What batteries should I use to power this circuit?

Discussion in 'The Projects Forum' started by bigapplehades, Nov 5, 2015.

  1. bigapplehades

    Thread Starter New Member

    Nov 5, 2015
    5
    0
    I'm trying to power a circuit that is composed of a string of LEDs (30 LEDs in parallel, 2.1-2.2 forward voltage, each with a 47 ohm resistor) and a 3v mini DC motor. I can either do this on two separate circuits connected by a DPST switch or on one circuit with a SPST switch. What size battery should I use for either option, and which would be the better choice?
    I'd prefer the SPST just because I want to use a momentary push button switch and I already have some (I'd have to order a DPST online), but I want max brightness for the LEDs and sound/vibrations from the motor. Either way the circuit(s) will only be closed for a few seconds at a time with the momentary switch.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Get a data sheet for AA and AAA batteries. With 21mA per LED times 30, plus another 100mA?? for the motor, (ie 3/4A) you will likely need the AAs for the peak current. They won't last long.
     
  3. bigapplehades

    Thread Starter New Member

    Nov 5, 2015
    5
    0
    How many AAs would I need to run it as one circuit as opposed to two circuits?
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,220
    It depends upon how long you need the batteries to last(?).
     
    Last edited: Nov 6, 2015
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,850
    767
    Assuming that the led is 2.2V/20mA/3mm and using 80% as 16 mA, I = 16 mA*30 = 480 mA.
    So maybe you need to use an adapter or buy a 18650/3.7V/3100 mAH chargeable battery.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,170
    395
    Assuming a load of 700 mA @ 20% duty cycle, would use 3 AA Ni-Mh of 2000 mAh giving about 14 h.
    Could add a I A diode in series to reduce V closer to 3 V.
     
  7. bigapplehades

    Thread Starter New Member

    Nov 5, 2015
    5
    0
    The motor is 380mA and each LED is 20mA... The circuit will only be closed for a few seconds at a time whenever the button is pressed (it's a model, it's for show and not going to be used very often or for very long). Could I use a 9v battery with a different motor and resistors?
     
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
  9. bigapplehades

    Thread Starter New Member

    Nov 5, 2015
    5
    0
    Similar project but I'm making it with 30 individual 5mm LEDs and twist the leads between two parallel wires... I've made this before with two AA batteries and a SPST switch but the LEDs aren't that bright and the motor isn't that loud. I don't have all that much technical knowledge.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,850
    767
    Alkaline batteries from 1700 mAh to 3000 mAh
    Assuming that you choosing two 1700 mAh AA batteries.
    You can calculate the resistors for each LED.

    I_total = I_led + I_Motor = 480mA + 380mA = 860 mAh
    Assuming that you press 10 sec then
    I = 860 mAh(10 sec/3600sec )
    = 860 mAh*0.0028
    = 2.4 mAh
    Times = 1700 mAh/2.4 mAh = 708
    So you can press 708 times for 10 seconds.
     
  11. bigapplehades

    Thread Starter New Member

    Nov 5, 2015
    5
    0
    Should I run the LEDs and the motor on two separate circuits with two separate sets of batteries?
     
  12. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    One circuit for everything is fine, nothing to be gained by splitting it up.
     
Loading...