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Sphynx electrolysis development journal entry #4: boards but IRL this time 🦾 - Hexbear
hexbear.netHello, comrades! I'm mostly not sick any more! Sorry so much for the week plus
since my last post, most of the time I spent in that timespan was doing other
things while I waited for components to arrive (especially because there's stuff
I forgot in the first order and had to make a second one screm-a
[https://hexbear.net/pictrs/image/120ccc38-035f-4901-8b34-fa93f333a6d6.png]).
Components got here, I put everything together, and things... work more than
they don't work spongebob-party
[https://www.hexbear.net/pictrs/image/495ba447-99c5-418b-9258-ed30a9fc1222.gif]
Last Post [https://hexbear.net/post/2085333] # What I've been up to Behold:
Show
[https://hexbear.net/pictrs/image/28959be3-1a39-4fbe-9f58-b7719401b0fa.jpeg]
Show
[https://hexbear.net/pictrs/image/406f87fa-554f-4605-bd3a-0f9d6d4d5e52.jpeg]
The boards are in and I made them and they work trans-heart
[https://www.hexbear.net/pictrs/image/aec9b839-d11e-4611-9263-60a84384ac89.png]
This segment of the post is going to be dense and technical, so feel free to
skim and ask questions if you feel lost and want to get caught up! Both boards
have minor bugs that I will be addressing in the next revision - the next
revision being the Alpha 1 version of the Sphynx Lite. The bugs I have are as
follows: ## Current pump bugs: - The knob is wired backwards. fix: make the knob
go forwards, or more specifically, switch pins 1 and 3 on VR1. Ez. - The knob
doesn't have a zero offset - instead of 0.1 mA to 2 mA, it goes 0 mA to 2 mA.
This is annoying; if you turn the knob all the way, the device won't do
anything, you have to bump it a tiny bit. The solution is to put a resistor of
roughly 11 kiloohms on the low-side leg of the potentiometer (exercise for the
electronics-curious reader to figure out why! check the schematic from my
previous post!). The wide tolerance on the potentiometer means there will be
somewhat of a range on the actual minimum current, but it should be a pretty
small variation, around 0.08 mA to 0.12 mA worst case variation. Since this
error 1) gets stacked up with the error of the user's body being different from
everyone else's, and 2) is only error on the low side, not on the high side, I'm
deeming it tolerable. - Not a circuit bug per se, but I designed this around a
INA350CDS, which has 50 times gain, and then I accidentally bought some
INA350ABSes, which only have 20 times gain. This part is a new addition that
uses R1 as a shunt for active current measurement. It works exactly as intended,
just with 20 gain instead of 50 gain. This can be used for a lot of things, but
particularly, an automatic shutoff of the board if the current draw ever goes
above, say, 2.5 mA. More safety! However, this component has another problem,
which is... - The INA350 is fucking impossible to solder angery
[https://www.hexbear.net/pictrs/image/e639b50e-a115-48d7-af0b-1958b2397480.png]
All these new space saving packages are absolute nightmares for hand assembly. I
didn't even need space savings, it was just a cheap instrumentation amplifier
that worked nice. I think I might have to ditch it because it doesn't come in a
usable size. The same thing can be done with a dual opamp and some bonus
resistors, which is less than $0.50 and I already have them on the board. ##
Digital timing bugs: - Backwards knob again... - Ignoring backwards knob, the
range, instead of being 2 seconds to 10 seconds like I expected, is actually 2
to 7.5ish seconds. I haven't actually figured out why on this yet - I think it's
because of the relatively simpler resistor divider thing I did with R2/R4/VR1,
and there's some other current path I haven't thought about yet through the
potentiometer. Could be something else too. In any case, the move here is
probably to do something more like in figures 8 and 9 of the LTC6993 datasheet
with an opamp current sink and a potentiometer, and something less like what I
did, which came from the unlabeled figure on the last page of the LTC6993
datasheet. Again, one more part, but opamps are pennies and I use them
elsewhere, which drives the cost down further. - The LTC6993 is one of the
microtiny packages too. This is a little worse than the other microtiny package
in the INA350, in that I can just buy another inamp or an opamp for that one,
but the LTC6993 is fairly unique and I can't just shop for another part that
does the same thing. I'm not sure what to do about this. It's too late to add a
microcontroller, but this timing issue just won't let up. I think the difficulty
to solder this one might just be a thing to fix in the Lite 2. Technically, you
need this level of coordination and fine motor skills to do the actual
electrolysis hair removal anyways, and I was able to do it with nothing but an
iron, a solder sucker, and a magnifying glass, so it's not impossible and anyone
with even slightly better tools than me should be okay, but it's still a bummer.
