Re: watering system, second attempt.

From: andy (news4_at_earthsong.free-online.co.uk)
Date: 08/05/04 

Date: Thu, 05 Aug 2004 01:36:35 +0100

On Wed, 04 Aug 2004 23:05:14 +0100, Terry Pinnell wrote:

> andy <news4@earthsong.free-online.co.uk> wrote:
>
>>after the comments on my first version, i've redesigned the circuit using
>>CMOS ICs and a mosfet to switch the coil. I was meaning to get it working
>>before i posted the new design, but i'm waiting for some parts, so i'm
>>posting it now to see what people think. as before, i would appreciate any
>>comments on how i've done it.
>>
>>mostly what i want to know is if there's
>>anything that will stop it working the way i'm expecting, cause parts to
>>fail, cause a hazard, or if i've missed any obvious simplifications to the
>>design i've come up with. it's a bit more complex than before, because
>>i've added a feature to water in the morning, evening or both.
>>
>>I've tested the part of the circuit that gets the light on/off signal from
>>the LDR using the 393, and xors it with the 'force watering' switch
>>signal, but then a chip failed and i'm waiting to get another one.
>>
>>http://www.niftybits.ukfsn.org/electronics/daily-water-with-chip.jpg
>>http://www.niftybits.ukfsn.org/electronics/daily-water-with-chip.png
>>
>>if you still think it's junk then say so, but i'd like to know why.
>
> I'd have been interested to study your circuit - but it's too large
> for comfortable reading. And if I try resizing it 50%, your dark
> colour scheme produces a barely readable result. What's wrong with
> conventional black on white, with occasional colour for special
> purposes?

that's just the way the program i have writes out an image.
I've run it through a paint program to make it b/w and smaller:

http://www.niftybits.ukfsn.org/electronics/daily-water-with-chip.jpg
http://www.niftybits.ukfsn.org/electronics/daily-water-with-chip.png
 
> Also, if you really want the feedback you've specified, a full text
> description of your objectives and approach would be helpful. Or are
> you expecting readers to remember the previous thread, or search for
> it?

It's meant to send a 1s 13A pulse through an electromagnet at the
beginning and/or end of every day, depending on whether the 'morning' or
'evening' switches are set. Will also trigger if the 'force' button is
pressed. there is a holdoff of up to 2 hours 40 min to stop the circuit
retriggering once a pulse has been triggered. The day/night sensing is
with an LDR, with a variable light level setting and a bit of hysteresis.
The 'reset' button resets the holdoff timer.

When the pulse is triggered, this will release a magnet which is holding a
ball*** valve closed in a water ***, and set off a single cycle of
filling and emptying the tank through some drip hose.

The design i'm going for is like this:

        | :#: iron cored electro- |
        | :#: magnet. |
        | m |
========|X------O ball*** valve with |
        | magnet (m) on top. |
        | |
        | |
        | |
        | |
        | |
        | |
        | |
        | |
        | |
        | |
        | |
        | =============
        | | slow drain
        '--------------------------------' into drip hose.

The tank is normally empty, with the magnet holding the ball*** shut
against the electromagnet's core. Then a short pulse through the
electromagnet should make the ball drop down. The tank fills quickly,
the valve latches shut again, and then the tank drains slowly through the
drip hose.

the idea is:

a) a short pulse to cancel the magnet's field should take less current
than having to open a valve against friction. and much less than having to
hold a solenoid valve open for the whole watering cycle.

b) the system always comes back to a stable off (no watering) state by a
purely mechanical process once the initial pulse has been sent. i.e. if
the power goes, it can't get stuck open.

c) it's easy to build out of common or garden parts - don't have to buy an
expensive electrically operated valve.

andy. 

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