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

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.

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.

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.

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.

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.

Get rid of the missus ?

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 :-)