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How to Use a Capacitor to Clean Up a Noisy Signal

A capacitor can filter out unwanted signal noise.
Capacitors. A close up. It is isolated on a white background. image by Andrey Khritin from Fotolia.com

If you’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 don’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 you’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 circuit’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 filter’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 resistor’s leads to your circuit’s positive or “hot” signal point.

Solder one of the capacitor’s leads to the free resistor lead.

Solder the other capacitor lead to the circuit’s negative or ground signal point. Trim excess leads with the diagonal cutters. Use the free resistor lead as the new “hot” signal output.

High-Pass Filter

Solder the capacitor to your circuit’s signal “hot” output point.

Solder one lead of the resistor to the free capacitor lead.

Solder the other resistor lead to your circuit’s signal grounding point. Trim excess leads with the diagonal cutters. Use the free capacitor lead as the new “hot” signal output.

Things You'll Need:

  • Noisy circuit
  • 10K-ohm ¼ watt resistor
  • 1000-ohm ¼ watt resistor
  • Calculator
  • Assorted capacitors
  • 25-watt soldering iron
  • Diagonal cutters
  • Electronics solder
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