Dave always says "Make a product" so, i've made one

[miceuz]

Hi,

I'm a programmer/systems architect by education and by the way of conduct, but somewhat 3 years ago I've got an electronics bug - had a sidejob to fix a LED board, got into microcontrollers, then made myself comfortable in analog.

Several times in his videos Dave told - go on, make a product. So i've made one - even probably built "down to a price" at some extent.

It's a "plant watering alarm" - you set the dry point, then water the plant and then it starts to chirp when the plant dries out, rare chirps at the beginning and more frequently when the water level really goes down. It has an LED hooked up as a light sensor, so it does not make noise when it's dark.

I've used capacitive sensing technique where square wave is fed into RC filter formed by known resistance and unknown capacitance as a wide track on the board. Filtered waveform is triangle-like. It is half wave rectified by a diode and stored into a capacitor that has a bleeder resistor. When the capacitance changes, the peaks of the waveform become lower or higher.

I'm feeding 1MHz from internal rc oscillator of attiny44 micro. I've measured my sensor to be around 4.3pf - 75pf, used the same board to measure this, so i might be a bit off. I've put some effort to calculate and measure the most optimal filter resistor value, so I get the widest range and then I looked for the shape of the sensor so it gives me more range. I decided against opamp as I was building this down to a price ,) The range I got is 508 - 833 LSB which is pretty enough for the application. Now I see that maybe by adding a full bridge rectifier I might gain more. http://wemakethings.net/2012/09/26/capacitance_measurement/

The isolation from moisture part was a bitch - water seeps in almost thru any stuff eventually. The combination I've found to work pretty ok was a scotch tape and some varnish designed for electronics. Epoxy works too, but lowers the range and the thing is not really designed to be covered in it.

The great and ineresting challenge to extend the battery life. I've set myself a goal of 1year on a cr2032 battery and learned many tricks how to get current consumption down - slowing down CPU speed when not measuring, sleeping all the time with only watchdog running, turning off every peripheral that is not used at that moment, etc - this was real fun.

Regarding manufacturing - waiting for orders from china, screwing up with gerbers and playing mail ping pong between western new year and chinese new year - man, what can I say - this stuff is hard 

I'm using it myself for a half a year and surprisingly - it still works the flowers have died though as we left the flat for vacation for two weeks 

What do you think - does it look as a product?

It was a journey anyway, i've learned so much, thanks Dave!

[Yaksaredabomb]

Very neat!  I might be in for one at Tindie's price.  If you could get the cost down a little further - maybe half that - I might be in for 5.
 
If a kit would be easier/cheaper I'd be up for that - surface mount not a problem as long as there aren't any tricky packages that hide their leads under their body.
 
Anyhow, very cool.  Thanks for sharing and good luck with the new versions.
 
Edit June 6th: My display name has recently changed from "jneumann" to "Yaksaredabomb"

[mimmus78]

Nice project man.

[EEVblog]

Yes, very neat!
And it does look like a real saleable product, well done.
Heaps better than most first project attempts!

[lemmegraphdat]

Folks of a certain age might wonder if the thing will short out if the "wires" get wet and if they will get a shock.

[Dave]

Folks of a certain age might wonder if the thing will short out if the "wires" get wet and if they will get a shock.

And when you try to explain to them that a 3V coin cell battery can't hurt them, some moron always shows up and shouts out that evergreen moronic sentence: "It's not the voltage that kills you, it's the CURRENT!"

Very neat project, I like it.
Did you manage to get the consumption low enough so a battery lasts one whole year?

[miceuz]

Very neat!  I might be in for one at Tindie's price.  If you could get the cost down a little further - maybe half that - I might be in for 5.

My BOM price for a batch of 25 is 3euro, this makes a retail price of 10euro quite appropriate, because wrapping every item in scotch tape takes a lot of hand-handling. SMD kit might be an option, but once in AmpHour Dave said "you have to be crazy to do an smd kit" and I somewhat agree - that SOD523 diode is reallty tiny

[miceuz]

Did you manage to get the consumption low enough so a battery lasts one whole year?

In general the consuption is ok as long as you water the plant, most of the power goes to chirping of course. On another hand, I don't have an access to a digital storage memory oscilloscope, so I can't measure exact current consumption during significant power draw. Regarding sleep time - from crude measurements I made, it's acting exactly as per attiny44a specifications and this should last for more than a year on cr2032.

