Build Your Own Electronic Canary: Circuit Diagram & Chirp Secrets

A delightful design, perfect for fostering new connections with this altered hartley oscillator. It can also serve as a suitable alternative signaling device.

Circuit Diagram

Notes:

This circuit represents a revised hartley oscillator incorporating several supplementary elements. The transformer utilized is a compact audio transformer, type LT700. Its primary winding is center-tapped, exhibiting an impedance of 1Kohms at 1KHz. The secondary winding possesses an impedance of 8 ohms. The incorporation of R1 and C1 contributes to the oscillator's distinctive "chirp" effect. As the 100uF capacitor charges through the 4.7K resistor, the transistor's bias is suppressed. Consequently, oscillation ceases, and the capacitor discharges through the transistor’s base-emitter circuit, initiating oscillation once more. Modifications to these components affect the frequency of the chirp. Furthermore, the chirp’s behaviour is sensitive to voltage variations. When the push button switch is activated, the 100uF capacitor is charged. Upon release, the oscillation diminishes, and the chirp accelerates.

More about components

The circuit leverages a **4N35 transistor**, a general-purpose NPN transistor frequently employed in amplifier and oscillator circuits. This particular transistor is known for its good gain and relatively low saturation voltage, making it ideal for this oscillating application. It operates as the core of the circuit, controlling the flow of current to generate the characteristic chirp. Its primary function is to amplify the signal created by the capacitor charging and discharging process, boosting the amplitude of the waveform and enabling the oscillatory behavior. The 4N35’s performance characteristics, including its current gain (hFE) and collector-emitter saturation voltage, are crucial factors in determining the overall oscillator frequency and stability. The transistor ensures stable and predictable operation within the circuit.