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Help wanted in designing a switching circuit. Answered

I have 3 inkbird controllers monitoring 3 beer fermenters. When any of the fermenters require chilling the Inkbird switches 230v to a pump. Problem is that there is a combination of any one on at a time or all three at once or none on. I can’t connect all 3 outputs together to drive the pump as the inkbirds will get damaged. Can anybody draw me a circuit whereby if any combination of of inkbirds switch 230v only one output is switched to the pump. Have attached a diagram showing all outputs connected together but have been told that mains connected across the Inkbird output will probably damage it.


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1 year ago

Your problem is not to know how these things switch - e.g. relay, mosfet, SSR...
Since you drive mains voltage directly the usual diode logic won't work.
IMHO the best and easiest way out would be to use a relay on each inkbrid output.
So they switch on their connected relay, but not the motor directly.
With NO contacts on the relay you would connect the common contact to mains live for all three relays.
The closing contact of all three relays is wired to the L (live) terminal of the motor, N goes to mains neutral.

Like this the pump will be engaged whenever one or more inkbirds say so with no problems of the inkbrids being damaged.


Reply 1 year ago

Hi Downunder35m
Thanks very much for your reply. I’m not sure what switches the Inkbird output but the manual says dual relay (heating and cooling) so given the size of the unit I assume it’s SSR. Would appreciate it if you could draw me a circuit diagram.

many thanks.

Jack A Lopez
Jack A Lopez

1 year ago

Naively, I am thinking the outputs from your, what you call, inkbirds, could be connected in parallel, and not get hurt in the circumstance where two outputs switch the mains on at the same time. Also the main pitfal, easy to avoid, would be not mixing up the (L)ines and (N)eutrals.

Although, there are some assumptions in that thinking, that might not be true. The problem is, I do not have a clear picture of the insides of the inkbird. At the moment, all I have is that black box you have drawn on notebook paper. That plus some intuition, gained from looking at other similar gizmos.

So far I have not gone to the trouble of seeking out documentation for your inkbird controller, especially since you did not mention which one you are asking about.

In any case, the logic you want is strict OR.

If I follow what you're saying, you have only one pump, and you want it to turn on, if any inkbird output says turn on.

At the risk of being pedantic, I will draw a truth table for this.

P = B1 OR B2 OR B3

B1 B2 B3 P
0 0 0 0
0 0 1 1
0 1 0 1
0 1 1 1
1 0 0 1
1 0 1 1
1 1 0 1
1 1 1 1

In other words, the only time the pump (P) is off, is when all three inkbird outputs (B1,B2,B3) are off.

One way to implement this, and to achieve some isolation between the inkbird controller outputs, is to build a new switch, commanded by a light signal (e.g phototransistor, logic gate, BJT transistor, relay).

Then you put the light controlled relay (or just the phototransistor sensor), into a light proof box, with three LED indicator lights, each of which is turned on by a inkbird output.

If you are wondering about this, what I call, "light controlled relay," I could draw you a circuit diagram for how that works.

I am not sure if my "light controlled relay" is a module you can just buy, from someone. Like a gizmo that turns the lights on when the sun comes up... I dunno. Such a thing might be out there. I do not know, because I have not tried searching for it.

Also there might be an easier way (e.g. like I mentioned at the beginning, if connecting inkbird outputs in parallel is possible) way to do this, but for some reason, this idea for OR logic by way of light signals, is just what popped into my head.