VARIOMETER_RADIO

Published:2009/7/17 2:39:00 Author:Jessie | From:SeekIC

The schematic of the complete variometer radio is shown in the figure. An antenna can be connected to the radio through either of the two points labeled ANT: either directly to the circuit or through a 100-pF capacitor. The ground connection can be made at any of the points marked GND.There is a reason for the preceding options: By varying the antenna capacitance, the ground connection, and the position of the sliding coil, the entire AM broadcast band can be tuned. Depending on the antenna and ground connections, it might be necessary to add a small capacitor, C3, at the point indicated in the schematic. If so, experiment with values between 25 and 200 pF (separately or in parallel) to find which gives the best result. If you build the variometer using Fahnestock clips (as explained later), adding the capacitor(s) after the radio is built if the need arises should be easy. When a signal is selected by adjusting the antenna, ground connection, and position of L2, the signal is passed on to the diode-detector part of the circuit, composed of D1, which demodulates the signal. The signal then goes through bypass capacitor C2 to the earphones. Only high-impedance earphones should be used with the variometer. L1 and L3 are 86 turns of #22 wire on a 1.25-inch form, and L2 is 74 turns of #22 wire on a 1.75-inch form.



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