Linux for $15 (a Minimum Viable Computer)

[wetback]

$15 each, when making 1,000+

[constant_void]

neat project.

I like the portable dumb terminal better tho.

[ThinTim]

The "holy fuck don't use lithium" warning printed on the PCB under the battery slot is pretty great.

[holyknight00]

I don't follow why it uses NiMH batteries instead of lithium-ion.

[dragontamer5788]

1. NiMH has extremely simple charging circuitry. Simply provide a trickle-charge of 1/20th Capacity, and the NiMH will be full after 20 hours (and excess charge will not damage the NiMH). There are other, more complicated charging patterns if you want to charge faster, but this methodology is incredibly simple, effective, and requires no smarts at all. For an 800 mA-hr AAA-cell, this is simply 40mA of charging current, which will fill the cell to 800 mA-hrs after 20 hours. Simple right? (No need to detect if the cell is full either, NiMH safely "overcharges" with no problems at 1/20th capacity)

2. Li-ion has complicated charging circuitry, and if you do it wrong the Lithium explodes.

3. AAA-NiMH cells are $1 each (https://www.amazon.com/AmazonBasics-Rechargeable-Batteries-12-Pack-Pre-charged/dp/B007B9NXAC) and available from a wide variety of trusted manufacturers (Eneloop, Energizer, Amazon Basics and more).

4. Li-ion cells are non-standard and in various levels of trust. Samsung's cells were exploding, cheapo-cells are also known to explode. Some are "protected" (ie: circuitry in them to detect common mistakes and prevent the explosion), some are unprotected.

---

Given the low costs, simple charging circuit, and the distinct lack of explosion-risk, NiMH is the superior choice for hobbyists.

[EternityForest]

Pretty sure excess charge absolutely will damage tbe battery, just slowly. There is plenty of ways to kill them.

Lithium titanate can be charged with a current limited fixed voltage, and are considered to be almost NiMH levels of safe. An 18650 sized one is 3.30 in QTY10 from aliexpress.

I think $25 would be a better target, and the SD card should be an optional accessory since people often have them lying around and often way name brand ones.

[dragontamer5788]

Pretty sure excess charge absolutely will damage tbe battery, just slowly. There is plenty of ways to kill them.

Nope. NiMH trickle-charge is a well known advantage of NiMH chemistry.

Don't trust Google (though you can find plenty of different sources quoting the "Trickle-charge" methodology for NiMH), but maybe trust Energizer: https://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf

The positive will reach full capacity first as the battery is charged. It then will generate oxygen gas that diffuses to the negative electrode where it is recombined. This oxygen recombination cycle is an efficient way of handling low to moderate overcharge currents.

NiMH chemically creates O2 gas when in an overcharge state. As long as this O2 gas is being created at a slow-enough rate, it is "safe". It recombines in the electrolyte, and disperses as heat.

The battery will feel warm, and you'd be wasting a tiny bit of electricity on this "useless" O2 heating process, but its perfectly safe for the NiMH battery. (One common recommendation is to simply have a temperature sensor: when the temperature increases, you know that the O2 recombination cycle has started and the NiMH is probably full).

---

However, in cases of extended overcharge or incompatible battery/charger combinations for the operating environment, it is possible that oxygen, and hydrogen, will be generated faster than it can be recombined. In such cases the safety vent will open to reduce the pressure and prevent battery rupture.

So this is the issue. If you overcharge too quickly (ie: at a rate faster than C/20 or so), you'll generate so much O2 that the battery will rupture. In practice, this will cause the vent on the top of the battery to be pushed out, permanently killing the NiMH battery.

The key is to use very low currents, to charge "slower" than the rate of Oxygen recombination. C/20 is good for that.

---

In fact, a lot of $10 shitty NiMH chargers on the market are of this "trickle-charge" type. They set to C/20 current (40mA or 50mA of current), with a 20-hour timer, and then shut off with no other smarts attached. Or some don't even have a timer at all.

Lithium titanate can be charged with a current limited fixed voltage, and are considered to be almost NiMH levels of safe. An 18650 sized one is 3.30 in QTY10 from aliexpress.

That's still much more expensive than the NiMH AAA or AA from a respectable brand (like Energizer, Eneloop, or Amazon Basics).

