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Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

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    Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

    I don't know the first thing about where to even start looking for the result I'm looking for ... but what I want, is a circuit ... or maybe a certain transistor ... that will remain "closed" when the voltage at the base is 5 volts, but when that voltage gets to 4 volts or less, I need that transistor to turn on - preferably with maximum saturation. I would prefer that it also does not even turn on a little as the voltage approaches 4 volts, but remains closed until that voltage hits 4 volts.

    Now, it would be OK if it had some kind of marginal transition from zero saturation to fully saturated once it hits that 4-volt threshold ... like say for example at 4 volts, it starts to turn on but is not fully saturated until it reaches 3.9 volts or whatever ... a 1-volt margin in that characteristic would be acceptable.

    I hope my question makes sense and that it's clear what I'm looking for. If not, please let me know.

    Thank you,

    See attachment for a simple visual.
    Attached Files

    #2
    Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

    you could start with something like this then adapt it .. maybe long way round though . https://www.homemade-circuits.com/lo...uit-using-two/

    Comment


      #3
      Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

      You need to study and understand how Transistor work and how to bias it. You will blow-up that E-B junction, if you understand Transistor then you would not have the BASE CONNECTED DIRECTLY TO 5V source like that. How did you come up with the connection like that?
      https://www.badcaps.net/forum/attach...5&d=1578528302
      You SHOULD to this thread: https://www.badcaps.net/forum/showthread.php?t=81784
      You lack basic understanding of electronics.
      Do the experiment by hooking up the high current 5V power supply directly to the E-B junction of that NPN Transistor and see what will happen.
      Last edited by budm; 01-08-2020, 07:19 PM.
      Never stop learning
      Basic LCD TV and Monitor troubleshooting guides.
      http://www.badcaps.net/forum/showthr...956#post305956

      Voltage Regulator (LDO) testing:
      http://www.badcaps.net/forum/showthr...999#post300999

      Inverter testing using old CFL:
      http://www.badcaps.net/forum/showthr...er+testing+cfl

      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
      http://s807.photobucket.com/user/budm/library/

      TV Factory reset codes listing:
      http://www.badcaps.net/forum/showthread.php?t=24809

      Comment


        #4
        Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

        go back to your original thread,
        and learn about schmitt-triggers and the difference between transistors and mosfets.

        Comment


          #5
          Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

          Transistor Theory 101 from like the 1960?

          Comment


            #6
            Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

            well the simple fact that transistors are current-driven and fet's are voltage driven is enough here.

            Comment


              #7
              Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

              Originally posted by EasyGoing1 View Post
              I hope my question makes sense and that it's clear what I'm looking for. If not, please let me know.
              Why don't you describe what you're trying to *do* instead of a particular circuit topology (that may or may not work, towards that goal)

              Comment


                #8
                Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                Originally posted by brethin View Post
                Transistor Theory 101 from like the 1960?
                I came here looking for help. I admitted my lack of knowledge while stating what I am trying to accomplish ... and you post a comment about 1960 transistor theory? Are you insulting me? Because I can't see any other purpose in your response other than sarcasm.

                Comment


                  #9
                  Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                  https://www.badcaps.net/forum/attach...5&d=1578528302
                  If you study the theory then you would not have made this kind of connections.
                  Heck, I study from books older than 1960 and I never stop learning.
                  How can you gain knowledge if you think it is insulting?
                  It is obvious that you did not do any studying.
                  Last edited by budm; 01-10-2020, 03:02 PM.
                  Never stop learning
                  Basic LCD TV and Monitor troubleshooting guides.
                  http://www.badcaps.net/forum/showthr...956#post305956

                  Voltage Regulator (LDO) testing:
                  http://www.badcaps.net/forum/showthr...999#post300999

                  Inverter testing using old CFL:
                  http://www.badcaps.net/forum/showthr...er+testing+cfl

                  Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                  http://s807.photobucket.com/user/budm/library/

                  TV Factory reset codes listing:
                  http://www.badcaps.net/forum/showthread.php?t=24809

                  Comment


                    #10
                    Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                    Connecting a transistor as you have drawn will make smoke
                    People want help - but "give me the answer" instead of "I'm trying to learn more about electronics".
                    The difference is a person will learn the basics about a transistor, it only has 3 leads.
                    We can't do much if you withhold a part number or details etc. even after people ask for more info.

