How to build a low-battery indicator (and other circuits)

Discussion in 'General Modding & Electronics Information' started by daftmike, Jan 18, 2010.

  1. daftmike

    daftmike Electronics Whizz!

    Here I'll detail what I did to build my low-battery circuit, hopefully some of this will also apply to more general circuit making. First, start with a schematic diagram; here is mine.

    [​IMG]
    This circuit will show a green led when the battery is ok, and switch it to show a red led when the battery voltage is low. There's more explanation at the end of the post in this thread.

    The first step is to find out what parts you need, all circuit diagrams should use the same symbols for components. This page http://www.kpsec.freeuk.com/symbol.htm has a good list of the most common symbols. In my case I need: 6 resistors, 5 npn transistors, 2 leds and a potentiometer. Now that you know what components you'll require, you need something to build the circuit on.

    There are a few types of prototyping board available, the type I use is called "Veroboard" or stripboard. It's the type that has rows of copper strips, there are other types that have separated copper pads and are just as useful but quite expensive where I live. I find 9 x 25 stripboard is usually enough to build most small circuits on.

    [​IMG]

    Next you need to translate the circuit diagram to your stripboard layout. This is the tricky part. I usually do this by hand on graph paper, because I find it easier to work with. This page http://www.zen22142.zen.co.uk/Prac/vero/vero.html has some useful templates that you can use if you want to do it by copy and pasting in paint.
    You need to think carefully about where you place each component and where you may need to cut tracks or add wire links to complete the circuit. You have to take into account the size of each part and where the leads are. There is software to do this "automatically", but I find for small circuits it's not really worth the effort in setting up and it always requires manual tweaking anyway.

    Here is what I came up with for the above schematic:
    [​IMG]

    The plus and minus symbols are where the battery is connected, the blue lines are wires, the brown squares are where the track underneath is cut and the part on the left is the potentiometer used to set the voltage the low-battery led comes on at. It's important to note the order of the pins on your transistor because they may not be the same as on my diagram, eg. from left-to-right; 2N4401 is E-B-C, and BC547 is C-B-E.

    Now to start building the circuit. When using stripboard you should begin by cutting the tracks, this can be done with a dremel or by carefully twisting a screwdriver or drill-bit inside the holes until the copper track is broken. It's a good idea to clean the board after this step to remove any residual copper.

    For the low-battery circuit, make the cuts according to this picture.
    [​IMG]

    Next gather the components:
    [​IMG]

    To make it easier to solder, start with the flattest components first working your way up to the tallest. I start by making the wire bridges. I keep the offcuts of components legs for this purpose and bend them into shape with needle-nosed pliers. Alternatively you can use any type of wire for this.
    [​IMG]

    With the copper side down, place the wire bridges according to this pic.
    [​IMG]

    When soldering it will sometimes help to use a crocodile clip to keep the piece in place:
    [​IMG]

    After soldering the wire bridges, trim the leads as close to the solder joint as possible without actually cutting into the solder. If you cut too close it will feel soft, you should hear a loud *snip* as you cut the excess leads(unless you are using plain wire for this part).
    [​IMG]

    When you have done all the wire bridges, move on to the resistors:
    [​IMG]

    After that you can solder the rest of the components.
    [​IMG]

    And there you have it, one finished low-battery indicator circuit.

    edit: underside view
    [​IMG]
     
  2. zeturi

    zeturi DEMON PIE!

    How funny, I was just reading your low voltage indicator and wishing I were educated enough to understand it. Even I should be able to make this circuit, since there's pics nao. Thanks for posting this, daftmike! You're my hero!
     
  3. robm

    robm Billy Mays Staff Member

    Nice Daftmike, adding to the sticky.
     
  4. XCVG

    XCVG ModRetro Legend

    Nice, a picture guide. I love those!

    Also, on another note, I was making all three battery indicator circuits into printed circuit board layouts, but I don't know if I finished them all. Of course, not many of us have PCB-making equipment.
     
  5. zeturi

    zeturi DEMON PIE!

    The sharpie board vomitsaw posted a while back should work fine for this.
     
  6. da_monkey

    da_monkey Newb

    why don't you have the green led and its resistor in parallel with the 10k on the collector of the transistor you have labeled the pins on(the one doing the voltage sensing) and the red led and its resistor instead of the 10k along.
    then you can dump the next 3 transistors and remove one current path during normal operation.
     
  7. Basement_Modder

    Basement_Modder He who mods in thy basement.

    I like this guy already. ^
     
  8. daftmike

    daftmike Electronics Whizz!

    You could simplify the circuit as mario did here, but the way you suggest would lose functionality, the green led wouldn't switch off when the red came on.

    The 'extra' transistors are there so you get a sharp change between the green/red leds and not a gradual fading where both would be on at the same time as the voltage reaches the tripping point.
     
  9. aHanwell

    aHanwell Member

    This is a great guide, thanks. That's a different perf board than what I've seen before. What is it called?
     
  10. XCVG

    XCVG ModRetro Legend

    Veroboard or stripboard.
     