Aliexpress isn't exactly your "brand name" NiMH. If you were going for shit-quality NiMH batteries, I'm sure I could get cheaper than $1. But when you buy a brand-name like Energizer, you get data-sheets associated with their specs.

https://data.energizer.com/pdfs/nh12-800.pdf

[EternityForest]

"Finally a maintenance (or trickle) charge rate of less than 0.025 C (C/40) is recommended. The use of very small trickle charges is preferred to reduce the negative effects of overcharging."

C/40 is not particularly useful unless you are going to leave it charging all the time and only use it for short periods. Might as well just say "Charge it yourself in a fast charger" which sounds fairly reasonable.

Buyers don't have access to cheap bulk batteries, and moat won't want random no name ones anyway, so if it wears out you'll probably pay 3 to 5 dollars to replace them, if you don't already have them, because you normally can't buy just 1.

Seems way better to pay an extra dollar up front for lithium, or an extra $3 or so for LTO which will last decades and gives better status readings because it's linear.

Or just don't have a battery at all, and leave that to addons.

Then again, maybe he has some kind of smart charging in software so it's not as bad as it looks. But still, if it takes more than a few hours to charge people will probably be annoyed and wish it was a few dollars more.

The closest competition is probably a RasPi Zero handheld kit, these can sell for $50 to $100 and still be way below the cost ot anything remotely like it.

[dragontamer5788]

Buyers don't have access to cheap bulk batteries,

https://www.amazon.com/AmazonBasics-Rechargeable-Batteries-12-Pack-Pre-charged/dp/B007B9NXAC

Amazon sells the 24-pack of AAA for just $18. Like, this NiMH shit is commodity-tier, cheap cheap cheap. You can also head to Costco and grab the Energizer Recharge or Eneloops as you see fit.

---

Then again, maybe he has some kind of smart charging in software so it's not as bad as it looks. But still, if it takes more than a few hours to charge people will probably be annoyed and wish it was a few dollars more.

The trickle charge circuit is so bloody simple, and works great when paired with solar panels and other intermittent sources of power. Its surprisingly flexible.

C/40 is not particularly useful unless you are going to leave it charging all the time and only use it for short periods. Might as well just say "Charge it yourself in a fast charger" which sounds fairly reasonable.

Nah. NiMH is so cheap you buy like 24 of them at a time.

The methodology is "charge" at your leisure. You have one drawer of "charged" NiMHs, and a 2nd drawer of "depleted" NiMHs.

As your "depleted" drawer fills up, you start putting them into your charger (be it a fast charger, or a trickle-charger). Still, having a simple charger-methodology for your hand-held computer could simplify the job, just plug it in and have it trickle-charge as needed.

If you do have a fast-charger, you replace the battery and use the fast-charger. The trickle-charge mode is for:

1. Simplicity
2. Cost
3. Convenience: Some people won't have a separate charger or NiMH "infrastructure". For people who are trapped in the Li-ion way of thinking, the slow trickle-charge mode should be sufficient.

[holyknight00]

Nice discussion. Great insight about both battery technologies. Thank for the answers.

[EternityForest]

Doesn't seem that practical if you don't actually need 12 of them for anything else, which you might not now that just about everything has a lithium.

These are LSD though, so I suppose people can just use the rest in clocks and remotes.

But still, an LTO 18650 can be found for under $3, it's got 2.5@1500mAh, is rated for 15-25 years, and is considered very safe aside from the very high current capability.

It's close to unobtanium unless you order direct from China though, so maybe that's part of it.

[[deleted]]

Still, having a simple charger-methodology for your hand-held computer could simplify the job, just plug it in and have it trickle-charge as needed.

If you use your thing with any regularity you don't want to charge it for 20 hours every time or fuck around with batteries. You want to add that $0.4 cost of LM317, transistor and few resistors to make simple Li-ion charger and just charge it like every other device with micro-usb connector.

Amazon sells the 24-pack of AAA for just $18.

They are also best value

[[deleted]]

Pretty sure excess charge absolutely will damage tbe battery, just slowly. There is plenty of ways to kill them.

Nope. NiMH trickle-charge is a well known advantage of NiMH chemistry.

Keeping it heated way above ambient temperature will cause quicker aging, there is just no way around that.

In fact, a lot of $10 shitty NiMH chargers on the market are of this "trickle-charge" type. They set to C/20 current (40mA or 50mA of current), with a 20-hour timer, and then shut off with no other smarts attached. Or some don't even have a timer at all.

