RC Filter Cutoff Frequency Guide

A first-order RC low-pass filter is one of the most useful circuits to have in your mental toolkit. Here's the practical version.

Cutoff frequency: f_c = 1 / (2π × R × C). At the cutoff frequency, the signal is attenuated by 3dB (about 70% of full amplitude). Above the cutoff, attenuation increases at -20dB per decade. Example: 10kΩ and 100nF gives f_c = 159Hz — useful for filtering audio noise above the voice range. 1kΩ and 100nF gives 1.59kHz.

Practical tips: use this for ADC input filtering to remove high-frequency noise before digitizing. Use it on GPIO outputs driving long cable runs to reduce EMI by slowing signal edges. Use it for debouncing alongside software debouncing — a 10kΩ / 100nF combination gives ~1.5ms time constant which smooths most contact bounce. Remember the RC filter has non-zero output impedance; if you need to drive a low-impedance load after the filter, add a buffer.