Battery charging and led indicator circuit help!

Discussion in 'The Projects Forum' started by jackdavies, Aug 2, 2010.

  1. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    Hi Everyone

    I am 14 and am designing a new product for the technology market. I have a basic knowledge of electronics and I am currently studiying electronics at school.
    *
    I need a circuit that will be cheep to produce (it will be manafactured in China) and as small as possible.*
    I want it to be able to charge 2 AA rechargable batteries from USB (roughly 5v) and have 5 LEDs that light to show the charge status of the batteries. If someone could come up with a circuit that is small and cost effective there maybe a donation if the circuit is good enough! :)

    If you have any questions please don't hesitate to ask. *

    Thanks*
    Jack*
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    213
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
  4. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    Hi

    Thanks for the quick reply, although this is for my own project during the school holiday here in the UK and not for corsework! :)

    I thought about using Zener Diodes assending in value but was unsure which values I would need for two AA batteries.

    Any Thoughts?

    Thanks
    Jack
     
  5. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    You'd need comparators to make the LEDs light. But why not use a microcontroller for everything? You could control the charging by a temperature sensor (look up ΔT charging for Ni-MH batteries) and use the ADC to measure voltage indicating SoC (state of charge.)

    Remember a battery's voltage response is not necessarily a reflection of charge. A flat 9V battery can still have 8.5V across its terminals, indicating it is only slightly discharged, but when you apply a load, you find the internal resistance has dropped the voltage to 7V, making your device unable to operate. Also, you will have to consider the effects of the charger on the sense circuit. The best way to measure charge is to measure charging current (ΔI), but this has the disadvantage that you cannot test batteries that have already been charged.
     
  6. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    hi
    I wanted to stay away from microcontrollers if possibe as I will be manafactureing in china and simple is better!

    Is there a way of using somthing similar to the zener or the zener but in a different configeration?

    I just want something that will light led's in a "series" to show the charge status of the 2 AA bateries.

    Thanks
     
  7. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Well you could go with four outputs with a quad comparator, some resistors and some clever logic. Then you'd need a charging circuit, if you're not using a micro it will definitely have to be a slow charge circuit or else your parts count will be massive...

    It will be much cheaper and easier to take a low pin count microcontroller with ADC and program the design into it. Any analog circuit will be MUCH more complex than any microcontroller based system.
     
    jackdavies likes this.
  8. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    Thanks tom66, would it be possibe to put together a circuit and parts list for the idea u came up with for me?
    It would be great if you could, I need to be able to charge the two AA batteries from USB aswell and have the 5 LEDs show how much the batteries are charged.

    Thanks again
    Jack
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You also need to be very specific about what kind of batteries you are dealing with. Actually reading material at the link provided in post #3 may shed considerable light on your project.
     
  10. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Jackdavies, I'll give it a shot, but there are a few things to note:

    A) I've never ever built a battery charger and all my research is from the 'net. I'm not responsible if it or the batteries blow up, so I'll expect you to take all reasonable precautions and analyse the circuit carefully. Two sets of eyes are better than one.
    B) You'll still need a programmer and stuff to build it on. Do you have those?
    C) It will be a linear charger most likely. Some kind of LM317 constant current charger, with a cut off switch. No temperature sensing!!
     
  11. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Here is a basic schematic.

    The charger should switch on, apply 2.7V to the batteries and wait for the delta-I to reduce to zero. A voltage sense pin is provided for state-of-charge and the voltage will vary slightly over charging. You will need to write the program for the PIC, I cannot and will not do that. It has a primitive over temperature protection system using a thermal fuse. If the temperature is exceeded, the fuse blows, preventing the charger from catching fire.

    I am not certain about this design because as I mentioned I have never designed a battery charger. I would be appreciative if someone could review the design.
     
  12. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    Hi thanks tom66 for taking on my project :)
    I have some experience with the PIC microcontroller so if possibe I would prefer to use that.
    Thanks
    Jack
     
  13. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Yes that's why there is a PIC12F675 at the core, a midrange low pin out microcontroller.

    One thing I realised later is that there should probably be a jumper between ICSPDAT and the battery voltage sense capacitor, otherwise the chip will not easily program.
     
  14. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    Hi
    Could u list the parts I would need to build the circuit?
    I am going to Maplin electonics to get some bits!
    I will try to piece the circuit diagram! :)

    Thanks
     
  15. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    You'll need all the components in the circuit, plus a PICkit 2 programmer or similar. The programmer is the most expensive when I bought it it cost me £40. I have added Maplin order codes for you.

    • C1: 100uF 16V or 25V electrolytic (Maplin: VH37S)
    • C2: 100nF 50V ceramic (Maplin: BX03D)
    • C3: 100uF 16V or 25V electrolytic (Maplin: VH37S)
    • C4: 100nF 50V ceramic (Maplin: BX03D)
    • C5: 100nF 50V ceramic (Maplin: BX03D)
    • D1: 1N4148 diode (Maplin: QL80B)
    • U1: LM317T 1.5A adjustable voltage regulator (Maplin: UF27E)
    • U2: PIC12F675 flash microcontroller (Maplin: N12BH)
    • R1: Resistor 330 ohms metal film ±5% 0.6W (Maplin: M330R)
    • R2: Resistor 330 ohms metal film ±5% 0.6W (Maplin: M330R)
    • R3: Resistor 330 ohms metal film ±5% 0.6W (Maplin: M330R)
    • R4: Resistor 270 ohms metal film ±5% 0.6W (Maplin: M270R)
    • R5: Resistor 330 ohms metal film ±5% 0.6W (Maplin: M330R)
    • R6: Resistor 150 ohms metal film ±5% 0.6W (Maplin: M150R)
    • R7: Resistor 27 ohms wirewound 3W (Maplin: W27R)
    • MISC: USB connector
     
  16. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Oh, and I almost forgot:

    • F1: 85°C thermal fuse (closest 93°C) (Maplin: RA14Q)
     
  17. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    Thanks so much for all the help you have given me! :)
    What program did you use for the circuit diagram and would it be possible to attach the file so I can make improvements?
    Thanks also for the parts list, ive got a bit of shopping to do!!

    Jack
     
  18. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    I used gschem (part of the GPL EDA toolkit), it's a Linux only program, but it's pretty powerful. I'm still getting used to it. It's not the easiest program to use, but I have kind of got attached to it. If you run Linux and have gschem, you can use the file I've attached.
     
    jackdavies likes this.
  19. tom66

    Senior Member

    May 9, 2009
    2,613
    213
    Ugh, I realised I've forgotten MORE things including the LEDs and the transistor. You can pretty much substitute any small signal LED and any transistor (>500mA collector current, 2N2222 will do - 2N3904 will NOT.)
     
  20. jackdavies

    Thread Starter New Member

    Aug 2, 2010
    19
    0
    Thanks Tom
    What would the small signal LED do and where would it be placed along with the transistor? Also what is ment by small signal?

    Jack
     
Loading...