An air variable capacitor is most at home working as a tuner in a radio circuit. Because of the relatively small plate area, its overall capacitance is low, which is ideal for radio frequency circuits. The variable capacitor is one half of a resonant network, the other half being an inductor. It is easier to make a variable capacitor than a variable inductor, so that's what vintage radio equipment uses. You can learn how to connect an air variable capacitor by building a simple one-transistor radio oscillator.
- Skill level:
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Things you need
- Solderless breadboard
- 2N3904 transistor
- 40 to 250 picofarad air variable capacitor
- 47K-ohm 1/4-watt resistor
- Radio frequency (RF) transformer, 0.5 to 350MHz
- 22K-ohm 1/4-watt resistor
- 1K-ohm 1/4-watt resistor
- .047-microfarad 50-volt capacitor, two
- 22-gauge solid wire
- Wire strippers
- 9-volt battery clip
- 9-volt battery
Insert the transistor into the breadboard, placing its emitter, collector and base leads into separate columns on the board. Set one of the variable capacitor's pins into the board so it connects to the transistor's collector. Insert the other variable capacitor pin into an unshared column. Insert the battery clip's red wire into this column.
Set one lead of the 47K-ohm resistor into the board so it connects with the battery clip wire and variable capacitor. Insert its other lead so it connects to a free column. Insert one of the transformer's secondary wires so it connects to this column. Connect the other transformer secondary wire to the transistor's base.
Insert one lead of the 22K-ohm resistor so it connects to the column shared by the 47K-ohm resistor and transformer secondary wire. Set the other lead into an unshared column. Connect one .047-microfarad capacitor in parallel with the 22K-ohm resistor. The capacitor's two leads will connect to a corresponding resistor lead.
Connect the transformer's primary wires in parallel with the variable capacitor. Set one lead of the 1K-ohm resistor into the column shared by the 22K-ohm resistor and .047-microfarad capacitor only. Connect the other lead to the transistor's emitter. Connect the second .047-microfarad capacitor in parallel with the 22K-ohm resistor. Insert the black battery clip wire into the column shared by the two .047-microfarad capacitors and two resistors.
Cut two 12-inch lengths of wire and remove about 1/2 inch of insulation from each end. Insert one wire so it connects to the black battery clip wire. Insert the other wire so it connects to the transistor's collector.
Turn the oscilloscope on. Set its horizontal sweep speed to 10 microseconds per division. Set the vertical gain to 1 volt per division. Clip the oscilloscope probe wires to the free ends of the 12-inch wires coming from the breadboard. Snap a 9-volt battery into the clip. You should see a sine wave on the oscilloscope screen. Turn the shaft on the variable capacitor and observe that you're changing the sine wave's frequency.
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