4 Replies Latest reply on May 30, 2020 7:03 PM by kkazem

    How can I automate circuit on/off based on a voltage threshold in another circuit?




      I'm new to circuit design and I'm trying to figure out an easy way to automatically switch circuits on and off.  Ideally, I would like for the switch to happen when a certain voltage is reached across a battery that is recharging.  When the threshold voltage is reached I want to turn off a circuit and turn on another circuit.


      There are actually three circuits involved so when circuit 1 is switched on, circuit 2 and 3 should be switched off. 


      The ON and Off of the circuits will rotate like this:


      circuit # 1 on switches 2 and 3 off 

      circuit # 2 on switches 1 and 3 off,

      circuit # 3 on switches 1 and 2 off


      then repeat back to circuit # 1 on switches 2 and 3 off, etc., etc.


      FYI, Each circuit will be running off its own 12V battery


      I was trying to avoid using Arduino as that seems like it may take a while to learn.  I thought Mosfets as switches might work but not sure how to configure them.


      Hope this makes sense. Any help would be greatly appreciated.

      Thanks for your time.


        • Re: How can I automate circuit on/off based on a voltage threshold in another circuit?

          I hope you aren't trying to make batteries charge each other


          You don't tell us enough.



          Do the three batteries have a common 0V connection or are they fully isolated.

          What is the current.

          Do you need make before break or break before make switching.

          What kind of batteries, are there any safety issues.


          Best bet is explain what you are trying to do overall and fill in the details. A block diagram would help.


          My first thought is that an Arduino or similar would be the ideal control element - because you'll find that you need hystereisis, timeouts and other features. You will still need power switches, relays are much easier to manage than MOSFETs.


          Doing it with pure analogue is always possible but much harder to modify and tune.


          If it's a serious commercial application you could use PLC type controllers - much more expensive than Arduino, and no easier to learn.





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            • Re: How can I automate circuit on/off based on a voltage threshold in another circuit?



              Thank you for your response.  Yeah, I was pretty vague with my question.  Let me explain.  This is not a commercial application.  I am a theoretical physicist and I'm doing some experiments to test the properties of electricity.  So I want to set up three circuits that will each, in turn, run a small load like a computer fan and I will be adding different components to different circuits and taking measurements from each separate circuit to compare.  I haven't set this up yet.  I'm just in the design phase.


              I would probably use lead-acid rechargeable batteries and have them totally separate. I just want to automate the on off so that only one circuit at a time is running the load.


              You said you hope I'm not charging batteries with other batteries?  Is there a problem with that? That may be something I would experiment with in the future.  Are there safety issues with doing that?  Right now I just want to run a small load with one circuit at a time but switch them automatically because each circuit would be running the same load but only one at a time.


              I will see if I can write up a more detailed question and give you more information, maybe upload a diagram too.





            • Re: How can I automate circuit on/off based on a voltage threshold in another circuit?

              This wouldn't be that hard to design and there are many ways to do this.

              1. An Arduino may be the easiest. The coding would be fairly simple. But the Arduino is for the control. You still need MOSFETS or BJT transistors with proper gate or base drive to do the actual switching. The details of how much current and voltage you're switching would need to be known to select the proper components.

              2. An FPGA would also be a good choice, but again, there is some programming. You need either an FPGA with analog comparators or use a quad comparator in front of the FPGA.

              3. Discrete gates and comparators would work as well, but there would be more breadboarding perhaps.


              Make a diagram with voltages and current loads and we can then talk specifics.