraw, please let me know!