Adding a basic amplifier circuit to your crystal radio allows multiple people to listen at once, by replacing the standard high-impedance, low-volume standard earphones with a loudspeaker. Although crystal radios do not require batteries, this amplifier circuit will require a small battery. This method of amplification is not designed to reduce the interference that is a natural result of the radio's high bandwidth. This is solely a basic means of amplification and an alternative to the limitations of earphones.
- Skill level:
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Things you need
- 9-volt battery
- Connector with leads for 9-volt battery
- Output transformer (1,000-ohm primary, 8-ohm secondary)
- .12 uF Mylar capacitor
- 15 uF, 10-volt capacitor
- .15 uF disk capacitor
- 150-ohm, 0.5-watt resistor
- ECG 129 PNP transistor
- SPST switch
- 3-inch 8-ohm speaker
- Solderless breadboard
- Jumper wire kit (22-gauge with alligator clips)
- 22-gauge wire
Remove the earphones from the radio circuit and leave a few inches of wire on both sides to work with. Strip back some of the insulation for the new connections. Connect the positive side of the earphone section to the base (B-side) of the transistor. The emitter (E-side) goes to the 150-ohm resistor, with the 15 uF capacitor connected across the resistor.
Ground one side of the disk capacitor and connect the other end of the resistor in Step 1 to the same ground circuit. Connect one end of the .12 uF Mylar capacitor to the wire between the base side of the transistor and the positive earphone side (from Step 1), and the other end to the same ground circuit.
Connect the other side of the disk capacitor to the transformer and connect the collector (C-side) of the transistor to the same wire, between these two points. The 8-ohm side of the transformer goes to the speaker and the other end is connected to one end of the switch.
Attach the 9-volt battery connector to the battery. Connect the positive terminal's lead to the other end of the switch and ground the negative terminal. Ground the other side of the earphone circuit. It is preferable for all grounds to come together at a common point on the chassis.
Turn on the radio, flip the switch on the amplifier circuit and enjoy your crystal radio.
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