LED chaser circuit and a mysterious heisenbug.

So in my previous post i had a 555 timer circuit making a square wave. (There was actually one problem I found with that circuit, since corrected.) I took that circuit and added a HFC4017B decade counter chip, and used the outputs of that chip to drive 8 LEDs. This works, (and here’s the weird part) so long as I have an oscilloscope probe connected between the 555 timer output and ground. The oscilloscope doesn’t have to be turned on. If I disconnect the oscilloscope probe, it quits working. I’m suspecting that the probe is acting as a very low value capacitor, and maybe if I stuck a few picofarad capacitor between the 555 timer output and ground, it might make the circuit happy — but WHY???? (As an experiment I just tried adding a 22pF cap between 555 timer output and ground, and no, it doesn’t work.)

I had been kind of thinking that all this electronic stuff I’ve been doing has been going too smoothly, too problem free. I do remember from my college days trying to debug circuits and pulling my hair out. I guess THAT’s starting to come back to me as well, LOL.

A video:

Update: playing around with the circuit a bit more, I found that if I disconnect the 4017 from the 555 timer circuit, and if If I just grab the wire lead feeding into the 4017’s clock pin, even on the insulated part, the circuit kind of works — the led’s chase — though of course the rate is not variable by means of the potentiometer controlling the 555. However, the rate is sort of sporadically variable just by lightly or firmly touching the wire lead connected to the 4017’s clock pin. So, I think maybe the lead is just floating highish, kinda sorta and triggering the chip to step through its sequence. If I let go of the lead, the sequence stops. Meanwhile, my 555 timer circuit has quit working, LOL.

Update again: I measured my finger with my oscilloscope, and I see I’m putting out around 0.5 to 1.5v of 60Hz sin-ish wave (measured peak-to-peak).. Measured with my voltmeter in AC mode, and got around 300mv RMS. So yeah, I have a superpower: apparently, I can drive the clock input of a 4017 decade counter IC at 60Hz with my finger. What a useless superpower.

Update again; I figured out what was wrong with the 555 timer circuit. I had those two long green wires going over to the other breadboard where another two wires connected to the potentiometer. One of the wires on the pot had come loose. So, now it’s all working, The lights chase correctly without the oscilloscope attached, and I can vary the speed with the pot. I’m still left with a little bit of that nagging thought of “how do I know it’s really correct, and not just apparently correct,” but… I don’t know of anything wrong with it at the moment.

But, I still am not sure what made the heisenbug go away, actually., as I’m sure that when I first posted the video, even though the circuit didn’t work properly, I could control the rate of the chase (with oscilloscope attached) with the potentiometer. For that to work, I’m pretty sure the pot would have had to have been connected at that time (and I know I was getting a square wave out of the 555 at that time.) So, the disconnected pot explains why the 555 timer circuit had stopped working, but I still don’t have an explanation for why the timer-driven chaser circuit began to work with the oscilloscope unattached, or why, when it wasn’t working, attaching the oscilloscope made it work. And now the bug is gone, so… might never know.

But, I did learn some things. 1) your test equipment can definitely influence the circuit under test. 2) Your body can act as a 1-3v 60Hz sin-ish wave generator — enough to tickle the inputs of some IC inputs if they are floating. 3) Don’t forget about the parts of the circuit that are off-board, as they can break too.

~ by scaryreasoner on July 13, 2009.

2 Responses to “LED chaser circuit and a mysterious heisenbug.”

  1. Hmmm, that was interesting. Looks like somethings will always remain mystery.

    I myself have been trying to solve the mystery of this legend for a while now. Could not understand much though.

    Let me know in case you get to understand the mystery of the Old Hound and the Legend

    By the way, good writing style. I’d love to read more on similar topics

  2. you get a 60Hz frequency because of the water molecules in your body responding to the 60Hz frequency of the US powergrid.

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