Most often used for television and radio broadcast,the frequency modulation (FM) band has many different purposes, including FM radio stations between 88 megahertz (MHz) and 108 MHz) It also formerly supported analog television channels 0-72 prior to the digital television conversion. A microphone can be bought or constructed that takes advantage of the FM frequency by broadcasting over it.

Instructions

1.Create the radio frequency (RF) coil by wrapping the insulated wire in loops around the 1/4-inch diameter bolt five times. Make sure the loops are close together and tight.

2.Cut off the ends of the wire. Use the knife to scrape off enough enamel to leave 1/2 inch available to solder onto the circuit board.

3.Unscrew the bolt from the coil. Press the loops together to eliminate any gaps. Use the schematic to layout the audio preamplifier and the radio frequency stages.

4.Cut off the legs from one of your components. Solder the legs onto the pads of the electret microphone. Mark the pad that has a connection to the outside of the microphone as the negative lead. Install it on the end of the perforated board so that it is not blocked by other components or in conflict with the RF stage.

5.Affix the components of the audio preamplifier stage onto the perforated board by bending the component leads and connecting them together. Test the audio preamplifier by soldering a temporary wire to test point 1 and hooking test point 1 and its ground to the battery. Attach the headphones to the test point 1 and ground connections and listen for a rumbling sound as you blow into the microphone.

6.Add the components for the RF stage according to the schematic. Set the FM radio to a dead FM frequency near 100 MHz and place the microphone close to it. Use the plastic screwdriver to turn the trimmer capacitor until you hear a loud thump on the radio.

7.Connect the battery. Complete the microphone by using tape to attach the battery to the circuit board's back.

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Next:The History of Commercial Radio

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