Or any logic controlling device such as PLC or Smart Relay?
Max.

Sure... but even if he were to use an MCU, it always boils down to logic gates... the question is which is more practical, and economical.
If he needs more than 5 or 6 relays, then a small generic PLC would be a better idea.

 

States:
Switch 1 (at 50 gallons) Switch 2 (at 80 gallons) Motor State
0 0 On
0 1 Off (Latch till 50 gallons)
1 0 On

Problem being, lever hits the 80 gallon switch and you lose power. The latching relay is latched into the "motor off" state. Say the liquid depletes all the way to 20 gallons with no power. Once power is re-established the latching relay is still keeping the motor off, but I really want "motor on"

Reason I can't have "00" state just reset the latch, is there is a time when the liquid is depleting from 80 to 50 gallons that the "00" state occurs (no switch is hit) and I want the latch to still be enabled not turning the motor on.

Hopefully that clarifies things.

You need to add a third column, whether the relay is on or not. Then you should have 8 combinations of states, and one column with the desired output - the next state of the relay - should it turn on or off or stay where it is.

 

the difference between a latching relay and a relay wired to be self latching is simply one of power to the relay coil. a latching relay coil does not need to be powered on a continuous basis to maintain itself in the latched position, where as, a relay that is wired to self latch will unlatch if power is removed from its coil.

 

does a latch relay exist that does what I want, fail to a specified state upon loss of power.

By definition, no. See Kermit2's post above.

 

Switch 1         Switch 2         Motor State
(at 50 gallons) (at 80 gallons)   
0                 0                  On
0                 1                  Off     (Latch till 50 gallons)
1                 0                  On

Looks to me you want to turn the motor on when less than 50 gallons, then then turn off when you reach 80 gallons.

True dat? If so a simple self latching relay circuit will be all you need. <50 turns it on, >80 turns it off, and loss of power also turns it off.

 

After reading everyone's response it seems I need a self latching relay that does what ernieM describes. Can anyone show me a simple self latching relay picture so I can visualize how it works?

 

See post #13. Or http://www.reuk.co.uk/OtherImages/make-latching-relay-with-a-dpst-relay.jpg for example.

 

 

Switch 1         Switch 2         Motor State
(at 50 gallons) (at 80 gallons)  
0                 0                  On
0                 1                  Off     (Latch till 50 gallons)
1                 0                  On

Looks to me you want to turn the motor on when less than 50 gallons, then then turn off when you reach 80 gallons.

True dat? If so a simple self latching relay circuit will be all you need. <50 turns it on, >80 turns it off, and loss of power also turns it off.

That was exactly my first impression... see post #4

 

After reading everyone's response it seems I need a self latching relay that does what ernieM describes. Can anyone show me a simple self latching relay picture so I can visualize how it works?

A self-latching relay works by making one of its own contacts drive its coil. Once you press a button to energize the coil, the contact will close and keep the coil energized even if you let go of that momentarily-closed (normally open) button. If the relay loses power then the coil will be de-energized and the relay will open, also opening the contact that kept the coil latched. So that when power comes back, the relay will not be activated until you pressed the button that momentarily energizes the coil again.

 

The circuit is latched, my motor is running. Bang, power fails... I have coffee... Power returns, do you want my motor to start automatically or do you want the circuit unlatched and motor not running?

You can have either.

Ron

In your scenario, I want the power loss to cause the latched relay to fail to motor not running state.

States:
Switch 1 (at 50 gallons) Switch 2 (at 80 gallons) Motor State
0 0 On
0 1 Off (Latch till 50 gallons)
1 0 On

Problem being, lever hits the 80 gallon switch and you lose power. The latching relay is latched into the "motor off" state. Say the liquid depletes all the way to 20 gallons with no power. Once power is re-established the latching relay is still keeping the motor off, but I really want "motor on"

Hopefully that clarifies things.

No, not really.

does your pump fill the tank or drain it?
what kind of level switches are you using, that only give a pulse? Most level switches will stay switched when fluid is > switch level (your 50-level switch would remain switched whether your tank is filling from 50 to 80 or draining from 80 to 50).

 

No, not really.

does your pump fill the tank or drain it?
what kind of level switches are you using, that only give a pulse? Most level switches will stay switched when fluid is > switch level (your 50-level switch would remain switched whether your tank is filling from 50 to 80 or draining from 80 to 50).

Oh now wait a moment with the no, not really. My post which you quoted was only the second post in this thread. When I made my comment / analogy it was spot on at the time in my defense. It was only later that the original poster began to add more and more information, most of which should have been included in the original post which is nothing like this thread has evolved into with now tank and gallons. Hell, this was the original post:

I am looking to use a latching relay that also includes a fail safe state if power is loss. I know normal latching relays remain in the state they currently are in if power is lost, but I am looking for one that fails to a certain state upon loss of power.

I have searched for a component that does this but have come up empty. Does anyone have any ideas?

The term Fail Safe State in this case implies if power fails when power returns lives or property will not be endangered. None of what this thread has evolved into even remotely resembles that original post and that is why I quit responding to the thread.

Ron

 

Quick update - my circuit works finally. Thanks for the help in this thread

 

Could you post that circuit?

 

Could you post that circuit?

Yeah, an explanation of what finally happened would be nice

 

Oh now wait a moment with the no, not really. My post which you quoted was only the second post in this thread. When I made my comment / analogy it was spot on at the time in my defense. It was only later that the original poster began to add more and more information, most of which should have been included in the original post which is nothing like this thread has evolved into with now tank and gallons. Hell, this was the original post:



The term Fail Safe State in this case implies if power fails when power returns lives or property will not be endangered. None of what this thread has evolved into even remotely resembles that original post and that is why I quit responding to the thread.

Ron

I don't understand the context of your response. I get the impression that you took my reply as some slight against you. If that's the case, you are mistaken. The point of my post which I tried to express using bold font highlights, is that you asked OP if the motor should come on after a power failure and he said "NO" then a few posts later he said "YES" and then he "hoped that clarified things", which it did not, to me.

 

I used the circuit in post 13 exactly

 

I don't understand the context of your response. I get the impression that you took my reply as some slight against you. If that's the case, you are mistaken. The point of my post which I tried to express using bold font highlights, is that you asked OP if the motor should come on after a power failure and he said "NO" then a few posts later he said "YES" and then he "hoped that clarified things", which it did not, to me.

I'm good with that. No offense was taken. I merely questioned the "no, not really" which you explained.

Ron