From now on, new rule, nothing smaller than a DFP package. If I get rid of the
INA350 and this, then there's no more. - The done alert doesn't work. The
intention was that LED D3 turns on and buzzer LS1 beeps for half a second when
the current pulse is done, alerting the operator that it's time to move to the
next hair. This circuit is everything connected to U4 in the schematic in my
last post. Not only did I pick the wrong kind of buzzer (very easy fix I just
need to shop better), but the whole timer sticks on forever instead of just
flashing on for half a second. I didn't know this at the time, but if I were to
read the damn datasheet [https://www.ti.com/lit/ds/symlink/ne555.pdf], I would
have seen on page 10 that: "Monostable operation is initiated when TRIG voltage
falls below the trigger threshold. Once initiated, the sequence ends only if
TRIG is high for at least 10 μs before the end of the timing interval."
Basically, the 555 wants to see a super quick off/on to start the timer, and I'm
just giving it an off with an indefinite delay after, causing the timer to stick
on. I don't really know how I'm going to handle this to be honest - my current
best guess is to make some kind of quick and cheap RC high pass filter thing
hooked up to a transistor that turns a high to low edge into a high to low to
high pulse. I'll have to breadboard or simulate it a little bit, but if I can
make this work it's a very cheap fix to the problem. I'll be fixing this list of
issues and moving both circuits to a new united design with test points and
jumpers this time (thank you @macerated_baby_presidents@hexbear.net
[https://hexbear.net/u/macerated_baby_presidents] trans-heart
[https://www.hexbear.net/pictrs/image/aec9b839-d11e-4611-9263-60a84384ac89.png]
) that will be, if it works, the first alpha version of the Lite. Progress! I
also cleaned up git - not sure if anyone has tried to pull down the repository
and look at the boards yet, but if you have, it was broken - it should be fixed
now, but you'll probably need to re-clone. If anyone tries this, let me know how
it works! # New Developments I bought a domain name! Meet sphynx.diy
[http://sphynx.diy] meow-melt
[https://www.hexbear.net/pictrs/image/e6149c4c-a145-40d5-a34c-e9cb9bb9bb9d.png]
Currently empty, just points to the git repo, but you have to start somewhere -
this is where I want to host blog posts like this one, the assembly guide, and
the usage guide and resources in the future. I'm not going to be doing much with
it immediately myself, but having this makes it so we can start working on an
actual site! I was thinking GitHub pages for hosting with a cute Jekyll theme,
or maybe readthedocs like mentioned last time - thoughts? Also! I used the
boards above to remove a couple square centimeters of hair as a proof of
concept! It's only been a couple days but I'm optimistic - I tested by tugging
on the hair with tweezers, observing it tightly connected to me by the follicle,
applying about 10 units of lye per Figure 2 of this very helpful resource I
found [https://electrologynow.com/wp-content/uploads/2021/05/English-GUIDE.pdf],
and then after current application, pulling again, for the hair to slide out
with no resistance, which is a very strong indicator that it worked! Stay posted
to see if they stay gone or if I need more juice, 10 units of lye is on the very
low side but I'm playing it safe. # Next up I'll absolutely be fixing up the
boards as mentioned above and making and designing an Lite Alpha 1.0 next! I
don't want to suggest that this one be used on human beings, but once I make it,
it might be fun to buy and assemble for research purposes? Probably not, maybe
best to wait until I at least make it to a beta version, but in any case I'm
excited for it! It's also not too early to get work on a manual started - this
is something that I'm going to try to lean on community help for. I'm also
exploring JLCPCB assembly - it would be much better for accessibility if I could
keep 100% to their parts catalog (plus I wouldn't feel bad about using microtiny
packages any more), and for now I think I'm close, but I know that there's at
least a potentiometer I use that isn't in their catalog. # Any ways to help?
There are starting to be more things to do! I'm kinda feeling like it might be
time to start pulling in folks for web stuff! If anyone has GitHub sites or
Jekyll experience, getting going on a place to host an assembly guide would be
awesome! This is also something that should be doable even without this
particular experience if you know web stuff, so don't feel intimidated - I think
this is accessible with a bit of new learning for anyone with intermediate web
knowledge. Another thing I could totally use help with is BOM management - I
have links for all the parts, but I don't have them linked anywhere and I don't
have them automated, tallied up based on cost, in a convenient one click buy
cart, or anything else like that - if anyone likes making bills of materials in
KiCAD, let me know, that would be super helpful! In a similar vein, having an
audit and maybe a port of my design to 100% JLCPCB catalog compliance would be
extremely nice, if either of those things sound interesting to you, let me know!
--- As always, stop by, hang out, say hi, ask questions, tell me what you've
been up to, design review me, however you'd like to be involved is good by me!
All the love and I'll talk to you in a week or so! meow-hug
[https://www.hexbear.net/pictrs/image/28d86083-caf1-46d9-b565-569425bb5451.png]
we’re back besties
Sphynx electrolysis development journal entry #4: boards but IRL this time 🦾 on !diy@hexbear.net
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lmao new Hexbear usability bug discovered
I’m gonna move the link from the URL section to the post body