# Switch off 2 power supplies using a single switch in a feedback loop

We have setup a solar stairwell project at home, using LED and salvaged batteries and this is what it looks like:

However it may look, the circuit can be oversimplified to:

simulate this circuit – Schematic created using CircuitLab

i.e., it's a battery at the top of my house, ( which is charged using solar power ) and all the LEDs run to the bottom of the house with all of them staying in a parallel configuration.

As you might have guessed, we did not have sufficient brightness at the end of the stairwell. The voltage at the bottom of the stairwell is around 8V. So, when we found another salvageable battery yesterday, we connected it to the stairwell, and it worked well.

The new circuit would look something like this:

simulate this circuit

However, we wanted to operate the whole stairwell using only a single switch, so I came up with an idea of using a relay to turn on the 2nd battery downstairs but I can't think of anyway to turn it off using the same switch upstairs, as it is creating a sort of feedback loop.

simulate this circuit

I used a 5V relay at the yellow wires as it was the only thing lying around in my house. I'm fairly certain that the blue section is causing a feedback loop, but I've already spent a whole night to think of way to solve it.

Both the batteries are heavy, so I cannot move them, they must remain upstairs and downstairs as I don't want to clog my stairwell. Since it's a stairwell, I can only add very minimal circuit in the middle as it's very difficult to work there, but if it's impossible to do so without tampering in the middle, I would live with it.

Apologies for the verbosity of my question, I'm pretty sure that someone from the industry might have faced this problem before and have a solution at hand. I'm just an enthusiast with no proper industry training and couldn't search for this problem precisely online.

All help would be appreciated. 😀

• Are you able to add an additional wire between the two batteries? Apr 25, 2022 at 3:31
• Depending on where you are, you may well be in breach of the wiring regulations. Here in the UK, there is a requirement to take voltage drop into account when sizing your cables. Done properly, you should either split this into separate subcircuits, or use an appropriately sized larger wire to feed the circuit. 12V is not really suitable for long runs. Apr 25, 2022 at 15:15
• @tsamridh86 I think the professional solution would be to just run extra, thicker wires. The professional would have calculated this before installing the wires! Apr 25, 2022 at 15:18
• As an enthusiast with no regard for building codes, at least you can consider running extra parallel wires somewhere convenient, and connecting them into the lights at the most convenient points only. No need to make them follow the lighting for the whole length. Apr 25, 2022 at 15:20
• @IanBland, don't worry sir, I'm in Nepal, where almost all of our top leaders are illiterate, so no one is going to come after me for not following building codes ( if that law even exists here 🤣), but I understand your concerns that it maybe causing an efficiency drop. As for safety, I knew it was just 12V so the worst that could happen was a spark and nothing harmful. I know 12V wasn't enough, but that's the only LED strip and the salvaged batteries that I had, and for the wires, I recycled some old telephone cables to keep the project under budget. I hope you can forgive me for doing this Apr 26, 2022 at 1:13

The lights look great! I have done similar things in my house. Is it possible to increase the voltage, if not ok. In my proposed solution the increase in voltage will reduce the current in the wire which I assume is sized properly for the currents involved. If you do not know please check, it could be a fire hazard. In either case I would then place a small sepic converters at each lamp or so? (suggest setting them at 11V) or less. I use them as they are inexpensive and work great.

These converters would be staged every few lights, requiring fewer of them. This approach would allow you to run them without the aux battery. The biggest problem would be how to hide them.

When you wire the converters starting at the beginning (solar/switch side) at each light or so you would cut the wire and insert the converter so it then powers everything downstream. A trick that may save you a lot of grief is by lowering the voltage to each light you will dramatically reduces its current requirement while increase its life exponentially. Try about 11V for a 12V light, the brightness will not change that much but the current will.

You could incorporate a wireless switch pair and have switches at both ends of the stairs.

• Thank you for your suggestion. I don't want to use wireless switch because I want it to be a cheap, green solution as all the parts that I'm using are salvaged / junk people don't want, I have hardly even spent $10 in this project because everything is just junk others don't want. That's the only reason why I don't want to use it, as for the sepic converters, I'll do a little bit of hunting around the market. Can you give me any hints where I can salvage those converters? Maybe i'll find something in the junkyard 😁 Apr 26, 2022 at 1:20 • I don't have experience manipulating voltage, I can keep batteries in parallel or series and measure using my voltmeter and that's where my knowledge ends. is there i guide where i can learn to change voltage to what i like? i frankly never paid too much attention in school, and suddenly this is interesting to me now. Sorry for all the trouble I'm giving you😅 Apr 26, 2022 at 1:23 • Search Aliexpress, they have a 3A device for US$0.97 each. There are many more. I used "Boost Buck DC-DC adjustable step up down Converter" as the search term. The one I am referencing was the first one found out of several hundred hits. No problem. The connection is in your capability. The power goes into the converter, you adjust the voltage using your meter and if it has current limit max that out. Use that output to feed the next converter and lights.S---C---C---C---X Start --- Wire, C converter, X last unit. That is the basics of the circuit. Do them one at a time and test.
– Gil
Apr 26, 2022 at 1:34
• Thanks, I'll give it a shot Apr 26, 2022 at 1:52
• @tsamridh86 I don't know where you would find a salvaged converter. You can make one from salvaged parts but it's not simple. It is probably much easier to salvage extra wire (don't steal it!). Apr 26, 2022 at 8:19

One way to make this work is to split the LEDs to only the way one is powered off of the second battery. In this case, you can use the switched first battery to operate the relay, acting as the switch on the second battery. Because you've separate the relay input and output, there is no feedback.

simulate this circuit – Schematic created using CircuitLab

If you're willing to add another wire from end-to-end and a diode, you can feed both batteries into all of the LEDs, as so:

simulate this circuit

Dioce D1 prevents the voltage from the downstairs battery from feeding back to the relay. Getting the isolated feed to the relay coil requires the extra wire from the switch to the relay.

• Do you think it will be better if I add the Diode D4 somewhere in the middle of my stairwell? or does it have to be at the very beginning? Apr 26, 2022 at 6:33
• @tsamridh86 It really doesn’t matter where it is as long as it’s between the tap for the third wire and and the first LED. Apr 26, 2022 at 7:45