Electronics Wizzes: Water Level Gap Controller

[kinnadian]

So I made my own ATO which works marvelously. Schematic is as follows:

When the switch closes, the gate on the MosFET completes the circuit for the 12V pump to flow, and it puts RO in the tank until the switch disengages. For redundancy I have two float switches, one above the other. If the lower one fails, the top acts as a backup until I can replace the first float switch (after having 1 fail I carry multiple spares now).

I want to expand this into an auto-water change system. A gap controller will control level in the return pump chamber; when lower float switch activates, the drain pump will stop and a pump will turn on which pumps fresh SW into the tank, and when the top float switch activates, this pump will stop and a drain pump will turn on. Effectively this will cycle for a given length of time (I will put it on a timer) to achieve a certain volume of water change.

The only real difference between this and my ATO is that the circuit has to "remember" to keep the pump running until it is told to stop.

I'm an electronics noob but as far as I can tell I need an IC Chip for logic gates for this purpose. I've looked online and found two somewhat similar designs (http://www.instructables.com/id/Build-a-simple-water-level-control/?ALLSTEPS and http://www.brighthubengineering.com/consumer-appliances-electronics/68342-build-this-simple-electronic-water-level-controller/#imgn_0) but these have feature-bloat, are way too complicated for me to understand (surely all the capacitors, semiconductors, etc are unnecessary!) and also function with a single copper wire for level detection, and I need to use a plastic float switch for obvious reasons.

Can anyone help?

 

[sleepydoc]

Nice - check out a 555 timer IC.

How much current does the pump take? I'd worry that the current draw would burn out the MOSFET. You could also have the MOSFET control a relay or SSR which would handle the current better.

I would also consider putting a shut off float switch so if the system gets stuck 'on' for any reason it would sense the high water level and shut off.

 

[kinnadian]

Pump current draw is only 0.3A, it should be fine no?

My actual design for my ATO has 2 float switches in series, as I wrote above. Top one is normally closed, when the bottom switch closes (level gets low) then the ATO pump turns on. If the bottom switch fails closed, the top switch acts as a redundancy to open the circuit when level gets high. I've had 1 cheapie ebay float switch fail in 1 year.

I did actually come to the realisation that a 555 IC chip would do what I am after.

I have the low floats on Trigger, high floats on Reset. Then the Drain pump is normally operating and when the low floats Trigger the 555, the Drain pump is turned off and the Supply pump turned on via the 555 output to another MosFET.

I've made the circuit up and just need to build it. It simulated to work properly.

Thanks for your help

I can upload the circuit if you're interested.

I've had to buy a solenoid so that the drain pump doesn't keep siphoning after turning off. The supply pump will just have 2 NRVs in the line.

When I've finished my build in a month or so (waiting on loads of stuff from ebay) I'll make a new post for others to use.

 

[sleepydoc]

300 mA may be fine, or it may not - it depends on what the transistor is rated for.

Go ahead and upload the circuit. It'd be interesting to look at. I was an electrical engineer in another life - I glanced at the instructables circuit and it looked like a NOR latch circuit from what I recall, but it's been a long time since I looked at logic circuits

Check the current draw on the solenoid, too, and make sure it's the proper kind for your use (normally open or closed)

 

[kinnadian]

http://i.imgur.com/zBc7ycm.jpg

Couldn't show the image since it is too big and the forum doesn't rescale it.

My electrical engineer mate reluctantly helped me with it after I designed much of it myself. I originally had 2 555 circuits to do drain and supply and he redesigned it with the transistors.

I was originally going to put it on a timer to run X length of time. However if the circuit continually stops at/near the low level switch more often than high, then when my ATO turns on it will fill with RO water, and eventually salinity will be too low.

I asked him if I could use a counter of some function to stop after X counts of "high" level and he didn't know how to do it easily. A friend of mine did Arduino and had a really hard time and so I really don't want to do it, I like to keep things simple.

Yep solenoid is normally closed. It is run off a separate 24v power supply, will have to use a mosfet or relay or something to turn it on.