watering system, second attempt.

[andy]

OK, my first thoughts are that it could be implemented with one cheap
chip, along the lines shown here:
http://www.terrypin.dial.pipex.com/Images/Greenhouse1.gif

thanks, that's quite neat.

 

[andy]

OK, my first thoughts are that it could be implemented with one cheap
chip, along the lines shown here:
http://www.terrypin.dial.pipex.com/Images/Greenhouse1.gif

some more thoughts.

the holdoff is a period during which the system won't retrigger. But
having some hysteresis on the input side might be enough for this. I think
some kind of RC circuit is needed on the input side to stop it triggering
on things like someone casting a shadow on the sensor though.

if i use this, i'll have to change the resistor values a bit - the overall
current needs to be kept right down so it can run from a small solar panel.

i like the way you've made the schmitt trigger and brought out inverting
and noninverting signals from it without any extra gates. also the diodes
on the nor gate outputs to make an OR gate.

overall it's a lot simpler than my design, so maybe i need to think about
why i was making it so complex.

 

[andy]

some more thoughts.

the holdoff is a period during which the system won't retrigger. But
having some hysteresis on the input side might be enough for this. I think
some kind of RC circuit is needed on the input side to stop it triggering
on things like someone casting a shadow on the sensor though.

if i use this, i'll have to change the resistor values a bit - the overall
current needs to be kept right down so it can run from a small solar panel.

i like the way you've made the schmitt trigger and brought out inverting
and noninverting signals from it without any extra gates. also the diodes
on the nor gate outputs to make an OR gate.

overall it's a lot simpler than my design, so maybe i need to think about
why i was making it so complex.

also, the point of the LEDs is to make it easier to calibrate and test the
system, not for when it's running normally.

 

[Active8]

some more thoughts.

the holdoff is a period during which the system won't retrigger. But
having some hysteresis on the input side might be enough for this. I think
some kind of RC circuit is needed on the input side to stop it triggering
on things like someone casting a shadow on the sensor though.

As I said, A quad op amp will have 2 amps left over for an
integrator AKA LPF.



| /
.-. </
| | <
| | ||
'-' +--||---+
| | || |
| | |
| |\| | |
+-------|-\ ___ | |\| |
| | >--+---|___|-+-|-\ | __
| +-----|+/ | | >--+-----| |
| | |/| | -|+/ |& |-
| | | |/| +-|__|
| | |
| | |
| ___ | AM
+---|___|---+
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

Something like that. The integrator will hold the charge on the cap
bewcause of the high input impedance of the op amp and gate (like
74HCT08.)

Maybe use Terry's one chip approach and work the integrator into
that on the first gate. I never tried that. An integrating schmitt.
Make it hold the valve open long enough. MAybe it'll oscillate, too.

 

[Rich Grise]

sorry, i re-did it and then forgot to change the link. it should be:

http://www.niftybits.ukfsn.org/electronics/daily-water-with-chip-bw.png
http://www.niftybits.ukfsn.org/electronics/daily-water-with-chip-bw.png

All this is to operate your daily tank-dumper?

I'd use a $3.95 coffee timer. ;-)

Cheers!
Rich

 

[andy]

All this is to operate your daily tank-dumper?

I'd use a $3.95 coffee timer. ;-)

Cheers!
Rich

what's one of them? how? can you run it off a small solar panel?

 

[andy]

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.

 

[Active8]

what's one of them? how? can you run it off a small solar panel?

A *small* solar panel is going to drive that solenoid? You must be a
giant What about the evening watering?

 

[andy]

A *small* solar panel is going to drive that solenoid? You must be a
giant

i am about 2 metres tall.

What about the evening watering?

Maplin Electronics do a 50mA, 12V panel for £24.99 (Can probably get one
cheaper than this).

Which makes 600mW @12V.
A 10W panel gives 0.7Ah/day in uk winters, so 600mW gives 0.042 Ah/day.
Equals 1.75 mA ave. current. take off .75 mA for the electronics, and
there is 1mA*24*3600 = 86.4 Amp seconds for the solenoid.

I tested it last night, and it's pushing the magnet off the solenoid with
about 12A for well under a second, so this should be OK.

The point of the design i've come up with is that all that's needed to
start a watering cycle is a single very short pulse through the solenoid -
it doesn't need to hold a valve open for any length of time - just push a
permanent magnet off the end of the solenoid core.

The only bit I'm still stuck on is finding a good bit of metal for the
solenoid core - I have an old drain cover opener which works OK, but I
don't want to cut it if i can find something better, and none of the bolts
I've found are magnetic enough.

Thanks for the suggestion about adding an integrator to the sensor - I've
redone it with a second op amp and an RC low pass filter between the two,
which seems to be working OK. (Putting the capacitor in the feedback loop
didn't seem like a good idea when I'm using the second comparator on an
LM393, which isn't like a normal op amp i think?)

 

[Rich Grise]

what's one of them? how? can you run it off a small solar panel?

