High water, pump failure warning light

[tkc100]

I would really appreciate some help with a simple circuit design.

Here at the park we have a lift station that has a very simple warning light circuit. (See attache file) It's just a normally open mercury float switch that turns on the warning light when there is a high water condition. What it is actually signaling is a failure in the lead pump. If the water continues to rise the backup or lag pump come on and pumps the water out. The problem is that it is quite possible if not probable for the light to go unnoticed during the time from the high water condition and the lag pump coming on.
I need a means or method so that once the water has risen to the point of closing the float switch the warning light remains on until manually reset.
I know there must be a simple and elegant solution to what seems like a simple problem but my attempt this far have not been successful. I attempted to use a latching relay but it was designed for only a momentary signal contact and if the float remained closed for a period of time the relay just sit there and vibrated.
Any help you all can provide will be greatly appreciated.
Thanks!

 

[Reloadron]

All you need is a simple relay latching circuit. Unfortunately I am traveling till tomorrow afternoon and I don't have my circuit making software on this laptop. No idea why when you tried the relay vibrated but what you want is a very simple circuit scheme. If nobody post one I'll have one by tomorrow evening for you.

Ron

 

[Tony Stewart]

From line to 500V small signal diode , then bar or cathode to 47k to 1uF 250V cap non polar to neutral.

. Then from cap, mercury switch to 10k to gate of triac , then triac output1 to neutral, then output2 to light then to momentary line-rated off switch to line input.

Input diode RCR cct. creates high V DC to create constant gate On current at 1.2Vdc to gate when ON but half line voltage DC across mercury when off. Switch voltage rating must be given as high V or changes required with bigger cap, extra resistor divider across cap to < mercury switch rating.

 

[Reloadron]

Here is a pretty rough cartoon of what I had in mind earlier:

I would just use an 8 pin octal base plug in relay available from McMaster Carr Supply and dozens of other suppliers. I would buy a DPDT relay just in case I or someone wants to add something later down the road.

Relay McMaster Carr #7170K13 $15
Relay Socket McMaster #7122K19 $5.00

Granted this relay is overkill for a small indicator lamp but it leaves options. That or just get a smaller SPST relay. Either way you come in under $20.

Ron

 

[Tony Stewart]

Cheaper way is use smoke alarm and battery switch with external , then listen . In case of power failure, bypass test switch or use mercury across test switch. Use small 0.1 uF cap across switch at alarm to suppress RF, ESD.

Doesnt latch but gives wider alarm reach, and detects failure with disconnected AC.

 

[tkc100]

Reloadron

It’s only simple after the fact.
I had attempted to use a single coil magnetic latching relay (MAFNECRAFT 755XBXC-120A).
This relay has a single AC coil. It employs steering diodes to differentiate between operate and reset commands. The relay will operate in one direction when power is applied with one polarity, and will reset when the polarity is reversed. The magnetic holds it after the coil in de-energized.
Sounded like just what I needed however the set and reset pins were only designed for a short pulse. If in my case the set or reset signal was longer in duration the coil vibrated because of the pulsating DC current being directed at the coil.

That’s when the mental process went on the rocks, so I reached out to you guys.

Thanks for a simple elegant solution.

I will gather up the parts and make it happen tomorrow.

 

[tkc100]

Reloadron

I have been studying the schematic you posted and I'm wondering....
Shouldn't the reset switch be a N/C push button?

 

[Reloadron]

Reloadron

I have been studying the schematic you posted and I'm wondering....
Shouldn't the reset switch be a N/C push button?

OMG that is absolutely correct. I drew in a N/C and labeled it N/O. Yes, it should be a N/C push button reset.

Ron

 

[tkc100]

Ron

No big deal just wanted to confirm.

As it turns out this concept you have given me will work on another project I've been working on.
I need a control circuit built for one of our wells to protect it in the event the differential pressure switch failed.
A run away pump could produce and over pressure situation and damage a lot of the plumbing here.
In place of the float switch I will place an N/O preset pressure switch and use then use the other set of N/C contacts to disengage the pump should the pressure switch closed.

As a side note: What software do you use for your drawings. I could really use a simple piece of software for documenting these project. Save someone down the line from having to figure out what was done.

Once again many thank for your assistance!

Tom

 

[Reloadron]

Hey Tom, my pleasure.

I used Cadence Orcad Capture (A free version) for the drawings. Pryor to retiring I used the full versions of Orcad. The free version works OK. Unfortunately I can't build parts libraries in the free version but again, it is free. I also use LtSpice at home for circuit simulations. There are several free schematic making software out there.

Something to keep in mind is the relay I linked to and used in my example is only rated I believe for 7 Amps AC (inductive load). Not a big deal as they can be slaved to other larger relays or motor starters (like big pumps). Spent a good number of years working with some very large and very unusual pumps. Many of which also carried pretty high price tags so I know the drill with pump protection routines.

Ron