555 astable crash at low frequency?

Discussion in 'The Projects Forum' started by ALAS, Mar 12, 2014.

  1. ALAS

    Thread Starter New Member

    Sep 25, 2013

    I've been trying to use a 555-based oscillator from this schematic: http://www.555-timer-circuits.com/up-down-fading-led.html ...
    for the purpose of using the output as an LFO to modulate the filter and pitch of a synthesizer. I am using an NE555 and a 2N3904 transistor for Q1. Even though this circuit is intended to drive LEDs, I thought I would use it for this because it is so simple, and because I liked the shape of the waveform it outputs and the wide range of rates I could get by using a pot for R1.

    When I first made the circuit, I could slow it down almost indefinitely by increasing R1. But it suddenly stopped working correctly for reasons that I don't understand. Now, the moment I turn up the resistance of the pot I am using for R1 past about 25K Ω, the oscillation will sort of "freeze" and the voltage at pin 3 will hold steady. If I turn the pot a tiny bit in either direction, the circuit will continue to progress through the cycle of the oscillator for a fraction of a second, and then freeze again.

    I had made three more of this circuit a few months ago and had been powering them together on the same 9V adapter. I didn't realize it until just now but all three of those have the same problem, even though they started out working OK as well.

    I tried switching out both the capacitor and the 555 chip on the one I just made, but to no avail. I haven't tried switching out the transistor yet, but before I do, I figured I would ask if anyone could explain why this would happen. Did I somehow destroy the transistor? Is there a problem with using the 2N3904 transistor specifically? I figured any general purpose NPN would be OK.

    One thing I might mention: this may have started happening at the same time that I put in decoupling capacitors (100 uf and .1 uf in parallel across power and ground) so I could run the new 555 oscillator off the same power adapter as the synth. When I noticed the problem, the first thing I did was take the decoupling capacitors back out. Could they have irrevocably damaged something though?

    Thanks in advance for your advice.
  2. #12


    Nov 30, 2010
    The voltage at pin3 holds steady at what voltage? High, low, same as the capacitor?

    The circuit is valid. I think we're looking for something broken or something miswired. Maybe a worn out pot.
    I wonder what happens when you turn the power off but C1 is still applying 5 or 6 volts to pins 2 and 6. An unpowered IC with voltage on certain input pins can go bad. A bit outside my experience.
    Last edited: Mar 12, 2014
  3. MrChips


    Oct 2, 2009
    Have you tried using CMOS versions such as LMC555 or TLC555?
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
    Maybe as #12 says you could measure the voltages of 6,2,3,4 and the emitter when it is hung.
  5. ScottWang


    Aug 23, 2012
    The below values was what I measured just for reference, I was used Power supply providing 9V, here in Taiwan that we called it LED breathing light.

  6. ALAS

    Thread Starter New Member

    Sep 25, 2013

    I figured out what the problem was, but I don't know how it happened. It was the transistor after all. I replaced the transistor and it went back to working normally. But I want to figure out how and why it happened so I can prevent it from happening again.

    I also just realized when it might have happened. I was trying to mix the output of this oscillator with the output of the LFO in the synth. I thought I could probably just do it the way I would set up a passive mixer to mix audio signals, just passing them through resistors and then mixing them together. But is it possible that the voltage of the other LFO hit the emitter of the transistor when I was combining them together and damaged it? When mixing audio signals, "bleed" like this isn't really destructive, so I didn't even consider it.

    If that was the problem, what can I do to prevent it from happening again? Could I just put diodes into the "mixer" to prevent the voltage from each oscillator from sloshing backwards into the other?

    Or am I totally on the wrong track with my attempts to mix these LFOs?

  7. #12


    Nov 30, 2010
    Running the emitter of a transistor higher than the base by about 5 volts makes it break down backwards. A diode or two should fix the back feed problem.

    How bad depends on how much current goes backwards. I've seen it done with a few milliamps as a fake zener diode, but 100 uf of capacitor can pack quite a surge if you connect it when it's charged to an unpowered transistor.