lm3916 dot/bar display with transister TIP121

Discussion in 'The Projects Forum' started by paddyhughes086, Apr 5, 2011.

  1. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi i have searched the web and i have found a few circuits that helped me along the way but im stuck.. please help.

    I have the basic LM3916 datasheet (schematic) working with the 10 leds, so thats fine.

    But when I swap the led on say pin 1 to 10 on the led side with a TIP121, I connect the fly lead from pin one to the base of the transistor, connect a new power supply (I'm using 4 leds (in series) that use a voltage of 8v in total so my power supply is 8v so that's fine.

    I connect this power supply to the leds in series on the + side.

    They feed into the left leg of the TIP121 on the right leg i connect the - feed from the battery.

    When i do this i think this second power supply is sending power back down towards the lm3916 chip. How I know this is I left one of the led's still in place, and when I connected the tip121 to the lm3916, both the new 4 leds with the tip121 and the original led at the Lm3916 stay on?

    Can any one give me any help please?
     
    Last edited: Apr 6, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I've had to re-format your post a bit, because I had a hard time understanding what you were attempting to say.
    The LM3916's output pins do not source current; they sink it. An NPN Darlington transistor would require you to source current to the base in order for it to turn on.

    Your 8v supply does not share a common ground with the LM3916 IC, so it will be "floating", and the TIP transistor will not be able to be controlled by the LM3916.

    You really need the 8v - connected to the LM3916 ground.

    The TIP121 needs to be exchanged for a TIP126, a PNP Darlington. However, a Darlington will drop about 0.7 to 1.4v from collector to emitter, depending how much current is flowing through it. I don't know how many LEDs you are planning on operating with this set-up.

    Next, 8v won't be enough if your LEDs have a Vf of 2v @ 20mA, due to the Darlington's 0.7v to 1.4v drop from collector to emitter.

    You do not have any current limiting resistors on your LEDs.

    You do not have a current limiting resistor from the base of the TIP Darlington to the LM3916 output.
     
  3. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Sorry about the post it late I’ve been doing this all day. You sum how managed to understand what I wanted thanks. The Tip121 is a NPN I taught it had something to do with my project not working as for the resistors I didn’t put any into the drawing as I was having problems with the program IM currently using. I will try to get Some PNP tomorrow thanks for your help. As for now its good night from Ireland il let you know how i get on tomorrow
     
  4. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Hi I went into town today and got Tip126 first one works great but I was trying the other 3 for ages and the lads would not light up.

    When I looked closer to the Transistor I see the 3 of them are (TIP126m) so they don’t match the first transistor (TIP126). I am hoping someone can tell me that this is true cos they just don’t work with my circuit

    Please help

    Is there a difference between TIP126 AND TIP126M


    All help is greatly accepted thanks
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I can't seem to find TIP126M's online. I think you may have three fake transistors.
     
  6. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    Would a shop sell you fakes that sucks
     
  7. paddyhughes086

    Thread Starter Active Member

    Jan 30, 2011
    48
    0
    I have a new question now about the lm3916 ic and the TIP126.
    I have supply of 3 v going the the Lm3916 IC and to the Tip126 I have 6v
    Somehow the 4 lads that should be 2v and 20ma to run all seem to be running at full power true the TIP126.
    Question is are the Led getting power from both power supplies-meaning is it get 6v true the collector and emitter and getting the rest of its voltage from the 3v power supply true the IC chip to the Base of the TIP126.

    I have tried to put a meter onto it to see but it keeps going up and down as the LM3916 is being controlled by an audio signal
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sorry that your post got "lost" - it's been busy.

    And you have the negative terminals of the supplies connected together, correct?

    No, the LEDs are only getting power from the 6v supply. The LM3916 only sinks some current through the transistors' base when the input signal is high enough.

    You can supply a fixed voltage to the SIG input using a potentiometer, with a resistance anywhere between 5k Ohms to 50k Ohms.
    Connect one "end" of the pot to your +3v supply, the other end to ground, and the middle terminal (the wiper) to the SIG input. Then you can slowly turn the pot in one direction and watch the LEDs light up in sequence, then turn the pot the other way to watch the LEDs turn off in sequence.
     
Loading...