Can I bridge GND on a relay so I only need two wires?

Projects General Guidance
1

I put these two relays in a junction box. The corner of the box is mangled; never mind that. It's an old box that I'm repurposing. Without going into specifics as to why I'd like to limit the number of wires, I'd like to ask:

Can I bridge the trigger pin and the GND pin on each of the relays so I can have one common 5V between the two relays and I can trigger each individual relay by sending GND to the bridged GND+Trigger pin, thus allowing me to use both relays in this box by wiring them with only three wires?

Wires

  1. 5V Common
  2. Relay #1 GND+Trigger
  3. Relay #2 GND+Trigger

IMG_9056
IMG_90561920×2560 295 KB

2

No, the MCU output pin would have to carry the coil current.

3

Also noted. There is no strain relief visible on any of the wires.

Maybe draw a wiring / schematic of how everything is connected.

4

How far is the box from the Arduino? A piece of 4 conductor, 24 AWG telephone "station cable" would work OK as long as flexibility is not needed.

5

I would send GND to the GND pin with a bridge to the trigger. The GND is not coming from GPIO LOW; it is coming from GND.

The wire is 20 gauge and the junction box is about 6 feet from the MCU. The MCU will connect directly to an 8-relay module that will serve as the Pilot Relay (let's call the relays on that module Relay #A, Relay #B, Relay #C...), and Relay #G and Relay #H will have GND connected to Common and NO will connect to the 6-foot wire that runs to the two relays in the extension box.

So, more specifically:

Wires to junction box

  1. 5V Common for both relays in the junction box

  2. NO from Pilot Relay #G is connected to GND+Trigger on Relay #1 in the junction box. When the MCU GPIO is HIGH, Pilot Relay #G triggers, sending GND through NO to the GND pin of Relay #1 in the junction box. The GND pin on Relay #1 is bridged to the Trigger pin on Relay #1, so when Pilot Relay #G is triggered by the MCU, Relay #1 is triggered by GND from Pilot Relay #G.

  3. NO from Pilot Relay #H is connected to GND+Trigger on Relay #2 in the junction box. When the MCU GPIO is HIGH, Pilot Relay #H triggers, sending GND through NO to the GND pin of Relay #2 in the junction box. The GND pin on Relay #2 is bridged to the Trigger pin on Relay #2, so when Pilot Relay #H is triggered by the MCU, Relay #2 is triggered by GND from Pilot Relay #H.

I'm trying not to get into the weeds. I just want to know whether there's any problem with providing constant 5V and a bridged GND+Trigger to activate the relays in the junction box. I could do the same thing with an 2N2222, or probably with an opto... someone probably will think the 8-relay module is overkill but this is part of a bigger project and I'm just trying to focus on the feasibility of this "three-wire for two relays" portion. Thanks for your input.

I'll secure the wires a bit from strain. This is still a work is progress.

7

This is a picture of what I am planning. The Pilot Relay module will send GND to the GND + Trigger pins at the same time, so the relay will trigger as soon as it's powered on because it uses a low level trigger; thus, I can use three wires to utilize two relays (one common 5V and two distinct GND).

My question is whether this is ill advised.

relay
relay930×412 141 KB

8

If by trigger you mean a connection to the activating coil, then it would be standard practice to common one side of multiple coils to ground and drive each through the other terminals. Whether that works with an Arduino gpio is another question. You also need flyback diodes on each coil.

9

If your remote relay is just a relay with no drive electronics associated with it then supplying it with +5V and then grounding the other side of the coil will work just fine. Is there some reason why you can’t do that?

10

Why didn't you explain this in you first post? I suppose it would work as long as the 5V supply can handle the 150mA current of 2 relay coils.

11

Someone PM'd me and asked why I'm using redundant relays. It seems that context might be of benefit so here is a copy/paste of my response. Long story short, I have two enclosures and one houses the electronics while the other houses the power supplies. I want to toggle On/Off for two of the power supplies with as few wires running between the two boxes as possible. Please read further if interested.

I have a steel box mounted to my house's exterior. It's about 12"x6"x6" and it has a GFCI receptacle inside. It has housed my sprinkler controls and, more-recently, a 6-relay module that I have put inside with and ESP32 (with an antenna wire that extends out the PVC pipe alongside the exiting wires that lead to my sprinklers and landscaping lights). The sprinkler valves are covered by a large, hollow, fake rock. Everything works fine and is concealed well.

