LIBRARYReactive parts and filtering.
How capacitors and inductors react to changing signals, and how an RC filter's cutoff frequency picks what passes. With a live calculator.
Resistors treat every frequency the same. Capacitors and inductors do not: they react to how fast a signal changes, and that lets you build a filter that passes some frequencies and blocks others.
Reactance
A capacitor's opposition to current falls as the frequency rises, and an inductor's opposition rises. That frequency-dependent opposition is reactance. It is why a capacitor blocks steady DC but passes a fast signal.
The RC filter and its cutoff
A resistor and a capacitor together set a cutoff frequency, the point where the filter starts to roll off. In a low-pass filter, frequencies below the cutoff pass and frequencies above it are attenuated. The cutoff comes straight from the R and C values.
Inputs
Result
159.2 Hz
−3 dB cutoff frequency
First-order: the response rolls off 20 dB per decade past the corner. For a steeper skirt, cascade stages or use an active filter.
Time constant (τ = R × C)
1.00 ms
A mechanical picture makes the roll-off intuitive: an RC low-pass behaves like a shock absorber. A fast, sharp jolt gets soaked up and smoothed, while a slow, steady change passes straight through unaltered. Fast is above the cutoff and gets damped; slow is below it and gets through.
Where you meet it
An RC low-pass in front of an ADC is an anti-alias filter: it removes fast noise the converter would otherwise fold into the signal (Espressif ESP-IDF). The same RC also sets how fast a line settles, which is why a reset or button line often carries one.
Checkpoint
Quick check
One Thousand Drones engineering team · verified 2026-07
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