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Circuit diagram

This circuit is designed to transmit a continuous audio tone within the FM broadcast frequency range (88-108 MHz), potentially for applications such as remote control or surveillance. The circuit draws approximately 30 mA from a 6-9 volt power source and can achieve a reception distance of up to 100 yards. A 555 timer is employed to generate the tone (around 600 Hz), which then modulates a Hartley oscillator. A second JFET transistor buffer stage is implemented to isolate the oscillator from the antenna, minimizing the impact of antenna position and length on the frequency. Precise frequency adjustments can be made by altering the value of the 200 ohm resistor connected in series with the battery. The oscillator’s frequency is determined by a 5-turn tapped inductor and a 13 pF capacitor. The inductor was wound around an #8 X 32 bolt (approximately 3/16 diameter) and subsequently removed by unscrewing the bolt. The inductor was then stretched to about a 3/8 inch length and a tap was created near its center. The oscillator frequency is targeted to be near the center of the band (98 MHz) and can be shifted slightly higher or lower by expanding or compressing the inductor. A small signal diode (1N914 or 1N4148) serves as a varactor diode, enabling variations in capacitance in parallel with the inductor. This slight change in capacitance, occurring at the audio rate, results in a corresponding shift in the oscillator’s frequency (600 Hz). A ramping waveform applied to pins 2 and 6 of the timer is directed through a large (1 Meg) resistor to the reversed biased diode. This alters the capacitance of the diode in response to the changing voltage, thereby modulating the tank circuit’s frequency. Alternatively, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator, possibly requiring an additional pullup resistor to reverse bias the diode. The N channel JFET transistors utilized should be high-frequency VHF or UHF types (Radio Shack #276-2062 MPF102) or similar.

Varactor Diode

A varactor diode, such as the 1N914 or 1N4148, is a semiconductor diode whose capacitance changes with the applied reverse voltage. These diodes are frequently utilized in frequency-modulated oscillators and other circuits where a variable capacitance is required. The capacitance of a varactor diode is typically measured in Farads (F). This allows the circuit to dynamically adjust the oscillator frequency based on the voltage applied across the diode, as described above. They are crucial for achieving frequency modulation.

JFET Transistor

The N channel JFET transistors, specifically the Radio Shack #276-2062 MPF102, represent a high-frequency VHF or UHF type. JFETs (junction field-effect transistors) are voltage-controlled field-effect transistors. This type of transistor allows for the amplification of signals while offering low input capacitance and high gain, making them suitable for applications requiring precision and stability. They are commonly used in oscillator circuits to provide a buffer and isolation from the antenna.