Homemade Crystal Radio Notch Filters

Written by jason parnell
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Homemade Crystal Radio Notch Filters
Build a notch, or bandpass, filter for your crystal radio. (Jupiterimages/Photos.com/Getty Images)

A notch filter rejects a specific frequency from voltage or audio sources. The name comes from the visual representation of the filter's effect on a graph which represents a divot, or notch. A notch filter is a useful way to improve sound quality or reduce unwanted noise in a crystal radio. While some math is involved, the circuit for a notch filter is simple. Simply create the circuit and place it between your crystal radio's audio outlet and your crystal earpiece.

Skill level:
Moderately Challenging

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Things you need

  • 25-watt soldering iron
  • Rosin-core solder
  • Wiring pencil
  • Copper tape
  • Blank circuit board
  • Resistors
  • Capacitors
  • Transistor
  • Wire cutters
  • Scientific calculator

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  1. 1

    Download a schematic of a notch filter from the Internet. Most schematics will contain at least four resistors, two capacitors and one transistor, and few schematics will assign specific values to these electronic components. The value of the components depends on the frequency the user wishes to filter out and schematics will label the resistors and capacitors R1 to R4 and C1 to C2, respectively.

  2. 2

    Use the scientific calculator to solve for the values of R and C which result in a the frequency you wish to filter out. C1and C2 will also have same value and can be chosen from a list of standard values with the lower values being associated with lower frequencies and vice versa. R3 and R4 will have the same value and can be found by solving 1 / (2 x pi x C1 x frequency) where frequency is the frequency you wish to notch. R1 and R2 are equal to 20 x R3. Once you solve the values of your variable resistors and capacitors order the parts from an Internet retailer.

  3. 3

    Arrange your electronic components on your blank circuit board as represented in your notch filter schematic. Slot the connecting pins through the holes of your circuit board and flip the board over before bending each pin at a 90 degree angle to keep the component in place. Creating a grounding strip by applying a piece of copper tape to an unused portion of your circuit board.

  4. 4

    Solder the electronic components in place using your 25-watt soldering iron and rosin-core solder. Place the tip of a length of rosin-core solder against the bent joint of the electronic component and circuit board. Touch the heated tip of your 25-watt soldering iron to the solder and melt a small amount directly to the joint. Allow the molten solder to cool for several minutes before snipping the excess length of the connecting pin off with wire cutters.

  5. 5

    Use your wiring pencil to draw the wiring among components as represented by the solid lines in your circuit schematic. On the side of the board you soldered use the wiring pencil to melt solid lines of molten copper directly on to the board. Connect components to other components or the grounding strip as represented in your circuit schematic.

Tips and warnings

  • If you can't find the exact resistor or capacitor your calculations suggest use a similar component.
  • It's best to guess at the capacitor values and work from there then update your work until you achieve the desired results.
  • You can string several notch or band-reject filters in a line to create more precise results.
  • Always solder in a ventilated area due to toxic fumes.
  • Never work with a soldering iron or wiring pencil without protective glasses or gloves.

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