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  • I have an RC circuit delivering a square signal via a standard 555 timer set in monostable.
  • I am powering the 555 with 4.88V DC directly from the Arduino, which itself is taking its power from the Serial USB cable connected to my computer, running on battery power (i.e. = not mains).
  • I did my homework and selected appropriate decoupling/bypass caps directly across VCC-GND to help with power smoothing. All fine.
  • Arduino triggers a set of 6 consecutive reads to calculate/average the time constant and then goes quiet until next request. Nothing fancy.

Now: When I put a probe on the capacitor and observe voltage, I see little ripples that almost look like a heart beat...(Figure 1.) They are spaced 25.5uS apart.

Q: Could someone tell me the origin of the ripples?

Figure 1.

enter image description here

Schematics 1.

schematic

simulate this circuit – Schematic created using CircuitLab

Editions: - Added Schematics following requests.

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  • \$\begingroup\$ Nope. It might be a simple setup, but your words don't describe anything reasonable. Where were those measurements made? What is the blue trace? What is the yellow trace? Do you mean the shark fins, or the little blips on the fins? \$\endgroup\$ – JRE Apr 19 at 19:40
  • \$\begingroup\$ Show us a schematic anyways. I'm wondering if you did not place a decoupling cap in some place where you should have. \$\endgroup\$ – Toor Apr 19 at 19:44
  • \$\begingroup\$ That you get ripples only during capacitor charging is curious. Are these synchronous with the Arduino reads? Seems there's no ripples apparent on monostable digital output.. \$\endgroup\$ – glen_geek Apr 19 at 19:51
  • \$\begingroup\$ What does your circuit look like? \$\endgroup\$ – laptop2d Apr 19 at 20:50
  • \$\begingroup\$ @laptop2d, Thanks: I have added the schematics. IMPORTANTLY, note that the "heart beat" ripples disappear completely if the long wire & push switch (dotted box in Figure 2.), are removed and instead, C1 is connected immediately after the 555-Threshold/Pin6. Which leads me to this question? Could this be inductance related (I am thinking, the long section of // wire could act as a coil...) \$\endgroup\$ – Jeruinsky Apr 20 at 11:58
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Q: Could someone tell me the origin of the ripples?

The most likely situation is is conducted emissions from either the USB, PC or Arduino. The Arduino has 10mΩ's between it and the PC on the USB's cable ground and some inductance. If there is a shifting load (likely from the Arduino or USB packets being sent repetitively) then it causes a shift in current on the USB ground. The USB ground is not a superconductor, it is a resistor, and anytime you have a resistor with shifting current the voltage also changes. This would cause the entire ground of the system to change. It's not uncommon with digital systems to see shifts of a few mV.

The most interesting thing is that 'heartbeat' is not the yellow trace, this indicates that it's most likely a current local to the 555 timer through ground.

The scope it also likely contributing to the situation by creating a ground loop (unless your using a differential probe), as the ground of the scope is most likely connected to AC mains, and so is the ground in the PC.

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