Re: watering system, second attempt.

From: andy (news4_at_earthsong.free-online.co.uk)
Date: 08/07/04 

Date: Sat, 07 Aug 2004 14:09:48 +0100

On Wed, 04 Aug 2004 21:33:12 +0100, andy 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've simplified it to just 2 chips after the comments from terry and
active8.

see:

http://skullcap23.tripod.com/daily-water3-bw.png
http://skullcap23.tripod.com/daily-water3-bw.jpg

One thing i'm not sure about is whether i need the 2N3906 on the output,
or whether i could drive the MOSFET directly from the 4538 monostable's
outputs if i used the noninverting outputs and put the diodes the other
way round.

Apart from that, I'm going to stick with this design, just to get the
thing built. I had most of it working on breadboard last night, so I think
it will do the job even if it's not the best solution. 

-- 
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