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Introduction
This is a one-evening project that will result in a working medium-wave (MW) radio, and will also teach you a little about the way electricity can be generated from the right metals and a little (safe) acid. All you need are three lemons or other citrus fruit, three pieces of copper and three pieces of zinc (or galvanized metal) for your power supply.
Construction
Figure 1 shows the circuit and Figure 2 its layout on a simple 'plug-in' prototype board. The six pieces of metal are connected as shown, to wire the three lemons in series; use ordinary wire between each lemon and the next. If you have a meter to measure the total voltage, it should be about 1.8V. Use a standard ferrite rod, and wind on it about 40 turns of single conductor PVC-insulated wire.
Wire up the circuit on the board as illustrated in Figure 2. Soldering is not required with this type of board -- just plug in the components and the wires.
Only one transistor is needed. The tuning capacitor, VC1, selects the station you want to hear, and D1 helps to remove the carrier from the RF signal.
The resulting audio signal is fed to TR1, a small transistor audio amplifier, which makes the signal big enough to drive a crystal earpiece comfortably.
Walkman-type earphones will not work, so invest in a crystal earpiece which you can use in several other projects, too! If you use a smaller capacitor than that specified for VC1, you will need more turns on the aerial coil.
Tests on the prototype indicated that the radio will run for about a week on three lemons!
Figure 1 Most parts are plugged into the board as shown -- soldering is not
required
Figure 2 Three lemons power the radio, which gives good results for such a simple circuit
Parts list
Resistors: all 0.25 watt, 5% tolerance
R1 1 megohm (M ohm)
R2 10 kilohm
Capacitors
C1 1 nanofarad (nF) min. ceramic
VC1 250-500 picofarad (pF) variable
Semiconductors
TR1 BC107 npn (or BC108, BC109C)
D1 OA90, OA91 germanium (not silicon)
Coil
L1 2 meters of single-conductor insulated wire on a standard ferrite rod Additional items
Plug-in prototype board
Wire aerial at least 3m long
Crystal earpiece
Three juicy fruits