How? Bilge pump controller using 555 + 'dry' motor switching

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I was hoping to help a workmate out with a simple grey water pump
control using a cheap 12v DC bilge pump.

Starting with a 555 circuit and relay just to get the initial part
working, thought it'd be easy to find a way to switch the 555 off
using current feedback when the pump was 'dry running'.

No luck there in finding a 'simple' solution (and I Googled till my
fingers bled).
BTW, although I could use a microcontroller didn't really want to add
a 5v power supply to the board.

His missus doesn't want any extra wires hanging around, so float
switches etc. are out.

Any ideas on the best way to do this?


Re: How? Bilge pump controller using 555 + 'dry' motor switching



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Whether you can use the pump current to detect running dry will depend on
how clear the current change is. If it's big enough to reliably detect, then
you could put a low value shunt resistor in the supply lead that drops a
fraction of a volt. Then apply some filtering with an RC network, and apply
the resultant filtered voltage to a single supply comparator that will
operate down to 0V, comparing the signal with a suitable adjustable
reference that can be set with a trimpot. The output of the comparator can
then drive the timer.

A fair amount of filtering would help, however component limitations will
probably limit you to a few seconds or so, unless you use some more active
components.

I've seen plenty of these sorts of things malfunction because the change in
current reduced over time, but that may not be a significant problem.

I'd go for a microcontroller because it allows you to easily add things like
better input state detection, and a run timeout in case the dry current
detector doesn't do its job. If you have a PIC programmer, the 12F675 gives
you an ADC that can handle the current detection and digital filtering, plus
up to 5 other I/O pins, all in an 8 pin DIP, with a single 5V supply, don't
even need a crystal.
 



Re: How? Bilge pump controller using 555 + 'dry' motor switching


Hi Bruce, and thanks.

Yep, a micro seems to be the way to go. I wanted to keep it simple
(without 5v supply etc.), but agree you can 'kludge' a lot more things
with micros.

I'm on the other (dark) side with micros BTW, and have some
ATtiny13's, ATtiny26's ect. ready to release the smoke from.

I was given a small PCB mount pressure transducer today which should
help also.

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Re: How? Bilge pump controller using 555 + 'dry' motor switching


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 I have no idea if this is top or bottom posting & I'm cringing in
fear of the verbal abuse that might come from the "Toaster fixer"

However back to the problem, simple is better.  It may be possible to
put a restriction in the pump outlet to generate a small pressure when
the pump is pumping water, & sense this with a sensitive pressure
switch such as a water level switch from a washing machine. Pretty
easy to control the 555 then.

Hope this helps.

Ian S.


Re: How? Bilge pump controller using 555 + 'dry' motor switching



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Maybe just the delivery pipe backpressure would be sufficient to operate the
switch. Some of the small 12/24V bilge pumps (Hella??) have built-in float
switches.
I did a liitle googling about grey water recycling and gave up on it when I
read about the water going septic if not handled correctly. Just ignore the
toaster poster.




Re: How? Bilge pump controller using 555 + 'dry' motor switching


Thanks also Ian and Robbo,

Looks like a micro and pressure switch combined may be the go.

I also don't know if I'm 'bottom' posting or not. Maybe we all need a
tutorial on this from your friend. Failing that, to settle the dispute
and for some gladitorial fun, table knives & toasters at 2 paces
maybe :-)

Hmm, is septic water worse than blue-green algae which is due in our
pipes here soon? I hear lately we may be having water delivered anyway
after the dams run dry in mid-April. I'm assuming it's not going to be
the Perrier brand...

Again, thanks for the help. Much appreciated.

Cheers, Phil.

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Re: How? Bilge pump controller using 555 + 'dry' motor switching


Get rid of the missus ?

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Re: How? Bilge pump controller using 555 + 'dry' motor switching


Thanks also Andrew.

Due to his life insurance probably being paid up, I don't think the
missus removal  may be an option for him. I'm pretty sure she has a
hair-dryer (and maybe even a toaster) at her disposal to 'test the
waters' so to speak :-)



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