I am using a DSO138 oscilloscope for learning electronics. It is not much, but it is very helpful for me at present.

I was setting up a 555 astable circuit. I noticed that it was triggering even though the circuit was not complete. I have included a picture of the circuit as well as the oscilloscope trace. The oscilloscope ground is connected to ground, and the oscilloscope positive is connected to pin 3 of the 555 (output). tracing 1 breadboard circuit

The funny thing is that it does not trigger if my fingers are far from the white wire (which is connected to pin 2 of the 555, but nowhere else), but if they are close, I get the trace I included. If my fingers get really close to the white wire, the duty cycle goes to near 50%.
I am not sure what is going on. Can anyone help explain this?

I have also included a trace for when the oscilloscope ground is connected but the oscilloscope positive lead is not: trace 2

I assume this is noise in the power supply. I also assume it is related to the issue described above.

Any help would be much appreciated. Thanks in advance.

Here is a schematic: schematic

  • 1
    \$\begingroup\$ Please post a schematic and mark the missing components, the point that you touch/approach etc. \$\endgroup\$ Oct 10 '16 at 14:31
  • \$\begingroup\$ Are you using a bipolar 555 or the CMOS 7555? \$\endgroup\$
    – Janka
    Oct 10 '16 at 15:21
  • \$\begingroup\$ I am using a Bipolar 555. \$\endgroup\$
    – Fed
    Oct 10 '16 at 15:33

From your oscilloscope, the frequency of the output seems to be around 50Hz (or could be 60Hz if you're in the US or other countries - it's not that easy to see the exact period). When you put your hand near to the white wire, you could be coupling mains electromagnetic radiation into the circuit. The 555 timer can be used as a schmitt trigger in certain conditions - generally with the trigger (pin 2) and threshold (pin 6) pins together.

If the threshold pin isn't actually connected to anything, it might be capacitively coupling to the trigger pin.

In a sentence, your body acts as an antenna for mains, bringing your body near to a circuit with floating inputs will couple some noise into the circuit, potentially producing an effect like this.

  • \$\begingroup\$ Thank you for that explanation. When I connect the trigger and threshold together, the oscillation remains, at near 50% duty cycle. Does this make sense as well? \$\endgroup\$
    – Fed
    Oct 10 '16 at 18:01
  • \$\begingroup\$ Yep, so when you connect the two pins together, its actually then configured properly as a Schmitt trigger, if you put your scope on pins 2 and 6, then also on the output (pin 3), you should see the output go low at the peak of the input sine wave. Unconnected pins on a chip are a bad idea because they cause a load of spurious effects! \$\endgroup\$
    – Pyrohaz
    Oct 10 '16 at 21:30
  • \$\begingroup\$ But there is still no input. So what is doing the triggering? \$\endgroup\$
    – Fed
    Oct 10 '16 at 21:40
  • \$\begingroup\$ Because your body is large and partially conductive, it acts like an antenna (albeit a poor one) to mains noise (50/60Hz from your power lines). When you bring your hand near to the wire, this noise couples into the circuit - most likely capacitive where your body acts as one capacitive plate and the wire acts like another. It is likely that this noise is causing the triggering. \$\endgroup\$
    – Pyrohaz
    Oct 11 '16 at 20:20

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