A first-order RC filter’s corner frequency is fc= 1 ÷ (2πRC). That is the −3 dB point, where the filter starts to bite. A 10 kΩ resistor with a 100 nF capacitor rolls off above 159 Hz. Set your own values below.
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
The formula
One resistor and one capacitor set a single corner. Below fca low-pass passes the signal; above it, the response falls off at 20 dB per decade. Swap which element goes in series and you get a high-pass with the same corner. The corner is set by the product RC, so a 10 kΩ / 100 nF and a 1 kΩ / 1 µF land at the same place. The two values still matter for loading and noise: bigger R is a higher source impedance, bigger C is a stiffer node.
From a real board
The OTD L1.05 board reads analog signals on the ESP32’s ADC. A small RC low-pass at the input rolls off noise above the corner before the ADC samples it, and it gives the converter a settled voltage to grab (One Thousand Drones, L1.05 design 2026). Set the corner above the signal you care about and below the noise you do not.
A first-order filter is gentle
One stage is a soft slope, not a wall: a decade past the corner you are only down 20 dB. If you need a sharp edge, cascade stages (each adds another 20 dB per decade) or move to an active filter with a defined response. For a simple anti-alias or noise trim on a slow signal, one RC is usually enough.
References
- One Thousand Drones. ESP32 analog sensing (L1.05), ADC input filtering. Build the board.