Colorful Homemade TO-92s

Cursed sand and trans-istors! Messing around with one of electronics' most iconic form factors.

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I want to make components that look like TO-92s, but colorful! And I want them to work, of course. I'm eager to hear other people's suggestions!

image source: wikipedia

Even if you've never heard the term "TO-92," you're likely to be familiar with the shape. These friendly tripods are most often holding some type of transistor, but sometimes you see temperature probes, LDO voltage regulators, or even EEPROM in the TO-92 package. But whatever function it has, it's almost always the same boring black epoxy material. And the rare cases when it isn't boring black epoxy, it's some other color purely for utilitarian purposes. Yawn.

My original goal was to make my own TO-92 sound ICs. The quick summary is that there are some super-simple music chips with only 3 pins: power, ground, and audio output. I'm really happy with the songs I've managed to play off an ATtiny10, which critically has just 1 kB of flash and 32 bytes of RAM. There's so much I want to talk about with the ATtiny10 music chips, however, that I had to spin it off into another project writeup.

The one key detail from the sound IC project is that these sound ICs often come in TO-92 packages, which is how we got here. Originally I wanted make the most convincing production-line imitations for them, but ultimately I decided "fun colors" was way better than "authenticity". I started to want transistors that look like lee cyborg's amazing alligator clips.

What can we cram inside a TO-92?

There's enough room inside a TO-92 body to house all sorts of surface-mount parts. But then there needs to be a way to connect them to the three legs that lead out of the body. My first instinct, as usual, was "make a wee PCB".

These are flex PCBs from OSH Park. I put a SOT-23-6 footprint on one side, and 3 long vertical pads on the reverse. With this, I can turn an ATtiny10 into a music chip, a B-E-C BJT into an E-B-C transistor, or a G-S-D MOSFET into an S-G-D MOSFET (two traditional pinouts for their TO-92 forms). I suppose you could also make a C-B-E BJT or a D-G-S MOSFET if you enjoy chaos. But it still doesn't look right!

Improving the TO-92ness

While visiting a good friend and Montréal maker, I did this earnest, clumsy attempt at building a mold and casting a chip in 2-part epoxy.

I think if I kept at it, I could have perfected my technique and gotten consistent good results. Maybe. But things got in the way and I lost interest in this approach. And then I got consistent access to a pretty-nice SLA 3D printer, which meant I could make detailed thin-walled prints like this:

You can make them yourself, the files are on Printables. I'm so happy with these little caps, they fit so nicely over the wee PCB without any play. And printing in clear resin means I can dye each individual batch without much fuss!

Which is how we got these 🏳️⚧️transistors🏳️⚧️

And, critically, they're still functional!

Package markings

Every chip package needs some identifying marks, right? I can fit three lines of four characters on each package, more or less. Orrrr I could do something like this:

Emily Velasco found these Sumerian logograms: 𒀾 aš "curse," 𒅖 sahar "sand". Cursed sand, that's what silicon chips are, right?

Some of the other chips just say "HaD 22" because I plan to hand them out at Supercon 2022. Others have the Linear B 𐂂 deer ideogram 𐂂.

Future work: Mass production? Leadframes?

Right now, my only plan for these things is to give them away to friends. I'd LOVE to coordinate a short manufacturing run of colorful transistors and sell them as a fundraiser for some queer- and trans-focused charities I'm fond of. But I don't know how to turn this into something a factory can manufacture! Could I call up an electronics packaging factory and ask them to churn out some pink, purple, orange, etc. transistors?

For now if you really want one, your best bet will be to find me at Hackaday Supercon 2022, which is a little over a week away at time of writing.

  • 1
    How I assemble these (your mileage may vary)

    If you want to make your own TO-92s in the same way as I'm doing it:

    Get the 3d printed shell file from Printables and make a bunch of them. I printed them in clear resin on a Formlabs printer and dyed them in Jacquard Piñata alcohol inks. Nota bene for the alcohol inks: the inks are suspended in mostly ethanol, not isopropyl alcohol. The opaque "blanco" ink does not disperse in IPA! You have to add it to fairly concentrated ethanol, and even then you have to agitate it constantly to avoid getting big congealed lumps of pigment.

  • 2
    Step 2

    Get the little SOT-23 holder PCBs from the git repo or direct from OSH Park. Be sure to order a flex PCB, nothing else will work. The 4×4 array has score lines that make the boards easy to snap away from each other.

  • 3
    Step 3

    Find a sacrificial TO-92 to act as a leg donor, and line it up with the flex PCB on a hot plate, like so:

    I'm using a spare PCB to keep the legs roughly coplanar with the PCB.

    There's a via in the middle leg pad that can help you line up the PCB properly:

    I realize now that the text is unclear. "E-B-C BJT" and "S-G-D MOSFET" mean that, if you attach a SOT-23 transistor with Base-Emitter-Collector pin arrangement, or a SOT-23 MOSFET with Gate-Source-Drain arrangement, you can produce an Emitter-Base-Collector TO-92 or a Source-Gate-Drain TO-92. These are the common SOT-23 and TO-92 pin arrangements here in North America.

    As an aside, I have no idea what The "FKA-Style-A" PCB is supposed to be! It is not mine. Do you know what it does?

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Robert Langer wrote 11/06/2022 at 10:27 point

Wow, I love this! A source for the rectangular wire should be found, because, one day, there are no more of the guys to sacrifice for their own recreation! ;-)

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Sam Ettinger wrote 11/07/2022 at 17:07 point

I don't know how to find standalone legs in quanitites under ~100k! I'd like to try some kind of improv leadframes, something that combines the legs' and the flex PCB's functions, someday.

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Dan Julio wrote 10/31/2022 at 14:06 point

Another clever project, Sam.  Very fun.  My suggestion are photodiodes or transistors and LEDs of course!  With small enough components you could even pack fairly complex circuits in one.  

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Sam Ettinger wrote 11/07/2022 at 17:08 point

Thank you! I have been really enjoying hearing people's ideas from this :D

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