# How to Use a Capacitor to Clean Up a Noisy Signal

By John Papiewski

If youâ€™re working on an electronics project and encounter a noisy signal, you can often remove the noise with a capacitor. The capacitor functions as a filter, passing the signal frequencies you want while reducing the ones you donâ€™t want. You can choose between two basic kinds of filters: low-pass and high-pass. A low-pass filter removes noise above a specified point, called a cut-off frequency. A high-pass removes noise below its cut-off frequency. Once youâ€™ve determined the filter type, determine the cut-off frequency, calculate the capacitance and resistance values, build the filter and connect it to your circuit.

### Preparation and Calculations

Determine the frequency ranges of your circuitâ€™s signal and its noise. If the noise has higher frequencies than your signal, choose a low-pass filter configuration for the capacitor. If the noise has lower frequencies, choose a high-pass filter design.

Determine the filterâ€™s cut-off frequency. This is the frequency at which the filter begins to remove noise. A low-pass filter admits all frequencies below this point, while removing frequencies above it. A high-pass filter does the exact opposite.

Select a resistor. If you want a low-pass filter, choose the 1,000-ohm resistor. For the high-pass filter, choose the 10K-ohm resistor.

Calculate the capacitance value for the filter based on the cut-off frequency. Use the following formula:

C = 1 / (2 x pi x R x Fc) where C is capacitance in farads, pi is the constant, 3.1415, R is resistance in ohms, and Fc is the cutoff frequency in hertz. For example, if you have a low-pass filter, you have a 1,000-ohm resistor. If your cut-off frequency is 5,000Hz, the formula becomes:

C = 1 / (2 x pi x 1,000 x 5,000) = 32 nanofarads

Select a capacitor having the value you calculated in Step 4. In most cases, you can use a capacitor with a voltage rating of about 50 volts.

### Low-Pass Filter

Solder one of the resistorâ€™s leads to your circuitâ€™s positive or â€śhotâ€ť signal point.