Using Transistor Arrays Instead of 2N3904s

Discussion in 'The Projects Forum' started by OldSkoolEffects, Apr 7, 2014.

  1. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
    Okay, I have a circuit I've been making for a while now, but it's a pain in the butt to solder 19 2N3904s with their 10k resistors every time I make a board. I've had some good experience with 2004ANs on other projects and want to use them for this one. Before I start etching PCBs though, I want to make sure I haven't made any huge mistakes.

    The logic that drives the transistors has been tested extensively, so I haven't included it. There are 2x 4015's and a single 4017 being run with a 556 timer. The only change to this logic portion is to add 0.1uF caps to each 40xx chip (V+, GND). Also not included are the birds' nest of nets to wire into sockets that run out to warm white LEDS. Again, this portion has been tested and works.

    The smaller image is what I have used previously as it pertains to logic coming into a transistor to power an LED (3-3.4V, 20mA). This setup was just repeated 19 times, each from a different logic pinout. The larger image is the section in question. I currently plan for 3x ULN2004AN, and have eliminated the 10k resistors, as those are in the array, and I'm using 470ohm resistor arrays in place of the original individual resistors. Testing of the original circuit showed acceptable performance for VCC between 6 and 15VDC, but I'm not sure if those limits are good for the longevity of the circuit.

    To sum up, here are my questions:

    1) Are my transistor arrays hooked up correctly to replace 2N3904s?
    2) Is the ULN2004AN transistor a good choice, or is a ULN2003AN better for this application?
    3) Do my allowable voltages for VCC allow for a stable/long lifecycle?

    I'm not to do anything exotic, just reduce my parts count and assembly time but replacing similar individual components with arrays.
  2. MaxHeadRoom


    Jul 18, 2013
    ULN2805 has eight transistors, also there is the inverted version.
    BTW, instead of bipolar transistors for individual apps, I look to the 2n7000 first, often no resistor are required.
  3. SgtWookie


    Jul 17, 2007
    Yes, they're fine.
    The ULN2004A/ULN2804A has 10k Ohm base resistors on the inputs; this is appropriate for being driven by 4000 series CMOS devices operating within a range of 6v to 15v.
    6v to 15v is within the range of 4000 series CMOS ICs.
    However, I can't see the value of your current limiting resistors for your LEDs; I don't know what the Vf and recommended If is for your LEDs.

    However, I suspect that you are attempting to make an automotive taillight turn signal sequencer, which is against the ToS of this website; I caution you that 4000 series CMOS ICs will be destroyed by transient voltage spikes present in automotive systems, and the LED current and resulting brightness will be unpredictable; therefore completely unsuitable for automotive use, and likely in violation of traffic safety laws in your area.
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    I'd go with the 2004. The 2003 is one of my favorite parts and will work with no problems, but you would be hitting the base a little hard.

  5. OldSkoolEffects

    Thread Starter Member

    Nov 18, 2009
    This is for an improved version of Ghostbusters Proton Pack lights I make. I barely have confidence in making dasblinkenlites, so I'd never putz around with my car electronics, regardless of the ToS that I'm already aware of.

    The LEDs are rated at 3-3.4VDC @20mA, and currently I use 470 Ohm resistors.
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Assuming VCC @ 12.5 V, yes to 1,2 & 3.
  7. SgtWookie


    Jul 17, 2007
    470 Ohms will be OK for up to roughly 13.9V, after that the LEDs will receive more than 20mA current.