Circuit for a beginner

Discussion in 'General Electronics Chat' started by Soomro, Nov 20, 2012.

  1. Soomro

    Thread Starter New Member

    Nov 20, 2012
    Hello everyone,

    I am a beginner of electronic study, through it has just started few days ago but it has inspired me a lot and I am looking to learn and implement things before I get to learn from the teachers. In that regard, I have come across an idea and would appreciate experts around here to suggest me some material or the circuit diagram itself.

    Basically, I have two 4v batteries with 2amps. I want to make a circuit that will light a LED from 1st battery until the battery consumes (drains), second battery is only used when first one gets empty. I have been searching this some few days but without any success, I know it requires transistor and resistors, which I already have gathered (2n3904 NPN transistor). I be happy if anyone out here can help me.


  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    your having a laff mate...:D

    Active Member

    Jun 29, 2012
    Hello Soomro a first I have a question,

    Your saying you have 2 batteries at 4 volts 2 amps and you want to power an LED with one battery and once that battery drains the other one will kick in. Now how much power are planning on giving this LED because Ic max on a 2n3904 is 200 mA.
    Last edited: Nov 20, 2012
  4. vk6zgo

    Active Member

    Jul 21, 2012
    There are circuits to do what you want,but they are not usually used in the simple case of two identical batteries,because it is easier & more efficient to simple connect the two batteries in parallel.

    Both batteries will last as long (probably slightly longer),as with your changeover circuit.

    Where the type of circuit you asked about is used,is in providing backup power to a system which normally runs off a mains sourced low voltage supply.
  5. SPQR


    Nov 4, 2011
    I think I know what you'd like to do - You'd like to put together a simple "beginners" circuit that turns something off and on depending on the voltage level of a battery, right?

    Here is a nice voltage level detector circuit that turns on an LED when voltage falls to a level determined by the Zener and R1.

    So if you built that one, you could then figure out how to change that LED to a "relay" (mechanical SPDT, or some type of transistor) that could switch between the two batteries.

    It uses a zener, some passive components, and two of the 2n3904s.

    Do you think this would get you started?

    If you had access to other components, you might think about using an IC voltage comparator, but I think your project is an interesting one.
    Last edited: Nov 21, 2012
  6. Soomro

    Thread Starter New Member

    Nov 20, 2012
    Thank you everyone for the explanation, here is what I exactly wanted to built. I got a LED charging light, when it is charging through AC (220v), its not illuminating and once its on DC, it illuminate automatically. It feels cool to me to make something like that without suing AC (I ain't gonna touch AC without understanding electricity properly) so I want to replace it with DC. Which makes it like if its charging from DC (battery A) then DC (B battery) is turned off and once I detach (remove or switch off) B battery them it starts glowing again.

    This is a cool idea and will help me in having good marks in coming semester as well in my understandings. I have just started working of schematically designs (not good yet) therefore would be highly obliged to anyone who can make me a schematic and brief over each component.

  7. Soomro

    Thread Starter New Member

    Nov 20, 2012
    Yes Exactly!
  8. SPQR


    Nov 4, 2011
    Here are some nice examples of switching circuits based on voltage:
    Using a MOSFET
    HERE is a nice one using a relay.
    Nice book chapter using Zener diodes.

    If you google with

    automatic switching circuit based on voltage level

    you'll see a ton of examples.

    Have fun!