Problems driving an NPN transistor from a 555

Discussion in 'The Projects Forum' started by AdamM, Nov 24, 2009.

  1. AdamM

    Thread Starter Active Member

    Mar 7, 2009
    Hi Folks,
    I've built a version of the IR LED driver circuit from here: and I'm having some trouble getting it to work.

    The timer is putting out a 38kHz signal, at 4.38v. (The duty cycle is wrong -- closer to 65% than 50% -- but that's a different question.)

    The problem is that when I connect the transistor, even with no load on it, the signal out of the 555 drops to about a .6 volt square wave, and the signal out of the transistor is about -5v relative to my positive bus, with about a .4v square wave on top of that -- in other words, the transistor is mostly on all the time, only varying the output voltage by about 0.4v.

    It seems vaguely similar to the issue mentioned in this post, though the application is fairly different:

    The transistor is a 2N2222 NPN, as specified in the design. The only difference between my components and the specifications is that I didn't have a 4.7k resistor on hand, so I've used a 5.6k between the 555 output and the base on the transistor instead. I also tried with a 3.3k though, and it didn't change the results I'm getting, so I don't think that's the problem.

    As anyone reading this has probably guessed, I don't know much about transistors! If anyone has an idea what I could be doing wrong, it would be great to hear about it.

    Thanks for your help,
  2. Wendy


    Mar 24, 2008
    What is the 555 number you are using? This circuit requires a CMOS version, but if you're using a conventional 555 it should work with 5V (no less).

    I'm one of the 555 pushers around here.

    Bill's Index

    The 555 Projects
  3. russ_hensel

    Distinguished Member

    Jan 11, 2009
    I think your resistor value is too low, it may not be what you think, it should be around 5000 ohms.
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    Try a 10k pull-up resistor from P3 to +5V
  5. SgtWookie


    Jul 17, 2007
    Both CMOS and BJT (transistor) versions of the 555 are much better at sinking current than sourcing current. Even at 15v, the CMOS version can only source 10mA, but can sink 100mA.

    At the same 15v, the bjt version can sink or source up to 200mA; but it's high output voltage is limited by the Darlington output configuration. Even under light load, Vcc-1.3v is about the best it can do. With a 5v supply, that's about 3.7v with a light load.

    As the voltage supply decreases, they both get much worse at sourcing/sinking current.

    I don't know offhand what your IR LED ratings are as far as typical Vf @ current goes, but 47 Ohms for R3 is far too low for a current limiting resistor.

    As a wild guess, I'd say your IR LED probably has a typical Vf of 1.2v @ 20mA.
    Figure Vce saturation voltage of the 2n2222 at around 0.2v.
    So, the limiting resistor (R3) needs to be:
    Rlimit >= (5v - (0.2v + 1.2v) / 20mA = 3.6/0.02 = 180 Ohms or greater.

    Oh, if you bought your IR LED from Radio Shack, don't believe the specs on the package. I bought one that said it was rated for 100mA; it burned up at 30mA. Apparently, the 100mA rating was peak current at a very low duty cycle, and the person responsible for putting the data on the package didn't know what they were doing.