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Ask HN: Most interesting tech you built for just yourself?

1901 points l2silver | 11 comments | 27 Apr 23 15:04 UTC | HN request time: 0.509s | source | bottom

Maybe you've created your own AR program for wearables that shows the definition of a word when you highlight it IRL, or you've built a personal calendar app for your family to display on a monitor in the kitchen. Whatever it is, I'd love to hear it.
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steve_adams_86 ◴[28 Apr 23 06:27 UTC] No.35738121[source]▶
I built myself an automated hydroponic grow tent.

It measures and corrects pH, electrical conductivity, oxidation reduction potential, temperature of the air and water, water level, and humidity. It also automates pumps, lights, and fans (I know people normally advise against this). None of it is particularly sophisticated, but I’m really proud of it.

I initially used a deep water culture and later moved on to the nutrient film technique. It produces a lot of greens and herbs — way more than I ever expected — and it’s remarkably hands off. I recently left it to do its thing for almost 3 months before I had to intervene, and the problem wasn’t the water, nutrients, or the system failing explicitly. The plants just got too big for their channels and as they became stressed, they developed some pest issues. It was such a cool and empowering experience to see real world automation Just Work.

The whole thing is powered by an Arduino Nano RP2040 Connect. It’s a great little controller.

I’m currently designing my first PCB to consolidate the system onto a single board so my friends can easily build their own. It’s not extremely cheap, but it’s not too expensive either and you get a tremendous amount of food from it. It’s such a fun hobby.

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1. system2 ◴[28 Apr 23 06:54 UTC] No.35738301[source]▶
How do you keep water touching sensors working long term? I tried similar sensors but they all get rusted / oxidized to work properly after a certain time.
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2. rytis ◴[28 Apr 23 07:04 UTC] No.35738359[source]▶
not OP, but one of the tricks is to activate the sensors only when measuring, so there's no constant DC applied to the sensor wires/pads. once you have that, reduce measurement frequency, so to mainimise the time when voltage is applied to the sensors. for example once every hour for moisture is sufficient, and 1/sec isn't really going to help much.
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3. hksoftware ◴[28 Apr 23 07:36 UTC] No.35738569[source]▶
I've seen a clever setup with the sensors in a dry container above the water tank. There is a hole in the bottom. Before testing, a pump fills the container up with the tank water, flooding the sensor probes. When the pump stops, the water drains back out into the tank.
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4. amelius ◴[28 Apr 23 08:44 UTC] No.35738962[source]▶
Would alternating the polarity work?
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5. jesprenj ◴[28 Apr 23 09:28 UTC] No.35739196{3}[source]▶
Off-topic but perhaps interesting:

That's what they do when performing catheter ablation (a medical procedure for curing cardiac fibrilation by destroying minute parts of muscle with electric current).

DC would work just as fine on this procedure, but due to electrolysis of water, oxygen and hydrogen bubbles would form, which could get stuck somewhere. Using a square wave AC quickly reverses the reaction every period, like you suggested for the moisture meter.

https://en.wikipedia.org/wiki/Catheter_ablation#Technique

I don't know the answer to your question, but it would be worth trying.

6. ljlukkar ◴[28 Apr 23 10:57 UTC] No.35739710[source]▶
You need industrial level sensors and the water needs to be flowing constantly through them. I built something similar about 15 years ago and tested many sensors. In the end I had to pay about 1000 dollars for ph and ec meters that did the job reliably. To be honest there is nothing new here. This is how big greenhouses have been operating for decades.

In small scale there is more work maintaining the automated setup and calibrating the sensors than it would take to do the measurements and dosing manually.

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7. ljlukkar ◴[28 Apr 23 11:12 UTC] No.35739834[source]▶
The ph sensor will die fast if it the membrane is kept dry.
8. netsectoday ◴[28 Apr 23 13:10 UTC] No.35740860[source]▶
You can use capacitive water sensors taped to the outside of non-capacitive containers (aluminum foil, a resistor, an arduino, and a plastic 5 gallon container), but honestly all you need are DNI timers to "automate" any grow operation. Put your lights and pumps on a schedule and there is absolutely no reason to get more creative. If you do anything besides low-level timers you're making it complicated and brittle with no added benefit.
9. steve_adams_86 ◴[28 Apr 23 14:42 UTC] No.35741840[source]▶
These sensors are designed to withstand contact with water and to minimize hydrolysis, and I haven’t had issues with that so far. I’ve been running this system for close to a year and they still seem to calibrate just fine.
10. steve_adams_86 ◴[28 Apr 23 14:54 UTC] No.35742018[source]▶
I think it can cost quite a bit less now, but you’re right — it isn’t cheap.
11. steve_adams_86 ◴[28 Apr 23 16:57 UTC] No.35743977[source]▶
You’d need to wash the sensors and return their caps with protective fluids. It would be totally possible to automate, but perhaps the same overall cost as buying industrial grade sensors which can handle long term submersion.

You’d also need to ensure the caps contained enough storage solution at the right concentration. Over time the probes would introduce drops of nutrient solution (unless you rinsed them with distilled water, in which case you’d dilute the storage solution), and you’d need to replenish it.