Comment by setq | Hacker News Reader

Comment by setq | Hacker News Reader

Parent Full thread

I love these projects. They are always destined to fail from experience but are great fun anyway. I've tried a few times to do similar things. I'll catalog my disasters quickly:

1. Seedlings need blowing around a bit when they pop out or they get all spindly. Cue wiring up two 80mm PC case fans to a chain of a single astable then two monostable 555's to generate an oscillating wind field that goes on for 5 seconds each direction after a delay of 10 minutes. Dried the compost out, blew most of it it away and then killed the plants dead. No tomatoes for me!

2. Watering robot version 1. Similar to above but with a 74hc390 dividing down the clock so it only ran once every day. Used an unprotected MOSFET to control a small water pump from ebay. Back EMF blew the MOSFET up and jammed it as a short. Emptied the entire water reservoir into the pot, down the wall and into the carpet.

3. Watering robot version 2. Same as above with problems fixed. Apart from I ran out of bipolar 555's so I used CMOS ones which are a little more tetchy about noise. Cue last 555 getting jammed in an on state and the same thing happening. This time, the tupperware box with the electronics ended up getting wet and the wall wart exploded.

Edit: meant to say to the OP - nice work. This is the spirit of all things interesting :)

Fun story: we're doing a project at work to do with agriculture.

We bought some pot plants to keep alive and some cheap soil sensors; they're just two electrodes and you pass a current through the soil, measure the resistance and get a proxy for soil moisture. We left the sensor continuously logging on Friday, came back on Monday to find the plant very unhappy and the probe contacts almost completely corroded. Within a day or two it was a stump, never recovered.

Turns out you're not supposed to leave these things powered on 24/7 and capacitative sensors are a much better solution. TIL you can electrocute plants.

Put a metal stick in the soil and check the travel time of the reflected signal:

https://en.wikipedia.org/wiki/Time-domain_reflectometer#TDR_...

https://en.wikipedia.org/wiki/Measuring_moisture_content_usi...

This is a really cool idea actually. However you're likely talking sub nanosecond rise times which makes things a little difficult in the signal processing space. For example my scope (knackered old 1971 Tek 475) can barely manage 1.8ns on a properly terminated transmission line which this will likely not be.

I don't remember the name of the peripheral off the top of my head, but there are some PICs that have a very fast time measurement unit in them. I don't know if they're fast enough for soil measurement, but there's app notes around for doing TDR-based liquid level sensing with them.

http://www.edn.com/Pdf/ViewPdf?contentItemId=4433411

Details above. Looks like you can get 3.5ps resolution with an off the shelf PIC after calibration which is pretty impressive!

Wow!

So the price is..

3$ for the microcontroller, 3$ for the 4 ns comparator, and a few cents for the resistors, capacitors, and 7 GHz transistors (I couldn't find the exact model), and probably some dollars for the rest of the materials such as steel rods and tubes. All in all it looks doable for <50$ easily. That's pretty amazing!

That's the one! I was blown away when I first saw it!

Provided you can get the rise time fast enough, you can use a TDC7200 http://www.ti.com/product/TDC7200 which will give you picosecond timing on the signal.

On the PIC solution you posted, how do you get around the fact that the comparator is only good to 4ns (regardless of whether you can time down to 4ps)? Lots of measurements and look at the stdev?

Probably not so much electrocuted as poisoned with metal ions.

I did wonder this, was it simply the corrosion that did it?

You plated your plant! :)

Seedlings need _light_ when they pop out or they get all spindly. The sun does something like 1 kW/m3, so if you're using artificial light, you will need plenty of (preferably LED/CFL) lighting. Source: the generations of spindly tomato plants who went before the thick-stalked purple-green-leaved ones.

Indeed. This was on the kitchen window sill with sunlight 50% of the day. We got there in the end unaided by technology but it was so much fun trying with it.

Ones that worked eventually: http://imgur.com/a/EDdwR - note cable ties :)

While that's true, the OP's solution is one way to help promote thicker stems if your lighting is sub-par. E.g. more light is best, but failing that for some reason (budget, space, whatever), stimulating the stems is a reasonable backup.

Solidarity. :-)

What a lovely way to learn about the world ... by taking copious notes about all of the (surprising) things that don't work.

It's fantastic. When it fails miserably I am usually in hysterics at the sheer carnage my creation unleashed. So much to learn.

I'm slightly tempted to do a 4th attempt now inspired by the original poster :)

That's a very positive outlook to have, the bit about unleashed carnage made me laugh :)

Have you seen https://www.reddit.com/r/shittyrobots great fun with the carnage

Yes this is a favourite bit of reddit for me. Highly recommended :)

you should play dwarf fortress.

Why not use a relay? Less to worry about for a side project like that. If you don't need sub-millisecond switching then you gain complete isolation, constant voltage output, and AC or DC switching

Relay has a considerably lower drive impedance than a MOSFET for switching so you end up having to drive the relay with something anyway because the 555 will probably die. Might as well skip the middle man and use a MOSFET which has a crazy high source impedance.

(you need to stick a transistor between the 555 and the relay anyway so you might as well just use a MOSFET)

Nope. 555 has drive capability of over 100mA. More than enough to drive a small relay directly. Just make sure you have a spike suppression diode in circuit.

Take a look at the output switching on different variants of 555s (there are about 6). This isn't always the case. Not only that if you pull lots of current through them, and 100mA is a lot, then the Pd of the sink transistor is pushed a bit hard. I've let the smoke out of a few over the years working on that assumption.

Good tip: measure voltage between the output pin and gnd or Vcc in both states and then have a think.

Plus when you're not an idiot like I was, a MOSFET is cheaper.

Fair point.

I sometimes overestimate the ability of the average electronics hobbyist to properly understand a datasheet.

I don't think many people read the data sheets to be honest even some of the professional engineers. It's mainly gluing canned circuits together and see if they work or not. This is a little difficult if you didn't start with analogue and work up.

I really want to get a Farm Bot to mess around with. Appears to be open source and you just order the parts to assemble for your space.

Looks like a cool project.

Reply on news.ycombinator.com