I apologise in advance if this topic has already been discussed here. I am no EE and have have no choice but to ask experts to move my project forward.

  1. I am developing a capacitive sensor via a simple RC circuit to read the capacitance of an epee/foil tip. This is done via a 555 Timer running in monostable mode (Figure.1). The circuit uses DC power at approx. 4.7V from the 5V output pin of an Arduino MKR1000.

  2. The expected capacitance of the sensor is small: 50 - 110 pF. I am using a 240K resistor as the RC time constant. T = RC is approx. 22uS as measured with scope and ties with calculations.

  3. When the weapon is not connected (switch open), the output of the 555 is stable and almost constant (reading approx. 70pF in 18uS). No resonance is observed at any sample frequency.

  4. When the weapon is connected (epee/foil described as C1 in the circuit), resonance for certain sampling frequencies is observed.

    • for high sampling frequencies (500uS per cycle, Figure 2.), capacitance oscillates, but stays "stable".
    • At lower sampling frequencies (above 1500uS per cycle, figure 3.), significant resonance can be observed and I am hitting harmonics at specific frequencies (2750uS/cycle, 3350uS/cycle, etc.).
  5. I am also experiencing significant resonance when noise has been injected in the system. Figure 4. shows capacitance obtained from the sensor after the tip of the weapon has been depressed on the opponent's lamé (i.e. a conductive metal jacket). Figures 5 to 9, show the capacitance read by the circuit under various sampling frequencies. High frequencies seem immune to resonance while lower ones feature significant distorsions.

  6. The circuit is in development (i.e. breadboard) mode and lines to the weapon system (denoted C1 in Figure 1, i.e. the connection sockets, 120cm twin-line wire connecting the circuit to the weapon and weapon blade) may constitute a source of significant Inductive Reactance, or at the very least parasitic Resistance.

So, my questions:

Q1- Since the circuit is a simple "class 1" RC (no inductor), what is the probable origin of the circuit reactance? (During my attempts to locate the origin of signal oscillation, I discovered that the main source is the body wire. Could a wire act as a coil and generate back emf voltage during the 555 phases?)

Q2- How can I reduce (eliminate?) resonance by making adjustments to my circuit? (i.e. other than simply avoiding resonance by way of "selecting" a sampling frequency which suits).

Q3- I understand that resonance is in part due to an amount of energy (or "noise") oscillating in the circuit which is not being converted. (Is this correct?) Is it possible to control noise in a circuit electronically?

Q4- Finally, how can I establish a frequency response function to identify the most significant resonant frequencies from the sampling of output capacitance (raw signal) only? (I realise this question is more of the signal processing kind, but I am asking here just in case someone has a ref. to share).

If you require more info/data, just ask, I'll do my best to provide it.

Figure 1.


simulate this circuit – Schematic created using CircuitLab

Figure 2. enter image description here

Figure 3. enter image description here

Figure 4. enter image description here

Figures 5-9. enter image description here enter image description here enter image description here enter image description here enter image description here

  • \$\begingroup\$ ... In other words, is there anything I should do to manage resonance other than trying to avoid sampling my sensor on resonant frequencies? \$\endgroup\$ – Jeruinsky Mar 20 at 18:50
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    \$\begingroup\$ Your drawing has a dead short across the +5 V and GND. Also, the Threshold input is tied to the+5 V. \$\endgroup\$ – AnalogKid Mar 22 at 15:26
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    \$\begingroup\$ Reset is tied to GND, so the 555 will not operate. \$\endgroup\$ – AnalogKid Mar 22 at 15:32
  • \$\begingroup\$ Thanks, AnalogKid, typo corrected. Looks accurate now. \$\endgroup\$ – Jeruinsky Mar 22 at 15:46
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    \$\begingroup\$ " the 555 operates at approx. 31.2kHz." If the 555 is being used as a monostable, what does this mean? " the Output oscillates, but is stable" By output do you mean 555 pin 3? None of the waveforms look like a 5 V digital signal. Without some form of positive feedback there can be no oscillation. Also, what is "high resonance"? \$\endgroup\$ – AnalogKid Mar 22 at 19:14

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