Hi, I reposted this from https://publiclab.org/wiki/coqui-a-simple-water-conductivity-sensor -- to make it into an activity that can be featured in the Coqui activities listing: https://publiclab.org/wiki/coqui#Activities -- to help @kgradow run a class based on this.

I hope I got it right -- if it should be authored by @kanarinka and co-authored by @ashkaya, please tell me and I can switch it. Thank you -- it's a WONDERFULLY documented activity!

This is a great overview! One suggested edit: Include an explanation of how component variability might impact the circuit. I've tried to build this circuit literally dozens of times and about half the time it fails because my resistor / capacitor values are incorrect or I haven't taken into account speaker resistance or I've gotten the wrong voltage for the 555 and burnt out the timer or it's simply not making a tone in the audible range.

One solution is to post a set list of components -- better would be to post some explanation of which components matter and what acceptable parameters are (for resistors, capacitors, tolerance on the 555, and power) so that people actually can drop in whatever components they have on hand. As written it suggests that that's possible, where there seem to be a lot of fail points that are relatively easy to hit if you're not taking ranges into account.

Thanks @warren! @kgradow1 - this is a great suggestion for improvement as I've also had that happen. How about @akshaya and I will make a digikey shopping cart with all the parts and then people can go look at it for reference? (Do those stick around for awhile?)

I'm intrigued by the speaker resistance - can that be measured with a multimeter - and what are the rules of thumb for not burning out the timer?

I'm actually not sure about this - @donblair - do you know the answer?

Hallo!

I haven't had great luck with Digikey shopping carts sticking around, so it might be better to put direct links to parts in e.g. a wiki ...

Re: burning out the timer, haven't had that happen myself (I don't think?) but I guess you'd just want to be careful to stay within the voltage limits of the 555. There might also be ways of applying a reverse voltage to the wrong pins which could make the 555 decide to curl up and die.

E.g., for this version, looks like the voltage inputs need to be between 2V and 15V.

@kanarinka - we're planning to order a bunch of these to test out at MakeFaire SF at the end of may -- getting ready for a big order, and printing up the guide as well -- quite a few of them. Happy to share!

Hi! I made a bookletized version of the pdf for printing on half-letter-sized paper --

Booklet_Sensor_Journalism_Final.pdf

Added a 3.5mm jack to record the wave ;D

Oooooohhhhhhhh!!!!!

This is really cool. A recording would mean you could calibrate it later, too -- if you maybe dipped it in 2 different high and low solutions of known conductivity at the start or end of each test? Like a 🎬 before a film clip!

Deffinitely! On wednesday i'll post the mod ;)

A bit late but here is how to plug the jack https://publiclab.org/notes/imvec/06-08-2018/adding-a-jack-to-the-coqui

Hello,

I know this a long time but, I just designed a PCB for this circuit that can be easily made at home and is exactly the size of a AA battery holder. https://github.com/sushil-sundaram/Public_Lab

Oh WOW! Do you have any photos of the board, or a screenshot you could post? Thanks!!

Hi @sushills - i reposted your design to Upverter (with attribution and license ref), and took a few pics:

https://upverter.com/jywarren/523e05396087d5a9/Coqui/

Hi all! With help from @zengirl2, @asnow and @bronwen, we've posted a new Coqui activity with a smaller breadboard and flatter wiring here: https://publiclab.org/notes/warren/03-01-2019/build-a-sound-generating-coqui-conductivity-sensor