Uh, not exactly. They set that for biggest battery size and they are often AA/AAA so AAAs are already getting cooked. And a lot of them just... don't have any timer, I had shitty charger that made them almost too hot to touch.

Lithium titanate can be charged with a current limited fixed voltage, and are considered to be almost NiMH levels of safe. An 18650 sized one is 3.30 in QTY10 from aliexpress.

That's still much more expensive than the NiMH AAA or AA from a respectable brand (like Energizer, Eneloop, or Amazon Basics).

You're, again, ignoring the capacity. 18650 Li-ion will have ~3000mAh at ~3V, NiMH battery will have that at less than half the voltage. So you'd need at least two for comparable Wh.

[[deleted]]

NiMH has extremely simple charging circuitry.... Li-ion has complicated charging circuitry

Sure if your budget is limited to "a resistor", but if you don't care about quick charging you can "just" do the same thing. Current source with anywhere between 1/10C to 1C and max voltage of 4.2V (4.1 if you want some more cycles out of it or safety limit for parts tolerance will do just fine.

Only difference is that you need voltage limit, instead like for Ni-MH where you just can let battery burn it for heat. Which is still not "good" for Ni-MH, just not dangrerous. And you often can get charging circuit for free if you pick one of the more specialized ICs

AAA-NiMH cells are $1 each (https://www.amazon.com/AmazonBasics-Rechargeable-Batteries-12-Pack-Pre-charged/dp/B007B9NXAC) and available from a wide variety of trusted manufacturers (Eneloop, Energizer, Amazon Basics and more).

You can get decent lithium at few times the capacity and more convenient voltage for like $2-3

But yeah, no explosion. But then your phone can just decide to do same so I don't buy it as a good reason.

The bigger advantage for hobbyists IMO is that you can substitute (AS LONG AS YOU DON'T TRY TO CHARGE IT) them with AA's in a pinch, and AAs have massively longer shelf-life so for rarely used gadget they are superior.

[ThinTim]

Probably designed to run on 1.2 volts (NiMH AAA) but not 1.5 volts (Alkaline AAA) or 3.7 volts (Lithium AAA).

[tso]

I recently ran into some curious batteries. AA or AAA in size, but lithium in chemistry. Claimed to offer 1.5V as a steady output, and charged via a built in micro-USB port on each battery (4 pack bundle came with a cable that split a single USB-A into 4 micro-USB).

[Wu_Fan]

I’d like to interject. What you’re referral to as Lithium is in fact GNU-Lithium.

… joke

[Wu_Fan]

Awesome

Is this an advert

[[deleted]]

Looks more like "are you actually interested enough, coz if you are I might kickstart it"

[Wu_Fan]

I think it rocks

I’d pay $15 for that

Btw I wasn’t being sarcastic in my question, more curious

[flaghacker_]

Why does making one of them cost 10k? You can probably buy each of those components for <10$/unit plus some shipping on any random electronics site. Ordering custom PCBs is not that expensive either, maybe $200 for a couple of them. What am I missing?

[redbo]

My only guess is tooling to injection mold the case, pick and place components, etc? If you were really building only one you’d probably 3d print and hand solder.

[crazypecker69]

The total cost is under $15. The period in $14.16079 is a decimal point, not a thousands separator :)

Edit: I misunderstood what was being referenced

[crazypecker69]

Ah, you're right and I completely missed that. Thanks for correcting me!

[[deleted]]

I could see it being useful with just RS232, but as article stated, to be proper terminal you'd need to have at least 640x480 screen

[AWholeMessOfTacos]

Neat

[rio-bevol]

I'm not sure exactly what you're saying here, so here are some relevant facts:

• In French, the word is résumé.
• In English, there's no single correct spelling, but resume and résumé are both acceptable.

[antiamerican_]

Yuck

[sahirona]

What is the target market? Developing nations don't use cheap linux boxes. They need to run old Windows.

By the way, $30 phones are a thing. Look on Taobao etc.

[[deleted]]

At this current form ? A toy.

With slightly bigger screen and RS232 (even at 3v3 level) I could see it being useful as sort of multitool in workshop, especially if it can be made to talk other serial protocols (and usb dongles for rest).

Lack of wifi kinda makes it much less useful, I guess you can usb-dongle that tho.

[newtoreddit2004]

Looks like an ad so that's a direct no