# Control one relay via multiple 12 V DC signals

I'm seeking help designing a circuit wherein we would have multiple 12 V DC signal inputs, with the goal of:

If one or more 12 V signals is ON, the main relay is energised and closed.


Basically, we have a vent fan that serves multiple appliances. When any appliance reaches a specified temperature, it sends out a continuous 12 VDC signal. This is intended to activate a relay to run a ventilation fan.

Because of the number of these appliances in use, we use one large fan, ducted to all appliances. We are manually switching the fan on and off at this time but I am aware of the 12 V heat-triggered output on each appliance and would like to take advantage of it (especially so the fan doesn't run all weekend when not needed).

What exceeds my knowledge is how to design the circuit with an "any or all" function, where either, any one 12 V signal by itself, or any number of 12 V signals at once, would turn on a relay.

(total # of collectively vented appliances, each with its own 12 V sender, is 7)

Each signal is sent via a two-wire DC lead (pos & neg).

Edit: the 12v signals need to be isolated from one another, so the most basic way I've thought of doing this is to have 7 "pilot" relays, one for each incoming 12v signal, and those relays would be switching a single "bus" 115v current that subsequently activates the main relay. But I am wondering if there's a better/more efficient design that would work.

• Depending on current requirements of the relay and the current drive of the signals, you may be able to tie them together with a Diode and a small-ish resistor. Do you have an "always-on" 12 V power line ? Jun 23, 2021 at 15:01

Since you need isolation, you will need a relay for each appliance.

Even if you didn't strictly need the isolation, it would be recommended since it might be hard to safely share the same voltage (ground) reference among all 7 appliances.

I would locate the individual relays at each appliance and only bring the parallel combination of the relay contact outputs to the fan control circuit. That way you won't have to worry about a voltage drop in the 12V circuits due to wire length.

Here's a schematic for two appliances. Expand for more.

simulate this circuit – Schematic created using CircuitLab

• Thanks. This makes sense. It's a short distance from all appliances to the fan control switch that we're building. I think the longest run would be about 30 feet from appliance to relay. Is there a specific length where 12VDC voltage drop becomes a concern, and can that be mitigated with heavier guage wire? (would be nice to keep all these signals runs at 12vdc for simpler wiring/conduit requirements) Jun 23, 2021 at 17:42
• @DavidB Wire gauge should be calculated based on the current and dropout voltage of the relay coil (from the relay data sheet) and wire length. Jun 23, 2021 at 19:09
• @DavidB BTW, on SE we like to have each question be just one question so I won't add more detail on wire sizing into this answer. If you search previous questions and still don't find guidance on how to size the wire from appliances to relay coils, post a fresh question. (PS On this question, click the checkmark next to the best answer to accept it.) Jun 23, 2021 at 21:42
• @DavidB Relays don't draw a lot of current. A typical 12V relay for a mains load would have something like 100mA coil current with a must-operate voltage of 9V, so you have a lot of headroom for v-drop. Even 24AWG wire would only have 0.5V of drop at 100mA over a 30m (100ft) run, so pretty much any hookup wire should be fine.
– J...
Jun 24, 2021 at 11:03

When any appliance reaches a specified temperature, it sends out a continuous 12 VDC signal. This is intended to activate a relay to run a ventilation fan.

Then those 12 V signals are powerful enough to power the relay's coil directly so then we can use a simple circuit like this:

simulate this circuit – Schematic created using CircuitLab

The diodes form a simple "OR" function meaning that if on one (or more) inputs a 12 V is present, the fan switch will close.

The diodes connect to the 12 V + of each wire.

All the - (negative) wires are connected together and connected to the relay as shown. The ground symbol is just a virtual thing, there's no need to actually connect to ground.

The diodes also prevent that input2 and input3 are pulled up to 12 V when only input1 is receiving 12 V.

Note how my circuit doesn't need any transistors.

Update:

Edit: the 12v signals need to be isolated from one another,

Thanks for mentioning that after me writing this answer.

Then what I propose cannot be done, you need multiple relays in parallel, see Theodore's answer.

• Yes, but separate appliances with separate power feeds on separate circuits should not share a common ground, in general. OP hasn't specified, but there's no guarantee that these are all sharing a common circuit. Tying the grounds together at the business end of the circuits will create ground loops - sometimes large ones that circle the whole building. This can be a catastrophe for noise pickup, so a solution that keeps the individual appliances isolated should be preferred, I think.
– J...
Jun 24, 2021 at 10:53
• but there's no guarantee that these are all sharing a common circuit. Actually the easiest solution is if all those grounds are not connecting yet so that you connect them at this circuit as shown above. Then there can be no ground loops. Without further detailed information it is impossible to say if isolation is needed at all. a solution that keeps the individual appliances isolated should be preferred If that is needed then yes it is, bit we don;t know. In that case I would simply use multiple relays (one for each signal) with their contacts in parallel. Jun 24, 2021 at 11:09
• No, I mean that these appliances could be individually powered by separate mains circuits in the building, each sourcing the same ground from the panel in a star configuration. If you electrically connect the appliance end of the commons together you now have created loop circuits from the panel to the appliances and back again that could potentially encircle large areas of the facility. Multiple relays is the correct solution - isolation is generally required between separately mains-connected equipment.
– J...
Jun 24, 2021 at 11:25
• This answer was written BEFORE OP mentioned that the grounds cannot be connected. Jun 24, 2021 at 11:37
• Yes, that's fine, and no DV from me - it's just an important consideration. Even without OP having said so, it's something we should always be thinking about in a facility context.
– J...
Jun 24, 2021 at 11:44

The critical question: do the three 12 V signals share a common Ground (12 V return, -12 V, etc.)?

And if so, do they share that ground with whatever supplies power to the relay coil?

If you are lucky, the various 12 V signals and power sources are either already common-grounded, or floating so all of their negatives can be connected together. If so, then Bimpel's answer with nothing but 3 diodes should work fine. Do you have a make/part number, datasheet link, or other information about the relay?

Update:

It sounds like the three control signals are floating, so their commons might be joinable. If that makes you nervous, then your are looking at four ground-isolated circuits, the three control signals and the relay power supply. This is the (relatively) worst case.

One way to solve it is with three opto-isolators. Depending on the opto and fan relay choices, they might be able to drive the relay coil directly. Otherwise, it's three opto output transistors in parallel, turning on a single driver transistor.

• The 12v signals don't share a ground - each is uniquely generated by the applieces, each of which has a transformer onboard taking 240v down to 12v for the computer controls onboard. The control relay(s) will be whatever we spec - we are building that controller. Right now the fan is just controlled via a 120vac single-pole switch. Jun 23, 2021 at 18:12

Something like this circuit may work, it uses mosfets on the low side of the relay to switch on the relay, if any of the mosfets are 'on' the relay will be on. If both of the mosfets are on, the relay will also be on/closed (if you select a normally closed relay, most relays come in normally open and normally closed versions.).

simulate this circuit – Schematic created using CircuitLab

The mosfets must be sized so that whatever switching voltage you use for their gate is high enough to turn the mosfets on, but not higher then the gates rated voltage. R1 might be necessary to get the right amout of current for the relay (but any 12V relay should work)

Edit: This answer was given before the isolation requirement was given.

If you need to use a transistor to energize a relay, you can do this:

simulate this circuit – Schematic created using CircuitLab