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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorBrian Faux
    • CommentTimeNov 11th 2021
    I have a hydro installation and I would like to do a bit of automation using an arduino or similar. First project is to build a depth comparator for switching a leaf filter on and off.
    Two depth sensors (on ebay about £30 each) that give out a 4 - 20mA output signal: compare depths and if different by a couple of cm then turn a relay on, turn relay off when depths synchronise again.
    It`s 20 years since I did any programming (in VB) so looking for someone to help so I can avoid steep learning curve for one project.
    • CommentAuthorJohn Walsh
    • CommentTimeNov 11th 2021
    That's quite a tricky ask - as you're prob aware there's always a lot of things to add into the mix when doing such projects. I googled 'raspberry pi depth sensor 4 - 20mA output signal' and the following looks an interesting start

    • CommentAuthorEd Davies
    • CommentTimeNov 11th 2021 edited
    I've done a bit of Arduino, ESP8266 and ESP32 programming and might well be able to help with that. Get in touch via https://edavies.me.uk/contact.html if you like.

    Also, do tell us all about your hydro installation.
    • CommentAuthorEd Davies
    • CommentTimeNov 12th 2021
    Posted By: John WalshI googled 'raspberry pi depth sensor 4 - 20mA output signal' and the following looks an interesting start
    Depending on exactly how the sensors are wired I'd think a resistor for each would be sufficient.
    • CommentAuthorBrian Faux
    • CommentTimeNov 21st 2021
    Thanks for your views- snowed under with work and computer problems at the moment...
    Ed has pointed out that the arduino will measure 1024 states which (given that depth sensor range is 5m) gives a resolution of around 5mm. For my purposes 5mm would probably be OK but a more accurate version would not do any harm and probably be more useful to the world at large.
    BTW waterwheel installation pics at Instagram (bdfaux) - some turbine pics too but having problems uploading...
    • CommentAuthorEd Davies
    • CommentTimeNov 21st 2021
    Yes, Brian and I had one exchange of email - I've still to reply to his last having had a couple of goodish days working on my roof and a few more days not feeling too bright and irritated by other things.

    Still, since what Brian wants to do is to compare to water levels to reasonably close resolution (10mm or so) over a range of about a metre measuring the levels separately before and after the leaf filter may lead to some problems if the sensors aren't linear (or are non-linear in slightly different ways).

    That could probably be overcome by storing some sort of calibration chart in the microprocessor, e.g., the raw A/D converter output for 100, 200, 300, … 1000mm depth for each of the two sensors.

    Another approach might be to directly measure the difference in pressures between the two levels using a sensor like this: https://www.ebay.co.uk/itm/254767611066?epid=1067948679&hash=item3b51553cba:g:bIMAAOSw6bVfoWcI , maybe one with a narrower range. Quite a few years ago I did the software, but not the electronics, for an electronic barograph (for use in gliding and other air sports) using non-differential sensors like this.

    They're not designed for exposure to water but are used in applications like sensing the water level in washing machines by using an air-filled tube to transfer the pressure, with negligible effect of gravity, to a point high enough that the sensor doesn't get wet. For a washing machine the tub gets emptied regularly so the air in the tube gets reset. For an application like Brian's I think you'd need to blow some air into the tube once in a while - I'm thinking of those little pumps used to aerate fish tanks.
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