You can learn a lot about FM radio by building a low-power transmitter. It produces enough power to broadcast a signal to different rooms of your home. The transmitter has a transistor that amplifies the signal from a microphone and sends it to a one-transistor oscillator. It runs safely on 9-volt battery power. You can have this circuit together and working in an evening or two.
- Skill level:
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Things you need
- 22-gauge magnet wire
- 2 plastic coil forms, each 5mm
- Wire cutters
- Soldering iron
- Solderless breadboard
- 2 transistors, 2N2222a or equivalent
- 9-volt battery clip
- Electret microphone
- 3 resistors, each 4.7K-ohm 1/4-watt
- 270-ohm 1/4-watt resistor
- 10K-ohm 1/4-watt resistor
- 100K-ohm 1/4-watt resistor
- .001-microfarad 50-volt capacitor
- 10-microfarad 50-volt capacitor
- 5.6-picofarad 50-volt capacitor
- 3- to 18-picofarad adjustable capacitor
- 9-volt battery
- FM antenna
- FM radio
Insert the end of the magnet wire in the base of a coil form and make five turns around its core. Cut the wire on the spool and burn the varnish off the wire end with the soldering iron. Slip the end into another base hole. In the same manner, prepare a second coil. Seat the 2N2222a transistors in the breadboard so each of the leads has its own column. Designate one transistor as "Q1" and the other as "Q2."
Seat the 2N2222a transistors in the breadboard so each lead has its own column. Designate one transistor as "Q1" and the other as "Q2."
Insert the battery clip's black lead into the board so it connects with Q2's emitter lead. Connect the negative lead of the electret microphone to Q2's emitter. Insert a 4.7K-ohm resistor so one lead connects to Q2's emitter and the other lead goes to Q1's base. Connect one lead of the .001-microfarad capacitor to Q2's emitter and the other to Q1's base. Connect the negative lead of a 10-microfarad capacitor to Q1's base and the positive lead to Q2's collector lead.
Insert one lead of the 5.6-picofarad capacitor so it connects to Q1's collector and the other to Q1's emitter lead. Connect the 270-ohm resistor to the emitters of Q1 and Q2. Place one lead of a 4.7K-ohm resistor at the Q1's collector and the other lead into an unused column on the board. Insert the battery clip's red wire into this column.
Insert one lead of the 10K-ohm resistor so it connects with the battery clip's red wire. Connect the other lead to Q1's base. Connect one lead of the adjustable capacitor to the battery clip's red wire and the other lead to Q1's collector. Insert one pin of a coil that you have made to Q1's collector and connect the other pin to the battery clip's red wire.
Insert the negative lead of the second 10-microfarad capacitor so it connects to Q2's base. Insert its positive lead into a free column on the board. Connect the positive wire of the microphone to this column. Connect one lead of a 4.7K-ohm resistor to this column. Connect the other lead to the battery clip's red wire. Insert one pin of the second coil to Q1's collector and the other pin to a free column. Connect the FM antenna wire to this column.
Snap a battery into the clip. Turn the FM radio on. Speak into the microphone while adjusting the radio's tuner. You should hear yourself on the radio when you find the right frequency.
Tips and warnings
- You can substitute capacitors with higher voltage ratings as long as the capacitance is the same.
- If you want to transmit only a few feet, you can probably omit the FM antenna.
- You can change the broadcast frequency by turning the adjustable capacitor.
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