  11. Re: How to build a low-battery indicator (and other circuits

    !!! DUDE !!!

    FLAMING-AWESOME CIRCUIT !!!

    Ive just registered on modretro for 2 MAJOR reasons...

    1. TO THANK YOU !!!

    I built the circuit by carefully following your plans and picture diagrams, and guess what, WORKED 1ST-TIME !!!!

    - I'm measuring a battery-pack of 4 x AA Rechargeable Batteries for my RC Buggy! Green Indicator Light came on straight away !!

    So a Major Thanks For taking the time to put these instructions up!

    Now, I know that this is a very old post and i may not even get a reply, but...

    2. The 2nd reason i registered to ask you why my RED LED doesnt come-ON when i adjust the pot right down (so-as-to set the low-battery setting) ???

    Please please please help as i need to build ANOTHER 5 CIRCUITS exactly the same as this !!!!!!

    My rechargeable batteries are Duracell 2450 mAh x 4 so i'd roughly say that 1.3v x 4 = 5.2v @ Fully Charged and 3.41v when flat (okay a bit too flat!) !

    Ive scoped-out your blog & you mentioned that this circuit will work with voltages from 3-12v as it is, please please please could you tell me what ive done wrong or why you think the red LED doesn't light up ?

    Many many thanks in advance, oh and here's what i used:
    (ALL Brand-New Components!)

    3mm Green LED
    3mm Red LED
    5 x BC547B Transistors
    4 x 10K Resistors
    2 x 1K Resistors
    10K POT
    Exact Same VeroBoard !
     
  12. Prog

    Prog Not a Meme™ Staff Member

    Re: How to build a low-battery indicator (and other circuits

    Mario simplified this circuit. Perhaps viewtopic.php?f=7&t=3640 would help.
     
  13. Aux

    Aux Well-Known Member

    Re: How to build a low-battery indicator (and other circuits

    Mario doesn't have the green LED though.

    First silly question- are you sure you didn't wire your red LED in backwards? It's a diode, it only works one way. Try flipping it around and see if that works. I'm surprised how often that's the problem, even with electronics pros.
     
  14. Re: How to build a low-battery indicator (and other circuits

    MAJOR THANKS folks !!!!

    I am so surprised at the SPEED of your help and i am EXTREMELY Grateful !!!!!

    Really appreciate the help as i have only started learning electronics this year and its been a right-headache so far mainly bcoz im on a heck-of-alot of pain medication (severely mobility-impaired!!!) at the mo...

    Thanks @ ProgMetalMan but i really needed this circuit as it is because i love the idea of visually being able to clearly-see in an instant that the batteries are good, plus i love LED's !!

    Also whatever i make circuit-wise, i also make for a good-buddy of mine so both our buggies look-great !!!

    & Many many thanks @ Aux !!!

    I had double-checked, tripple-checked & QUAD-Dang-CHECKED, lol x 2!, and i had followed daftmike's (simple) instructions to the letter and as it was the RED LED that was not working, yet the entire circuit worked as it should:

    - When the POT was turned TOTALLY LOW, the GREEN LED switched-off
    - The POT Worked as it should have

    So i went-over the area where the red LED had been soldered & as USUAL, as you MENTIONED @ AUX, lol !, IT WAS MY ERROR, always gotta check the soldering !!!!

    I had made a solder bridge on the tiny thin strip of copper on the edge of the veroboard that screwed up my RED LED's functionality ! (IDIOT!)

    [​IMG]

    I just tried to stick a pic here but i think it failed!

    Anyhoo - suffice to say that ive found the problem, alot LESS INFURIATED and very very happy with this circuit as its so Dang easy to put together so just in case daftmike is seeing this post --- MAJOR THAKS --- ONCE AGAIN !!!

    & A LARGE-PORTION of thats to AUX & ProgMetalMan for the extremely SWIFT responses - i am so grateful !!!

    Also, speaking of this circuit being so easy.... I TRIED to put it onto a breadboard and as soon as i plugged in a 9v battery, the battery started SHORTING and getting very hot - so AGAIN - I have TOTALLY SCREWED UP a simple job!!!

    Please could anyone refer me to any software that can decipher veroboard to breadboard or more importantly --- CIRCUIT diagram - to - Breadboard & vice versa for me bcoz by the looks of thing, I AM A SIMPLETON, lol x 2!

    Many thanks in advance Again & Apologies for the LONG-READ !!!!
     
  15. Re: How to build a low-battery indicator (and other circuits

    Okay, i have a new problem with this circuit!

    As im very poor, i cannot afford to buy anymore components for the next 3 months, at-least, times are very difficult & as with alot of people, i'm heavily in debt :-(

    Anyhoo, i only have two 10K trimmers but is there any way i could just substitute the 10K Trimmer/POT for a Resistor or a combination of Resistors to 'Permanantly Set' the low battery RED LED to start illuminating ?

    I mean, as i'm still learning electronics i'm still trying to wrap my head around it all, made a few circuits so far and i'm a quick learner but need a little help here & would be most grateful & appreciative!