Oopps, sorry, no. They plug into the wall. It's just a 24-hour clock
motor that turns a wheel with selectable on/off detents.

Glad to hear that you've got the thing going so well.

But it's still a lot of circuitry.

If it's solar powered, why do you need a timer? Aren't we all kinda
standing on one? ;-)

Cheers!
Rich

 

[Rich Grise]

The only bit I'm still stuck on is finding a good bit of metal for the
solenoid core - I have an old drain cover opener which works OK, but I
don't want to cut it if i can find something better, and none of the bolts
I've found are magnetic enough.

From what I've read, "soft iron" is good for electromagnet cores. You could
also look into transformer iron, or (I've read) silicon steel. You might
look in the yellow pages for metals recyclers and like that.

Good Luck!
Rich

 

[Rich Grise]

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.

Keep the 2N3906. It gives a good solid pullup to drive the gate cap. This
is what you want, isn't it?

And, it looks like it waters either
A) by timer
B) when you push either button

or is there something I'm missing?
Is the 4538 just one-to-a-package? If so, you could lose
one of them by just oring the switches together.

Cheers!
Rich

 

[andy]

Oopps, sorry, no. They plug into the wall. It's just a 24-hour clock
motor that turns a wheel with selectable on/off detents.

Glad to hear that you've got the thing going so well.

But it's still a lot of circuitry.

If it's solar powered, why do you need a timer? Aren't we all kinda
standing on one? ;-)

that's the point - it uses a photocell to detect morning/evening so as to
avoid problems with keeping a clock synchronised.

 

[andy]

From what I've read, "soft iron" is good for electromagnet cores. You could
also look into transformer iron, or (I've read) silicon steel. You might
look in the yellow pages for metals recyclers and like that.

I know that, but when i go into B & Q looking for 'soft iron' bolts, i
can't find any.

 

[andy]

Keep the 2N3906. It gives a good solid pullup to drive the gate cap. This
is what you want, isn't it?

And, it looks like it waters either
A) by timer
B) when you push either button

It waters in the morning when one switch is set high, and in the evening
when the other one is. Moving the switches does trigger it, but that's a
side-effect.

or is there something I'm missing?
Is the 4538 just one-to-a-package? If so, you could lose
one of them by just oring the switches together.

There are two to a DIL package.

 

[Rich Grise]

I know that, but when i go into B & Q looking for 'soft iron' bolts, i
can't find any.

Oh, don't use bolts. There's no such thing as a soft iron bolt.

I was thinking just a small random piece of scrap, or a chunk of
core from a discarded transformer, or an old actual solenoid.

And if you have access to an acetylene torch, you could heat a
bolt just short of the melting point and bury it in vermiculite
hot, and let it cool slowly overnight. That should anneal it,
and give it nice soft magnetic characteristics.

But it's probably more related to the alloy comp.; I am just kind of
making this stuff up.

Good Luck!
Rich

 

[Rich Grise]

that's the point - it uses a photocell to detect morning/evening so as to
avoid problems with keeping a clock synchronised.

Yeah, I know. I was just kind of thinking, well, isn't "there's
electricity coming from the solar panels" a pretty good indication
that the sun's probably up? ;-)

Cheers!
Rich

 

[Terry Pinnell]

i am about 2 metres tall.


Maplin Electronics do a 50mA, 12V panel for £24.99 (Can probably get one
cheaper than this).

Which makes 600mW @12V.
A 10W panel gives 0.7Ah/day in uk winters, so 600mW gives 0.042 Ah/day.
Equals 1.75 mA ave. current. take off .75 mA for the electronics, and
there is 1mA*24*3600 = 86.4 Amp seconds for the solenoid.

I tested it last night, and it's pushing the magnet off the solenoid with
about 12A for well under a second, so this should be OK.

The point of the design i've come up with is that all that's needed to
start a watering cycle is a single very short pulse through the solenoid -
it doesn't need to hold a valve open for any length of time - just push a
permanent magnet off the end of the solenoid core.

Novel, but I still think I'd have gone for something simpler than the
home-brewed solenoid. Not sure if it was included in the suggestions
you had in earlier thread, but one alternative that comes to mind (and
which would preserve your circuit design) would be a small geared DC
motor fitted with a cam, which would push the magnet off.

 

[andy]

Yeah, I know. I was just kind of thinking, well, isn't "there's
electricity coming from the solar panels" a pretty good indication
that the sun's probably up? ;-)

Cheers!
Rich

i thought of that, but they don't have the right Voltage/current/light
characteristic - the voltage stays pretty much the same regardless of the
light intensity. and measuring the current wouldn't work because it
depends on the battery level too.

 

[andy]

It waters in the morning when one switch is set high, and in the evening
when the other one is. Moving the switches does trigger it, but that's a
side-effect.

i don't think that would work anyhow - the 4538 triggers on a rising edge
of the output of the 'or' gate between inputs A and B. This will let you
trigger on either a rising or falling edge, by choosing which input to
connect the trigger signal to and using the other to gate it with. but i
couldn't see any way of triggering on both edges using just one 4538.