As I've said - it's after half a year it's still running at my home, so probably I'm not that far off my goal.

Thanks to all for a warm feedback!

[jucole]

What do you think - does it look as a product?

Absolutely; it looks very professional to me!  the website is great too! nice and clean looking.

I really like the little Cricket thing you have going on;  I would have tried to play on it more,  with the notion of chirping, perhaps like..

[jucole]

Regarding the capacitance measurements;  a while ago I was exploring some home-made capacitance touch buttons using a micro-controller using a single I/O pin and a cap and resistor; basically you output to the pin, thus charging the cap; then you read in the pin but using a timer to count up until the input pin changes from 1 to 0.  If you used more than one pin you could perhaps measure a wider range; not unlike an auto-ranging DMM.   Maybe?

[komet]

As it works capacitively (interesting twist, most plant alarms are resistive) you could conformally coat or heatshrink or encase the entire thing, which might help for its acceptance.

Does the LED light up along with the beeping? If you have 100 of them in a room and 3 of them are beeping, how easy is it to ascertain which ones are going?

[Psi]

I've set myself a goal of 1year on a cr2032 battery and learned many tricks how to get current consumption down - slowing down CPU speed when not measuring, sleeping all the time with only watchdog running, turning off every peripheral that is not used at that moment, etc - this was real fun.

And now you can add to your CV/resume
"Engineering ultra low power electronics for extended battery life"

[jucole]

Does the LED light up along with the beeping? If you have 100 of them in a room and 3 of them are beeping, how easy is it to ascertain which ones are going?

Not sure it does in the code  https://github.com/Miceuz/PlantWateringAlarm/blob/master/src/main.c  but you could easily tweak it;  that said, it's probably easier than trying to water 100 plants 

[miceuz]

What do you think - does it look as a product?

Absolutely; it looks very professional to me!  the website is great too! nice and clean looking.

I really like the little Cricket thing you have going on;  I would have tried to play on it more,  with the notion of chirping, perhaps like..

actually it's a curve of real capacitance measurements under the cricket in the original

[miceuz]

Regarding the capacitance measurements;  a while ago I was exploring some home-made capacitance touch buttons using a micro-controller using a single I/O pin and a cap and resistor; basically you output to the pin, thus charging the cap; then you read in the pin but using a timer to count up until the input pin changes from 1 to 0.  If you used more than one pin you could perhaps measure a wider range; not unlike an auto-ranging DMM.   Maybe?

the first idea I've tried before i've started with this project, was based on this - measure the time it takes for the capacitor to charge.

it's ok for making a capacitance meter circuit, as you can get pretty accurate with it (depends on the speed and precision of oscillator crystal), but for chirp it had a significant drawback - to measure the capacitance as small as a track on the board and not to run on many megahetz you have to put rather big known resistor. And then the sensor track starts to act as an antenna and picks up noise. I had a 510k resistor and the noise was really big.

[miceuz]

Does the LED light up along with the beeping? If you have 100 of them in a room and 3 of them are beeping, how easy is it to ascertain which ones are going?

Not sure it does in the code  https://github.com/Miceuz/PlantWateringAlarm/blob/master/src/main.c  but you could easily tweak it;  that said, it's probably easier than trying to water 100 plants 

Exactly, I've used an LED and not a phototransistor so someone could make it blink if one wants

[Yaksaredabomb]

Very neat!  I might be in for one at Tindie's price.  If you could get the cost down a little further - maybe half that - I might be in for 5.

My BOM price for a batch of 25 is 3euro, this makes a retail price of 10euro quite appropriate, because wrapping every item in scotch tape takes a lot of hand-handling. SMD kit might be an option, but once in AmpHour Dave said "you have to be crazy to do an smd kit" and I somewhat agree - that SOD523 diode is reallty tiny

Oh, I'm not saying your current retail price isn't appropriate.  I'm only saying that I may feel like buying 1 at the current price, and I may feel like buying 5 at half that price.  Supply and demand.  At 10 euro only demand for 1, at 5 euro demand for 5.

For 14 USD + 5 USD S&H (tindie) I'd be buying because it's neat and it would be cool to support you while getting a pretty polished and useful product in return - not because I'd feel it's actually "practical" at that price.  $20+ to be lazy about remembering to water a plant is a novelty/luxury/unnecessary thing *to me*.  Even at 7 USD * 5 units + 9 USD S&H = about 9 USD per plant.  Still a novelty at that price perhaps (to me), but hey I might "splurg" as some say on the 5-pack and give 3 or 4 as gifts.

