LM3914 based LED VU Meter Using Transistor for multiple Leds

Discussion in 'The Projects Forum' started by paddyhughes086, Mar 2, 2015.

  1. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi i am working on a LM3914 VU Meter with a student and he wants to have about 4or5 leds on each output pin of the Lm3914 could a Transistor be used for this and if so what one. or is there some other way of doing this thanks

    [​IMG]
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    You dont need the series 1K resistors from the led outputs, the current limit is set by the resistor on pin 7, as for extra leds, use a PNP transistor with a 1K ohms base resistor from the ouptuts and feed the leds with 100 ohms series resistors.
     
    Last edited: Mar 2, 2015
    Roderick Young likes this.
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,797
    1,103
    What is the typical Vf of the LEDs?
    Is a supply voltage greater than 9V available to power the LEDs?
     
  4. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi Dogydave

    Would a BC557B Diotec Bipolar PNP Transistor -45V or BC556B Diotec Bipolar PNP Transistor -80V work with this.

    so pin 1 to 10 on the IC use a 100 ohm on each pin out to the base of the PNP TRANSISTOR and then a 1k from the emitter or the collector
     
  5. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Yes the transistor BC556B is fine, put the leds and 1k resistors on the Collector to ground, Emitters to V+,
     
  6. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi Dodgydave
    attached are my circuits could you have a look over them and see if they look ok Thanks
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    looks ok , but you forgot the base resistors...
     
  8. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi Alec_t
    Sorry I missed your post thanks for the replay. The leds are Piranha
    Red- Forward Voltage: 3.0-3.4V (at 20mA)
    Yellow Forward Voltage: 1.9-2.1V (at 20mA)
    Green Forward Voltage: 2.8-3.6V (at 20mA)
    Blue Forward Voltage: 2.8-3.6V (at 20mA)

    and I will use 12v dc power supply if that helps
     
  9. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Thanks Dave I have added the 1k what controls the voltage to the leds
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Paddy, if your leds have a volt drop of 3V,then four in series is 12V volt drop, may be better to put 3 in series with 150 to 330 ohms resistor, that would give an led current of 10 to 20mA.
     
    Last edited: Mar 2, 2015
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,797
    1,103
    As Dodgydave says, 4 LEDs drop too much voltage (unless the supply is ~15V). The resistors in series with each LED string are rather high in value, unless you want only a couple of mA through each string?
     
  12. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Sorry to keep asking but could I just up the power supply to 12v. And when you say 3v drop do you mean that's 3v max down to the lowest working voltage
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Yes you can use a 12v supply , looking at the voltage drops of each colour, the yellow have the lowest and the blue the highest, so four yellow leds will be 8 V, whereas four blue leds will be 14V,

    So its better to put 3 leds in series,with resistors of 330 ohms for the yellow, and 100 ohms for the other colours,otherwise the brightness will be different on each colours.
     
    Last edited: Mar 2, 2015
  14. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi Dodgydave
    Just have a question on working out the resister values for each led string.
    Red- Forward Voltage: 3.0-3.4V (at 20mA)
    Yellow Forward Voltage: 1.9-2.1V (at 20mA)
    Green Forward Voltage: 2.8-3.6V (at 20mA)
    Blue Forward Voltage: 2.8-3.6V (at 20mA)

    if the voltage is 12v how are you working out the resister value and the mA

    thanks
     
  15. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Hi Paddy, each led has a different voltage drop, so 3 reds will give a voltage drop of approx 9v, whereas 3 yellows are 6v, so 20mA for the reds needs a 300 ohms resistor,and 20mA for the yellow needs a 150 ohms.

    Red resistor is 6v/20mA = 300

    Yellow resistor is 3v/20mA =150
     
  16. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    thanks dave

    so 3 red at 3v is 9v at 20mA... 9v/20mA =450

    if the mA of led is between 20mA-30mA would i have the value to 25mA
     
  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    three reds is 9v, subtract from the supply so 12-9= 3v, the resistor drops the remaining voltage and sets the current,

    thats 3v/20mA = 150 ohms
     
    Last edited: Mar 12, 2015
  18. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    Just a simple question here. You say "LM3914 based LED VU Meter" VU is Volume Units which are Logarithmic and the LM3914 is a linear chip. If you do in fact want a VU meter representation wouldn't the LM3915 be the better choice? That would afford 3 db log steps.

    Ron
     
  19. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Sorry to keep at you. How are you working out the 300 resister what's the equation
     
  20. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    what 300 resistor? if you mean the resistor to select for the red current, then as explained previously

    3 yellow leds in series gives a volt drop of 2Vx 3= 6V, its run from a 12V supply so we need to drop 12-6 = 6V at 20mA thats 6V/20mA = 300 ohms.
     
    Last edited: Mar 12, 2015
Loading...