    What i want to do is to attach a flat battery to the circuit, dial down the 10K Trimmer/POT until the RED LED comes ON, then use my multimeter to measure the __________ (Please insert what i need to do here!!! Please please please!!!) so i can substitute the 10K Trimmer/POT for Resistor/s and have the circuit permanently-set to whatever battery {or battery-pack} that i need each circuit to monitor!

    I need to make 6 of these circuits but have only completed 2 because i dont have anymore 10K Trimmers/POT's & being severely mobility-impaired i'm on benefits so income is a huge problem.

    Please does anyone know what im trying to do is even possible ???

    Many thanks in advance for any help in this last area for this circuit for me (& hopefully others who might want to do the same!)

    Kind regards,

    Oneil
     
  16. Prog

    Prog Not a Meme™ Staff Member

    Re: How to build a low-battery indicator (and other circuits

    You're going to want to measure the ohms, and get the resister with the closest amount.
     
  17. Hermaphroditus

    Hermaphroditus Well-Known Member

    Re: How to build a low-battery indicator (and other circuits

    Completely possible, but likely that you're gonna have some ugly resistance values on your pot.

    When you find the perfect resistance, turn your circuit off and take the pot out to measure its resistance not attached to anything else. A resistance measurement in a circuit is different than alone. This measurement is in Ohms, btw. The greek letter Omega (Ω).

    Then you will likely have to make a series/parallel resistor network to get a final resistance near what the target is. Or you could just find a standard resistor value close enough.
     
  18. Re: How to build a low-battery indicator (and other circuits

    Many many thanks Hermaphroditus !! I am very very grateful !

    Now-Armed with the missing info i can create more of these circuits for use all over the house (lol x2!) !!!

    And with less expensive components! - The 10K Trimmers (Or Trimpots, whatever, lol !) are quite expensive for someone living on a shoe-string, its like £4 for two of them, and thats on EBAY !! The big websites just kill me with the extortionate shipping costs which i'll never be able to pay and my local electronics shop nearest to me is 50 miles away - plus i cant walk !!!

    So using up even a bunch-load of resistors to get the right resistance for the circuit would be great as i get them in packs of 100 ! - Cheap as chips !

    Once Again, Many many thanks for the info, i really appreciate it !!!
     
  19. Re: How to build a low-battery indicator (and other circuits

    --- ONCE AGAIN ---

    MAJOR THANKS TO daftmike FOR THE CIRCUIT & all the effort put into making it EASY-TO-FOLLOW for a NooBie who only started learning electronics THIS YEAR !!!!!!

    --- AND ---

    FOR EVERYONE WHO HELPED ME VERY RECENTLY:

    A MAJOR THANK-YOU !!!!!!!!!

    i'm so impressed with the extreme simplicity of this circuit and more importantly the EASE of putting it together & the VERY VERY LOW-COST of making it and also thanks to everyone who has helped me to fill-in the missing info i needed to learn to find out how to measure and calculate adding a resistor/s per different low-battery/pack to this circuit so it can be 'permanently-set' to very specific batteries in all my different needs around the house!!!

    One thing i made was a very simple solder-fume extractor using a 12v pc fan & a 7812 voltage regulator found HERE

    http://makeprojects.com/Project/Mini-Fume-Extractor/21/1

    BUT...............

    ONE of the TWO 9 volt batteries it takes (i had only noticed this other day bcoz i was showing a friend how to accurately find out the exact voltage of a 9v battery!), DRAINS ALL THE WAY TILL 0.00 or 0.02 VOLTS and the second 9v battery drains to about 7 volts, which i think should be the time to charge it!, but was NOT happy that the Dang solder fume extractor was TOTALLY draining ONE battery !!!

    So, You See, THIS circuit will really save my bacon, lol, when it comes to needing to recharge the batteries inside it, i can have two of these circuits (one per battery!) and it'll tell me exactly - ish - when they are discharged to around about 7volts EACH and NOT allow only ONE battery from draining ALL-THE-WAY !!!

    I'm also very very surprised that people took the time to help me out as ive posted questions in all the ""BIG-NAME"" electronics forums and have just got plain-ignored which i think is the HEIGHT of RUDENESS, WHY FOOKING have the Dang forum if questions are gonna be ignored ? A-HOLES.....

    SO.........ONCE LAST TIME !!!!

    I am so very grateful for all your help here and promise i wont ask any stupid questions as i usually google like mad for weeks for my endeavors before i give up and ask a question in the forums!

    MAJOR THANKS TO EVERYONE !!!!
     
  20. How to build THIS low-battery indicator ON A BREADBOARD !!!

    Hello Folks, i'm back again, hopefully for the last time ! lol

    Now im trying to get this SIMPLE Circuit on my breadboard but fail epically !!!!

    I have tried over 20 TIMES and its really getting upsetting that i'm so inept that i can even follow a simple schematic to translate this onto a breadbaord...

    Please could anyone post any help on how to correctly get this onto my breadboard because every Dang time i try the circuit NEVER functions as it does after ive soldered it to a perfboard/stripboard, have cried many many times!

    Please help once again!

    Many thanks in advance for your gratefully & most-appreciated help!