                    Comment


                      #11
                      Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                      Originally posted by redwire View Post
                      connecting a transistor as you have drawn will make smoke
                      People want help - but "give me the answer" instead of "i'm trying to learn more about electronics".
                      The difference is a person will learn the basics about a transistor, it only has 3 leads.
                      We can't do much if you withhold a part number or details etc. Even after people ask for more info.
                      +100
                      Never stop learning
                      Basic LCD TV and Monitor troubleshooting guides.
                      http://www.badcaps.net/forum/showthr...956#post305956

                      Voltage Regulator (LDO) testing:
                      http://www.badcaps.net/forum/showthr...999#post300999

                      Inverter testing using old CFL:
                      http://www.badcaps.net/forum/showthr...er+testing+cfl

                      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                      http://s807.photobucket.com/user/budm/library/

                      TV Factory reset codes listing:
                      http://www.badcaps.net/forum/showthread.php?t=24809

                      Comment


                        #12
                        Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                        i like playing with this ...http://www.falstad.com/circuit/e-npn.html

                        Comment


                          #13
                          Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                          What about using a STM1061 series (STM1061N38) to drive one transistor (which would turn OFF when under 3.8 volts) and it could turn ON the output transistor.
                          There are theres like the ON NCP304A
                          Last edited by R_J; 01-10-2020, 07:02 PM.

                          Comment


                            #14
                            Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                            A LM311 with a resistor circuit will make a level sensor like a sync seperator .
                            Used one in my diode radiation detector I made last year.
                            Barry Wilkins

                            Comment


                              #15
                              Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                              Originally posted by budm View Post
                              https://www.badcaps.net/forum/attach...5&d=1578528302
                              It is obvious that you did not do any studying.
                              In the very early 1990's, I took two years of electronics, including electrical calculus. I even passed my CET ... but when you don't use the knowledge it tends to vacate the belfry... I understand transistor theory just fine, I simply didn't know how to design a circuit that is off at 5 volts, then turns on as the voltage gets lower ... I played with some circuit sims with pnp transistors and didn't get anywhere... I googled around for a while but didn't come up with anything ... so as per usual, I came in here as my last resort ... and it seems that some people would just rather make useless comments instead of answering a simple question, or even pointing me in the right direction... it's all too often like pulling teeth in forums...

                              Comment


                                #16
                                Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                                Originally posted by petehall347 View Post
                                i like playing with this ...http://www.falstad.com/circuit/e-npn.html
                                HEY - That's pretty nifty! Thank you.

                                Comment


                                  #17
                                  Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                                  Originally posted by budm View Post
                                  https://www.badcaps.net/forum/attach...5&d=1578528302
                                  If you study the theory then you would not have made this kind of connections.
                                  The cirucuit I drew was a pseudo-circuit to illustrate the general effect of what I was looking for ... it never was a literal circuit.

                                  Comment


                                    #18
                                    Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                                    Originally posted by EasyGoing1 View Post
                                    HEY - That's pretty nifty! Thank you.
                                    Try changing the voltage bounds of the base voltage and see why people are complaining that your circuit tends to generate magic smoke...

                                    I tried setting base voltage to 2VDC to see what the simulator does. When I do this, it starts printing weird things...

                                    Base current: 6149GA
                                    Collector current: 614897GA
                                    Emitter current: 621046GA

                                    What the... oh yeah SI units... G as in...giga...gigaamps.

                                    Okay, so 614 teraamps starts flowing through the collector. Ahhhhh.....

                                    Comment


                                      #19
                                      Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                                      1) So far you did not provide the spec of the IR receiver.
                                      2) The screen shot of wave form of the IR output.
                                      3) The real circuit that you actually made.
                                      4) Did you actually run the simulation of the circuit you made?
                                      Never stop learning
                                      Basic LCD TV and Monitor troubleshooting guides.
                                      http://www.badcaps.net/forum/showthr...956#post305956

                                      Voltage Regulator (LDO) testing:
                                      http://www.badcaps.net/forum/showthr...999#post300999

                                      Inverter testing using old CFL:
                                      http://www.badcaps.net/forum/showthr...er+testing+cfl

                                      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                                      http://s807.photobucket.com/user/budm/library/

                                      TV Factory reset codes listing:
                                      http://www.badcaps.net/forum/showthread.php?t=24809

                                      Comment


                                        #20
                                        Re: Need a circuit that switches ON when a 5 volt source drops to 4 volts or less

                                        If you are working with voltage detections above 2.5V and need a quick and easy way to accomplish this use:
                                        431 current shunt

                                        (i.e. TL431, KA431, AS431... and etc.)

                                        It's the easiest (IMO) 3-terminal "voltage detection" device to use for that function. If you have Anode connected to ground (how these 431 shunts are often used in PSUs) and the voltage on the reference pin goes above 2.5V, then the Cathode pin will short to ground. You can use the Cathode pin to drive a transistor circuit or whatever it is you are trying to drive (just mind the limitations on maximum current you can drive through the 431 shunt.)

                                        Now, to detect a voltage that is different (and above) 2.5V, use a simple resistor divider between your threshold voltage and the reference pin.

                                        Comment

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