So in this one example case, you could sell one and net 7 euro or you could sell five and net 10 euro.  Win-win.  Of course, your time and effort is worth something - BOM price isn't the only thing to consider.  It all depends on what you feel like and where you want to go with it.  Besides, this is based on market research with a very small sample size ("me").  Maybe others consider me "cheap" or can't believe I'd consider spending nearly 50 USD on these things.

Also, about the SMD kit aspect, as long as there aren't any packages that hide their leads under them (requiring stencil and reflow oven, or some other technique) it shouldn't be a problem.  I probably have more experience than most though, so maybe I am indeed being unrealistic.

As for making the kits, it looks like you've optimized for as low a part count as possible - less than 20 parts?  Sorting them into a kit shouldn't take more than 5 min per kit if you sort kits out several at a time.  Programming the attiny's would probably be the most annoying part - you'd have to build up a pod with a zif socket to do that out of circuit, though you may be able to program them a few at a time if you make the pod capable of that.  The same pod could be used to run a verification test after programming - load 1 or a few chips in their sockets, program them, push a button, get a green LED(s), take them out, fold them in a small piece of ESD foam, and you're done.

This may be far more than you're looking to get into though, and maybe it's not practical anyhow - you'd know better than me for sure.  Just thinking through it and coming up with ideas.

By the way, the "Buy it now" at http://wemakethings.net/chirp/ lists it for 17 euro + 3 euro shipping?  At least that's what I'm seeing.

My display name has recently changed from "jneumann" to "Yaksaredabomb"

[HackedFridgeMagnet]

Nice project, nice idea, nice looking build.
Is it annoying at night? You might not want to get out of bed to water it.
Could you put on a light sensor for people who don't want it beeping in the dark?
Corrected after reading subsequent post. It was late sorry.

[Dave]

You didn't bother reading the whole post, did you?

It's a "plant watering alarm" - you set the dry point, then water the plant and then it starts to chirp when the plant dries out, rare chirps at the beginning and more frequently when the water level really goes down. It has an LED hooked up as a light sensor, so it does not make noise when it's dark.

[lindworm]

Yeah cool stuff!
I agree with Yaksaredabomb, if it were a little cheaper (for example 5 Euros/ 7 USD) I'd just go ahead and buy 5 out of curiosity, and to give to some friends.
At the current price: maybe one to try it out, or none.
For 5€ I'd also go for the kit option, as long as there are no BGA parts, I can solder it. Well, I can solder BGA too, if I have to, but the success rate is lower and nobody wants that in a kit.

[miceuz]

For 14 USD + 5 USD S&H (tindie) I'd be buying because it's neat and it would be cool to support you while getting a pretty polished and useful product in return - not because I'd feel it's actually "practical" at that price.  $20+ to be lazy about remembering to water a plant is a novelty/luxury/unnecessary thing *to me*.  Even at 7 USD * 5 units + 9 USD S&H = about 9 USD per plant.  Still a novelty at that price perhaps (to me), but hey I might "splurg" as some say on the 5-pack and give 3 or 4 as gifts.
My display name has recently changed from "jneumann" to "Yaksaredabomb"

well, yes, I see your point and maybe i'll reconsider the kit idea - this way I wouldn't have to care about the manufacturing. I've done 25 by hand and don't want to do that anymore

I'm now a bit more interested in more useful projects than this one. I know it's a silly product really, but as a kit it would fit into "whynot sounds fun" cathegory. I'd consider project a win if it would be cloned by chinese 

[Yaksaredabomb]

...I know it's a silly product really....

Hardly!  The way you use the LED to *sense* light vs produce it is something I'd never heard of before - and I'm an electical engineer.  Maybe my education was lacking haha.  Also, using the capacitive method vs conductive is more complex but better and more technically interesting.  Sure it looks simple and serves a simple purpose but I wouldn't call it "silly" at all.
 
Keep us posted on future improvements/versions and let us know if you decide to offer kits.  Good luck with the "more useful" projects too! 

[jvdb]

nice project. 

seems like it would be an ideal application for small solar cells such as these:

[gerrysweeney]

Thats a nice project, +1 from me...