But I want to put some outdoor cameras in that vicinity and I cannot supply ethernet to that area (believe me, no way). My grandfather built this house himself and my dad and I cherish it as it is; no holes shall be cut into walls, etc. I'm using MoCA to supply wired ethernet to various rooms in the house. One such room is the kitchen, and the steel box is on the wall outside that kitchen. So here is my solution:

  • MoCA to the kitchen, to a Unifi AP
  • Unifi In-Wall AP in a large enclosure underneath that fake rock with the sprinkler valves
  • ESP32 and 8-relay module in that same enclosure
  • Peltier module mounted to the enclosure to keep it cool (ESP32 will have a temp sensor inside the enclosure and another inside the rock)

The AP in the kitchen will serve as a backbone to the AP in the plastic enclosure, so I can run PoE cameras over that dedicated backbone. The In-Wall AP that I'm putting in the plastic enclosure under the fake rock has two data ports, so I can connect two cameras to it.

The steel box is relatively small but the plastic enclosure that houses all of this stuff is nice and big, so it will house the electronics. The steel box, with the GFCI outlet, will house the power supplies (and convert them to DC before I send the electricity out to the plastic rock). Three power supplies:

  1. 55V for PoE Injector
  2. 12V for Peltier module
  3. 24VAC for sprinklers and landscaping lights

The 5V for the ESP32 will come from the PoE inside the plastic enclosure.

I have a 7x wire cable that I would like to make the most of, running between the steel box and the plastic enclosure. Now, those two relays in the junction box, which my thread is about... those are also (hopefully) going to fit inside the steel box so I can toggle the power to the 12V Peltier module when the weather is hot, and the 24VAC power supply to sprinklers and landscaping lights (so can turn off that transformer when sprinklers and lights are not in use). The PoE injector will always be powered on; no relay needed.

  1. 24VAC+ for sprinklers and landscaping lights (to 8-relay module #A #B #C #D #E)
  2. 24VAC- for sprinklers and landscaping lights
  3. PoE Injector 55V+ (it's like 1.6A so this wire is fine)
  4. PoE Injector 55V-
  5. 5V+ to Relay #1 #2
  6. GND+Trigger for Relay #1 (from Relay #G)
  7. GND+Trigger for Relay #2 (from Relay #H)

This leaves only the 12V power for the Peltier, which can reach 6A so I'm using some 14AWG wire that I already have.

I could use some more wire that I have lying around if I need another pair for the two relay triggers in the metal box, but I'm just aiming for efficiency.

In case you're interested, the PoE injector inside the plastic enclosure has four ports:

  1. Power to the Unifi In-Wall access point
  2. Power to the 5V splitter
  3. Camera #1
  4. Camera #2

The house's gutter has a down spout right next to the fake rock, and I've purchased some aluminum tube. I'm going to run the ethernet up the downspout of the gutter (I have some gaskets coming from AliExpress so I can put the aluminum tubes through the bottom of the downspout) so it's concealed inside metal throughout its run to the top of the house. My ethernet cable is heavy duty, outdoor cable.

Long story short, the ESP32, etc. don't fit inside the steel enclosure and I want to toggle AC power to the power supplies. So, I'm housing all the electronics elsewhere and sending On/Off signals to two of the power supplies inside the steel box ...and I'm aiming for efficiency. Yes, a couple more wires wouldn't be a burden if I need to separate GND and Trigger for the relays, but if I don't need to do that I'd rather not.

12

  1. Forgive me but I don't know what drive electronics are. I presume those would be electronics that are powered by the same power supply as the MCU? The relay is going to toggle power to some AC devices.

  2. "grounding the other side of the coil": Do you mean literally "grounding", as in connecting the GND on the relay pin to earth's ground?

Forgive me. I'm not well-versed in these sort of topics.

13

Your plan will work just fine using the diagram in post 8. Since you are using the electronically controlled relays the will already have the fly back diodes required incorporated into the circuitry.

I was having trouble understanding what you wanted to do until you posted the picture. In electronics a schematic is worth well more than a thousand words.

Good luck with you project

14

It's really hard to talk about your circuit without any part numbers for your relay modules.

I suggest you try what you propose and see if it works. It looks like it probably will.